Ch-5 Periodic Classification of Elements - Lakhmir Singh Manjit Kaur Solutions Part 1

Lakhmir Singh Manjit Kaur (chemistry) Solutions for Class 10 Science Chapter 5 Periodic Classification Of Elements are provided here with simple step-by-step explanations. These solutions for Periodic Classification Of Elements are extremely popular among Class 10 students for Science Periodic Classification Of Elements Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Lakhmir Singh Manjit Kaur (chemistry) Book of Class 10 Science Chapter 5 are provided here for you for free.

Page No 281: Question 1: (a) On what basis did mandeleev arrange the elements in his periodic table? (b) On what basis are they arranged now?

ANSWER: (a) Mendeleev arranged the elements in his periodic table on the basis of atomic masses. (b) In the modern periodic table, the elements are arranged on the basis of atomic numbers.

Page No 281: Question 2: State whether the following statements are true of false:

(a) Newlands divided the elements into horizontal rows of eight elements each. (b) According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic numbers. © The elements in a group have consecutive atomic numbers.

ANSWER: (a) False Newlands divided the elements into horizontal rows of seven elements each.

(b) False According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses.

© False The elements in a period have consecutive atomic numbers.

Page No 282: Question 3: Name the Russian chemist who said that the properties of elements are a periodic function of their atomic masses.

ANSWER: Dmitri Mendeleev was a Russian chemist who said that the properties of elements are a periodic function of their atomic masses.

Page No 282: Question 4: Rewrite the following statements after correction, if necessary:

(a) Groups have elements with consecutive atomic numbers. (b) Periods are the horizontal rows of elements. © Isotopes are the elements of the same group.

ANSWER: (a) Periods have elements with consecutive atomic numbers. (b) The statement is correct. © The statement is correct.

Page No 282: Question 5: Name the scientists who gave the following laws in the early classification of elements:

(a) Law of octaves (b) Law of triads

ANSWER: (a) John Newlands is the scientist who gave the Law of Octaves in the early classification of elements. (b) Johann Dobereiner, a German chemist gave the Law of Triads in the early classification of elements.

Page No 282: Question 6: A, B and C are the elements of a Dobereiner's triad. If the atomic mass of A is 7 and the of C is 39, what should be the atomic mass of B?

ANSWER: A, B and C are the elements of a Dobereiner's triad. If the atomic masses of A and C are 7 and 39 respectively, then according to Dobereiner's triad law, the atomic mass of B is the average of the atomic masses of A and C.

So, the average of the atomic masses of A and C =(7+39)/2=46/2=23 Therefore, the atomic mass of B is 23.

Page No 282: Question 7: X and Y are the two elements having similar properties which obey Newland's laws of octaves. How many elements are there in-between X and Y?

ANSWER: There are six elements between X and Y. Since X and Y are the two elements having similar properties and obey the Newland's law of octaves, the number of elements between X and Y, including both, must total eight.

Page No 282: Question 8: What was the Mendeleev's basis for the classification of elements?

ANSWER: The atomic mass was Mendeleev's basis for the classification of elements.

Page No 282: Question 9: In the classification of the then known elements, Mendeleev was guided by two factors. What are those two factors?

ANSWER: In the classification of the then known elements, Mendeleev was guided by two factors, which are: 1. Increasing atomic masses 2.Grouping together of elements having the same properties

Page No 282: Question 10: Name two elements whose properties were predicted on the basis fo their positions in Mendeleev's periodic table.

ANSWER: Gallium (Ga) and scandium (Sc) are the two elements whose properties were predicted on the basis of their position in Mendeleev's periodic table.

Page No 282: Question 11: The three elements predicted by Mendeleev from the gaps in his periodic table were known as eka-boron, eka-aluminimum and eka-silicon. What names were given to these elements when they were discovered later on?

ANSWER: (i) Eka-boron was named scandium (Sc).

(ii) Eka-aluminium was named gallium (Ga).

(iii) Eka- silicon was named germanium (Ge).

Page No 282: Question 12: Name two elements whose properties were predicted on the basis of their positions in Mendeleev's periodic table.

ANSWER: Germanium (Ge) and Scandium (Sc) are the two elements whose properties were predicted on the basis of their position in Mendeleev's periodic table.

Page No 282: Question 13: State one example of a Dobereiner's triad, showing in it that the atomic mass of middle element is half-way between those of the other two.

ANSWER: Lithium (Li), sodium (Na) and potassium (K) with atomic masses 7, 23, 39 constitute a Dobereiner's triad. The atomic mass of sodium (middle element) is half-way between those of the other two. This can be seen in the illustration below:

Average of the atomic masses of lithium and potassium = (Atomic mass of lithium) / (Atomic mass of potassium) =(7+39) / 2 =23

Page No 282: Question 14: Which group of elements could be placed in Mendeleev's periodic table later on, without disturbing the order? Give reason.

ANSWER: Noble gases are the group of elements which could be placed in Mendeleev's periodic table later on, without disturbing the order. This is because they are inert (unreactive) elements, as their valence shells are completely filled with electrons. Therefore, Mendeleev decided to include these elements in a separate group.

Page No 282: Question 15: Fill in the following blanks with suitable words:

(a) The basis for modern periodic table is …………… (b) The horizontal rows in a periodic table are called …………… © Group 1 elements are called …………… (d) Group 17 elements are known as …………… (e) Group 18 elements are called …………… (f) According to Newland's classification of elements, the properties of sulphur are similar to those of oxygen because sulphur is the …………… element starting from oxygen.

ANSWER: (a) The basis for the modern periodic table is the atomic number.

(b) The horizontal rows in a periodic table are called periods.

© Group 1 elements are called alkali metals.

(d) Group 17 elements are known as halogens.

(e) Group 18 elements are called noble gases.

(f) According to Newland's classification of elements, the properties of sulphur are similar to those of oxygen because sulphur is the eighth element, starting from oxygen.

Page No 282: Question 16: (a) What is meant by (i) a group, and (ii) a period, in a periodic table? (b) How many periods and groups are there in the long form of periodic table? © Give two examples each of (i) group 1 elements (ii) group 17 elements (iii) group 18 elements.

ANSWER: (a) (i) A group is a vertical column of elements in a periodic table. (ii) A period is a horizontal row of elements in a periodic table.

(b) There are seven periods and eighteen groups in the long form of the periodic table.

© (i) Lithium (Li) and sodium (Na) are two elements of group 1. (ii) Chlorine (Cl) and bromine (Br) are two elements of group 17. (iii) Neon (Ne) and argon (Ar) are two elements of group 18.

Page No 282: Question 17: (a) In the modern periodic table, which are the metals among the first ten elements? (b) What is the significance of atomic number in the modern classification of elements? Explain with the help of an example.

ANSWER: (a) Lithium and beryllium are the metals among the first ten elements in the modern periodic table.

(b) The significance of atomic numbers in the modern periodic table is that it helps in arranging the elements according to their electronic configuration. Since, the number of valence electrons is the same, elements placed in the same group show similar chemical properties.

For example, if we take the element lithium (Li), its atomic number is 3. Its electronic configuration is (2,1). As it has one valence electron, it is placed in the first group of the periodic table. Similarly, all the elements having one valence electron will be placed in group 1, and they show similar chemical properties. In this manner, classification of elements is made according to the electronic configuration of the elements.

Page No 282: Question 18: (a) How were the positions of isotopes of an element decided in the modern periodic table? (b) How were the positions of cobalt and nickel resolved in the modern periodic table? © where should hydrogen be placed in the modern periodic table? Give reason for your answer.

ANSWER: (a) The isotopes of elements are not given a separate place in the periodic table. This is because isotopes have the same atomic number and a different atomic mass, and the modern periodic table is based on the atomic number of elements. Hence, to give it a separate place, the elements should be arranged according to their atomic mass.

(b) As per Mendeleev's classification, elements are arranged in an increasing order of atomic masses, but cobalt, with a higher atomic mass was placed before nickel. Cobalt has an atomic mass of 58.9 and nickel has an atomic mass of 58.7, which is slightly lower than that of cobalt. Mendeleev could not explain this point. This problem was resolved in the modern periodic table. According to the modern periodic law, elements are arranged in the increasing order of their atomic numbers. The atomic numbers of cobalt and nickel are 27 and 28 respectively. So, cobalt with a lower atomic number should come before nickel.

© Generally, hydrogen is treated as a special element and placed alone at the head of the periodic table. The position of hydrogen in the periodic table is unclear because it resembles both the alkali metals and halogens in some of its properties. However, it is placed in the first group, above all the alkali metals in the modern periodic table because its electronic configuration is similar to that of alkali metals. But due to the small size of hydrogen, it exhibits properties different from that of the alkali metals.

Page No 282: Question 19: (a) On which side of the periodic table will you find metals? (b) On which side of the periodic table will you find non-metals? © What is the name of those elements which divide metals and non-metals in the periodic table?

ANSWER: (a) On the left side of the periodic table, we find metals.

(b) On the right side of the periodic table, we find non-metals.

© Metalloids are the names of those elements which divide metals and non-metals in the periodic table.

Page No 283: Question 27: (a) Why do we classify elements? (b) What were the two criteria used by Mendeleev to classify the elements in his periodic table? © Why did Mendeleev leave some gaps in his periodic table? (d) In Mendeleev's periodic table, why was there no mention of noble gases like helium, neon and argon? (e) Would you place the two isotopes of chlorine, CI-35 and CI-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are the same? Justify your answer.

ANSWER: (a) Elements are classified into certain groups in such a manner that elements belonging to the same group exhibit similar properties. This eases the study of elements, as we can reduce the study to a few groups of elements rather than studying the properties of all the 115 elements known at present, which is very difficult. This is the reason for classification of elements.

(b) The two criteria used by Mendeleev to classify the elements in his periodic table are: (i) Increasing atomic masses (ii) Grouping of elements which exhibited similar properties, placed under the same vertical column (group).

© Mendeleev left some gaps in his periodic table for the elements which were not known at that time, to ascertain that elements having the same properties fell in the same vertical column or group, without disturbing the arrangement of previous elements.

(d) There was no mention of noble gases like helium, neon and argon in Mendeleev's periodic table as those gases were not known at that time.

(e) The isotopes of chlorine Cl-35 and Cl-37 should be placed in the same slot because their chemical properties are the same. The two isotopes of chlorine have the same atomic number and chemical properties. We know that the arrangement of elements is made on the basis of atomic number in the periodic table. And chemical properties of the element depend on the atomic number of the atom.

Page No 283: Question 28: (a) State Mendeleev's periodic law. (b) What chemical properties of elements were used by Mendeleev in creating his periodic table? © State any three limitations of Mendeleev's classification of elements. (d) Besides gallium, which two other elements have since been discovered for which Mendeleev had left gaps in his periodic table? (e) Which group of elements was missing from Mendeleev's original periodic table?

ANSWER: (a) Mendeleev's periodic law: According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses. This means, when the elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at a regular interval of the periods.

(b) Mendeleev used the formula of the oxides and hydrides produced by the elements as the basic chemical properties of elements, in creating his periodic table.

© Limitations of Mendeleev's classification of elements are:

i) Mendeleev's periodic law could not explain the position of isotopes in the periodic table. ii) Mendeleev's periodic law could not assign a proper position to the element hydrogen in the periodic table. iii) Mendeleev's periodic law failed to explain the wrong order of atomic masses of some elements.

(d) Germanium (Ge) and scandium (Sc) are the two other elements besides gallium, which have since been discovered, for which Mendeleev had left gaps in his periodic table.

(e) Group 18 consisting of noble gases was missing from Mendeleev's original periodic table as the noble gases were not known at that time.

Page No 283: Question 29: (a) State modern periodic law. (b) How does the electronic configuration of the atom of an element relate to its position in the modern periodic table? © How could the modern periodic law remove various anomalies of Mendeleev's periodic table? Explain with examples. (d) Is it possible to have an element having atomic number 1.5 placed between hydrogen and helium? (e) Name the scientist who prepared modern periodic table.

ANSWER: (a) Modern periodic law: According to the modern periodic law, the properties of elements are a periodic function of their atomic numbers. This means, if the elements are arranged in an increasing order of their atomic numbers, the elements exhibiting similar properties will repeat after regular intervals of periods.

(b) The elements are arranged in an increasing order of atomic number, and it helps in arranging the elements according to their electronic configuration. Since elements having the same number of valence electrons show similar chemical properties, they are placed under the same group.

© As the modern periodic law is based on the atomic numbers of elements, all the anomalies of Mendeleev's periodic table are removed. For example:

1. The position of isotopes could be explained, which the Mendeleev's periodic table failed to explain. As all the isotopes of an element have the same atomic number, they can be placed under the same group in a periodic table.

For example: Cl-35 and Cl-36, which are the isotopes of Chlorine with the same atomic number 17, they can be placed in the same group of the periodic table.

2. Mendeleev could not explain why cobalt was placed before nickel. This problem was solved by the modern periodic law. The atomic numbers of cobalt and nickel are 27 and 28 respectively. Hence, cobalt with a lower atomic number was placed before nickel in the modern periodic table.

(d) It is not possible to place an element with an atomic number of 1.5 between hydrogen and helium. This is because elements are placed in the increasing order of their atomic numbers. Atomic numbers are always whole numbers and cannot be in decimals, as half an electron or half a proton cannot exist .

(e) Niels Bohr is the scientist who prepared the modern periodic table.

Page No 283: Question 30: In Mendeleev's periodic table, gap was not left for one of the following elements. This element is:

(a) gallium (b) beryllium © germanium (d) scandium

ANSWER: (b) Beryllium.

In Mendeleev's periodic table, a gap was left for gallium, scandium and germanium.

Page No 283: Question 31: The Newland's law of octaves for the classification of elements was found to be applicable only up to the element:

(a) potassium (b) calcium © cobalt (d) phosphorus

ANSWER: (b) Calcium. The Newlands' law of octaves for the classification of elements was applicable only up to the element calcium.

Page No 283: Question 32: According to Mendeleev's periodic law, the elements were arranged in the periodic table in the order of:

(a) decreasing atomic numbers (b) increasing atomic numbers © decreasing atomic masses (d) increasing atomic masses

ANSWER: (d) Increasing atomic masses. According to Mendeleev's periodic law, the elements in the periodic table were arranged in the order of increasing atomic masses.

Page No 283: Question 33: The three elements having chemical symbols of Si, B and Ge are:

(a) all metals (b) all non-metals © all metalloids (d) Si is metalloid, B is metal and Ge is non-metal

ANSWER: © All metalloids The three elements having the chemical symbols Si, B and Ge are metalloids.

Page No 283: Question 20: (a) Name three elements that have a single electron in their outermost shells. (b) Name two elements that have two electrons in their outermost shells. © Name three elements with completely filled outermost shells.

ANSWER: (a) Lithium (Li), sodium (Na) and potassium (K) are three elements (metals of group 1) that have only a single electron in their outermost shell.

(b) Magnesium (Mg) and calcium (Ca) are two elements (metals of group 2) that have two electrons in their outermost shell.

© Helium (He), neon (Ne) and argon (Ar) are three elements (inert gases of group 18) that have a completely filled outermost shell.

Page No 283: Question 21: What is Debereiner's law of triads? Explain with the help of one example of a Dobereiner's triad.

ANSWER: According to Dobereiner's law of triads, when elements are arranged in increasing order of their atomic masses, a group of three elements with similar chemical properties is obtained. This group is called a triad. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two elements.

For example, the set of elements: calcium (Ca), strontium (Sr) and barium (Ba), with atomic masses 40, 88 and 137 respectively form a triad because they have similar chemical properties, and the atomic mass of strontium is approximately equal to the average of the atomic masses of calcium and barium. Arithmetic mean of atomic masses of calcium and barium = (40+137) / 2=88.5

Page No 283: Question 22: What is Newlands' law of octaves? Explain with an example.

ANSWER: According to Newlands' law of octaves, when elements are arranged in increasing order of their atomic masses, the properties of the eighth element are a repetition of the properties of the first element. Newlands divided the elements into horizontal rows. Each horizontal row had 7 elements.

For example, let us take a row from Newlands' classification of elements. If we take the elements Li, Be, B, C, N,O, F and Na, lithium (Li) is the first element, and sodium the eighth. It has been found that the properties of sodium are a repetition of the properties of lithium. In short, both lithium and sodium have similar chemical properties.

Page No 283: Question 23: (a) Did Dobereiner's triads also exist in the columns of Newlands' law of octaves? Explain your answer. (b) What were the limitations of Dobereiner's classification of elements? © What were the limitations of Newlands' law of octaves?

ANSWER: (a) Yes, Dobereiner's triads also existed in the columns of Newlands' law of octaves. This can be explained by taking the second column of the Newlands' classification of elements as an example. The second column has the elements lithium (Li), sodium (Na) and potassium (K), which form a Dobereiner's triad.

(b) Limitations of Dobereiner's classification of elements: It could identify only three triads from the elements which were known at that time. It failed to arrange all the elements which were known then in the form of triads of elements with the same chemical properties.

© Limitations of Newlands' law of octaves: 1. Newlands' law of octaves was applicable only up to the element calcium and not beyond that, i.e, it only works for lighter elements.

2. According to Newlands, only 56 elements existed in nature and there would be no elements discovered in the future. However, this assumption was proved wrong with the discovery of several elements later on which did not fit into Newlands' law of octaves.

3. A unique slot was not given to all the elements, as Newlands put two elements which have different properties together in a single slot.

4. The element iron which resembles cobalt and nickel in its properties was given a position far away from them.

Page No 283: Question 24: (a) State the periodic law on which Mendeleev's periodic table table was based. Why and how was this periodic law changed? (b) Explain why, the noble gases are placed in a separate group.

ANSWER: (a) According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses. This means that when elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at regular periods.

However, Henry Moseley, a scientist, showed that the atomic number of an element is a more fundamental property and a better basis to classify elements than the atomic mass. The atomic number increases consistently by 1, from one element to the next, whereas the atomic mass does not change regularly. Also, the atomic number of an element does not change and remains fixed. This led to a change in Mendeleev's periodic law.

(b) Noble gases are placed in a separate group of the periodic table because they are inert elements. They are unreactive as their valence shells are completely filled with electrons. Their properties are different when compared to all the other elements.

Page No 283: Question 25: (a) State the merits of Mendeleev's classification of elements. (b) Describe two anomalies of Mendeleev's periodic classification of elements.

ANSWER: (a) The merits of Mendeleev's classification of elements:

1. Mendeleev's periodic law predicted the existence of certain elements such as gallium, scandium and germanium, which were not discovered at that time.

2. Mendeleev's periodic table could also predict the properties of various elements based on their position in the periodic table.

3. Mendeleev's periodic table could place group 18 elements, called Noble gases in the periodic table, when they were discovered.

(b) Anomalies of Mendeleev's periodic classification of elements:

1. Mendeleev's periodic classification of elements could not explain the position of isotopes in the periodic table: Since, isotopes of the same metal have similar chemical properties but different in their atomic masses. So, the isotopes of the same element should be given different place in periodic table as Mendeleev's periodic table was arranged in increasing order of their atomic masses. However, isotopes were not given separate place in his periodic table.

2.Mendeleev's periodic classification of elements could not assign a proper place for the element,hydrogen. Since, hydrogen resembled both alkali metals and halogens in some of the properties.

Page No 283: Question 26: (a) How do the properties of eka-aluminium element predicted by Mendeleev compare with the actual properties of gallium element? Expalin your answer. (b) What names were given by Mendeleev to the then undiscovered elements (i) scandium (ii) gallium, and (iii) germanium?

ANSWER: (a) The properties of eka-alumiunium, predicted by Mendeleev, which was undiscovered then, were almost similar to the actual properties of the element discovered later on. The comparison table below shows the similarities in the properties of the undiscovered element then and the actual element (Gallium).

b) (i) Scandium (Sc) was known as eka-boron. (ii) Gallium(Ga) as eka-aluminium. (iii) Germanium (Ge) as eka-silicon.

Page No 284: Question 34: In Mendeleev's periodic table, gaps were left for the elements to be discovered later on. An element which found a vacant place in the periodic table later on is:

(a) Be (b) Si © Ge (d) Se

ANSWER: © Ge. Germanium (eka- silicon) is the element which found a vacant place in the periodic table later on.

Page No 284: Question 35: The three imaginary elements X, Y and Z represent a Dobernener's triad. If the atomic mass of element X is 14 and that of element Y is 46, then the atomic mass of element Z will be:

(a) 28 (b) 60 © 78 (d) 72

ANSWER: © 78 The atomic mass of the element Z is 78. According to Dobereiner's law of triads, atomic mass of Y=(X+Z) / 2 Substituting the atomic masses we get:

46=(14 + Z) / 2

92=14+Z Therefore, Z=78.

Page No 284: Question 36: The atomic numbers of four elements P, Q, R and S are 6, 8, 14 and 16 respectively. Out of these, the element known as metalloid is:

(a) P (b) Q © R (d) S

ANSWER: © R. R (Si) is a metalloid since its atomic number is 14 and it has 4 valence electrons.

Page No 284: Question 37: Which of the following statement is correct in regard to the classification of elements?

(a) Elements in modern periodic table are arranged on the basis of increasing atomic masses. (b) Elements in Mendeleev's periodic table are arranged on the basis of increasing atomic numbers. © In modern periodic table, the element nickel of lower atomic mass is kept before the element cobalt of higher atomic mass. (d) In modern periodic table, the isotopes of chlorine having different atomic masses are kept in the same group.

ANSWER: (d) In the modern periodic table, the isotopes of chlorine, having different atomic masses are kept in the same group. The isotopes of chlorine, having different atomic masses but same atomic numbers are kept in the same group in the modern periodic table.

Page No 284: Question 38: Which of the following statement about the modern periodic table is correct?

(a) It has 18 horizontal rows known as periods. (b) It has 7 vertical columns known as periods. © It has 18 vertical columns known as groups. (d) It has 7 horizontal rows known as groups.

ANSWER: © It has 18 vertical columns known as groups. In the modern periodic table, there are 18 vertical columns known as groups.

Page No 284: Question 39: An element X forms an oxide X2O3. In which group of Mendeleev's periodic table is this element placed?

(a) group II (b) group III © group V (d) group VIII

ANSWER: (b) Group III. The element which forms the oxide X2O3 is placed in group 3 of the periodic table.

Page No 284: Question 40: The modern periodic table was prepared by:

(a) Dobereiner (b) Newlands © Bohr (d) Mendeleev

ANSWER: © Bohr. The modern periodic table of elements was prepared by Neils Bohr.

Page No 284: Question 41: The atomic particle whose number in the atoms of an element always remains the same and which forms the real basis for the modern classification of elements is:

(a) electron (b) proton © neutron (d) meson

ANSWER: (b) Proton. Proton is an atomic particle whose number in the atoms of an element remains the same always, and forms the real basis for the modern classification of elements.

Page No 284: Question 42: The atomic masses of three elements X, Y and Z having similar chemical properties are 7, 23 and 39 respectively.

(a) Calculate the average atomic mass of elements X and Z. (b) How does the average atomic mass of elements X and Z compare with the atomic mass of element Y? © Which law of classification of elements is illustrated by this example? (d) What could the elements X, Y and Z be? (e) Give another example of a set of elements which can be classified according to this law.

ANSWER: (a) The atomic masses of three elements X, Y and Z are 7, 23 and 39 respectively. The average atomic mass of X and Z is nothing but the arithmetic mean of the atomic masses of X and Z.

Therefore, the average atomic mass of X and Z = (7+39) / 2=23.

(b) From the above calculation, we observe that the average atomic mass of the elements X and Z is equal to the atomic mass of the element Y.

© Dobereiner's law of triads is illustrated by the above example. According to Dobereiner's law, when elements are arranged in an increasing order of atomic masses, a three element group (triads), having the same chemical properties is formed. The atomic mass of the middle element is equal to the average of the atomic masses of the other two elements.

(d) The elements X,Y and Z , having atomic masses 7, 23, 39 are lithium (Li), sodium (Na) and potassium (K). They belong to the alkali metal group (group 1), with the same valency of 1 and exhibit similar chemical properties.

(e) The elements calcium (Ca), strontium (Sr) and barium (Ba) are a set of alkali earth metals (group 2 elements), with atomic masses 40,88 and 137 respectively, which are classified according to Dobereiner's law of triads.

Page No 284: Question 43: In the following set of element, one element does not belong to the set. Select this element and explain why it does not belong: Calcium, Magnesium, Sodium, Beryllium

ANSWER: In the given set of elements, i.e., calcium, magnesium, sodium and beryllium, sodium does not belong to the set. This is because sodium is a group 1 element and the remaining three elements? belong to group 2. Elements of the same group show similar chemical properties.

Page No 284: Question 44: In the following set of elements, one element does not belong to the set. Select this element and state why it does not belong: Oxygen, Nitrogen, Carbon, Chlorine, Fluorine

ANSWER: In the given set of elements, i.e., oxygen, nitrogen, carbon, chlorine and fluorine, chlorine does not belong to the set. The element chlorine belongs to the third period while the remaining elements belong to the second period.

Page No 284: Question 45: Can the following groups of elements be classified as Dobereiner's triads?

(a) Na, Si, Cl (b) Be, Mg, Ca Give reason for your answer. (Atomic masses : Be 9 ; Na 23 ; Mg 24 ; Si 28 ; Cl 35.5 ; Ca 40)

ANSWER: (a) The atomic masses of Na, Si and Cl are 23, 28 and 35.5 respectively. According to Dobereiner's law of triads, when elements are arranged in the increasing order of their atomic masses, a three element group with similar chemical properties is obtained. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two.

The above set of elements does not form a triad because arithmetic mean of the atomic masses of Na and Cl = (23+35.5) / 2=29.25 This value is approximately equal to the atomic mass of silicon, but they do not form a triad because the elements do not exhibit similar chemical properties.

(b) The set of elements Be, Mg and Ca with atomic masses 9, 24 and 40 respectively, form a triad because they have similar chemical properties and the atomic mass of magnesium (Mg) is approximately equal to the average of the atomic masses of beryllium (Be) and calcium (Ca). The arithmetic mean of the atomic masses of Be and Ca = (9+40) / 2=24.5 which is approximately equal to 24.

Page No 284: Question 46: Consider the following elements: Na, Ca, Al, K, Mg, Li

(a) Which of these elements belong to the same period of the periodic table? (b) Which of these elements belong to the same group of the periodic table?

ANSWER: (a) Sodium (Na), magnesium (Mg) and aluminium (Al) belong to the same(third) period of the periodic table.

(b) Lithium (Li), sodium (Na) and potassium (K) belong to the same group(group 1) of the periodic table.

Page No 284: Question 47: Which element has:

(a) two shells, both of which are completely filled with electrons? (b) the electronic configuration 2, 8, 2? © a total of three shells, with four electrons in its valence shell? (d) a total of two shells, with three electrons in its valence shell? (e) twice as many electrons in its second shell as its first shell?

ANSWER: (a) Neon with atomic number 10. Electronic configuration:- Shells: K L Electrons arranged: 2 8

(b) Magnesium is the element with the electronic configuration of 2,8,2 as its atomic number is 12, since the atomic number is the sum of all the electrons present in the electronic configuration. Electronic configuration:- Shells: K L M Electrons arranged: 2 8 2

© Silicon is an element with a total of three shells and four electrons in its valence shell. Its atomic number is 14 and the electronic configuration is 2,8,4. Electronic configuration:- Shells: K L M Electrons arranged: 2 8 4

(d) Boron is an element with a total of two shells, with three electrons in its valence shell. Its atomic number is 5 and electronic configuration is 2,3. Electronic configuration:- Shells: K L Electrons arranged: 2 3

(e) Carbon is the element with twice as many electrons in its second shell as its first shell. The atomic number of carbon is 6 and its electronic configuration is 2,4. Electronic configuration:- Shells: K L Electrons arranged: 2 4

Page No 285: Question 48: Consider the following elements: Ca, Cl, Na, I, Li, Ba, Sr, K, Br

Separate these elements into three groups (families) of similar properties. State one property in each case on the basis of which you have made your choice.

ANSWER: (i) Lithium (Li), sodium (Na) and potassium (K) are three metals of group 1 having a valency of 1. (ii) Calcium (Ca), strontium (Sr) and barium (Ba) are three metals of group 2 having a valency of 2. (iii) Chlorine (Cl), bromine(Br) and iodine (I) are three non-metals of group 17 which are known as halogens, and have seven valence electrons.

Page No 285: Question 49: Mendeleev predicted the existence of certain elements not known at that time and named two of them as eka-aluminium, and eka-silicon.

(a) Name the element which has taken the place of (i) eka-aluminium, and (ii) eka-silicon (b) Mention the period/periods of these elements in the modern periodic table. © Write the group/ groups of these elements in the modern periodic table. (d) Classify these elements as metals, non-metals or metalloids. (e) How many valence electrons are present in the atoms of each of these elements?

ANSWER: (a) (i) Gallium is the element which has taken the place of eka-aluminium.

(ii) Germanium is the element which has taken the place of eka-silicon.

(b) Gallium and Germanium are placed in the 4th period of the periodic table.

© Gallium belongs to group 13 and Germanium belongs to group 14 in the periodic table.

(d) Gallium is a metal and Germanium, a metalloid.

(e) Gallium, which belongs to group 13, has 3 valence electrons in its atom, and germanium which belongs to group 14 has 4.

Page No 285: Question 50: A part of the early classification of elements has been given below:

H Li Be B C N O F Na Mg Al Si P S

(a) Which law of classification of element is illustrated by the above arrangement of elements? (b) Name the scientist who proposed such a classification of elements. © Why is such a classification of elements compared with a characteristics of musical scale? (d) State one limitation of this classification of elements. ANSWER: (a) Newlands' law of octaves is illustrated by the above arrangement of elements.

(b) John Newlands is the scientist who proposed this classification of elements.

© This classification is compared with a characteristic of the musical scale because the repetition of the properties of elements is just like the repetition of the eighth note in an octave of music.

(d) Limitation: Newlands' law of octaves could only be applied up to the element calcium.

Page No 302: Question 1: Give alongside is a part of the periodic table : As we move horizontally from left to right:

Li Be B C N O F Na Mg Al Si P S Cl

(i) What happens to the metallic character of the elements? (ii) What happens to the atomic size? ANSWER: (i) The metallic property of elements decreases as we move horizontally from left to right.

(ii) The atomic size of elements decreases as we move horizontally from left to right.

Page No 302: Question 2: How would the tendency to gain electrons change on moving from left to right in a period of the periodic table?

ANSWER: The tendency to gain electrons increases on moving from left to right in a period of the periodic table, because the metallic character decreases from left to right, and the non-metallic character increases.

Page No 302: Question 3: How would the tendency to lose electrons change as we go from left to right across a period of the periodic table?

ANSWER: The tendency to lose electrons decreases as we go from left to right across a period of the periodic table. This happens because the atomic size decreases from left to right and the force of the nuclear attraction on the outermost electron increases. So, it becomes difficult for the atom to lose electrons.

Page No 303: Question 4: (a) How does the chemical reactivity of alkali metals vary on going down in group 1 of the periodic table? (b) How does the chemical reactivity of the halogens vary on going down in group 17 of the periodic table?

ANSWER: (a) The chemical reactivity of alkali metals increases on going down in group 1 of the periodic table. (b) The chemical reactivity of halogens (non-metals) decreases on going down in group 17 of the periodic table.

Page No 303: Question 5: What property do all elements in the same column of the periodic table as boron have in common?

ANSWER: All elements in the same column of the periodic table as boron have the same valency of 3.

Page No 303: Question 6: What property do all the elements in the same group of the periodic table as fluorine have in common?

ANSWER: All elements in the same group of the periodic table as fluorine are halogens. Halogens are placed in group 17 which have seven valence electrons. Hence, their valency is 1.

Page No 303: Question 7: (a) What is the number of valence electrons in the atoms of first elements in a period? (b) What is the usual number of valence electrons in the atoms of the last element in a period?

ANSWER: (a) The number of valence electrons in the atoms of first elements in a period is 1.

(b) The usual number of valence electrons in the atoms of the last element (inert gas) in a period is 8. Helium (He) is the only inert gas having 2 valence electrons.

Page No 303: Question 8: State whether the following statement is true or false: On going down in a group of the periodic table, the number of valence electrons increases.

ANSWER: False On going down in a group of the periodic table, the number of valence electrons remains the same.

Page No 303: Question 9: What is the major characteristic of the first elements in the periods of the periodic table? What is the general name of such elements?

ANSWER: The first element in every period of the periodic table have 1 valence electron. The elements are known as alkali metals.

Page No 303: Question 10: How do the atomic radii of elements change as we go from left to right in a period of the periodic table?

ANSWER: As we go from left to right in a period of the periodic table, the atomic radii decrease.

Page No 303: Question 11: What happens to the metallic character of the elements as we go down in a group of the periodic table?

ANSWER: The metallic character of elements increases as we go down in a group of the periodic table.

Page No 303: Question 12: How does the number of valence electrons vary on moving from left or right:

(i) in the first period of the periodic table? (ii) in the second period of the periodic table?

ANSWER: i) In the first period of the periodic table, the number of valence electrons increases from 1 to 2, on moving from left to right.

ii) In the second period of the periodic table, the number of valence electrons increases from 1 to 8 on moving from left to right.

Page No 303: Question 13: How does the valency of elements change of moving from left to right in the third period of the periodic table?

ANSWER: The valency of the elements increases from 1(Na) to 4 (Si) and then decreases to 0 (Ar), on moving from left to right in the third period of the periodic table.

Page No 303: Question 14: How does the valency of elements vary in going down a group of the periodic table?

ANSWER: The valency of elements remains the same on going down a group of the periodic table.

Page No 303: Question 15: Name the element which is in:

(a) first group and third period. (b) seventeenth group and second period.

ANSWER: (a) Sodium (Na) is the element which is in the first group and the third period.

(b) Fluorine (F) is the element which is in the seventeenth group and the second period.

Page No 303: Question 16: How do electronic configurations of elements change in second period of periodic table with increase in atomic numbers?

ANSWER:

Page No 303: Question 17: Arrange the following elements in increasing order of their atomic radii: Li, Be, F, N

ANSWER: The elements in the increasing order of their atomic radii are as follows:

F

Page No 303: Question 18: Arrange the following elements in the increasing order of their metallic character: Mg, Ca, K, Ga

ANSWER: The elements in the increasing order of their metallic character are as follows: Ga < Mg < Ca < K

Page No 303: Question 19: Rewrite the following statements after correction, if necessary:

(i) Elements in the same period have equal valency (ii) The metallic character of elements in a period increases gradually on moving from left to right.

ANSWER: (i) Elements in the same group have equal valency.

(ii) The metallic character of the elements in a period decreases gradually on moving from left to right.

Page No 303: Question 20: Fill in the blanks in the following statements:

(a) The horizontal rows in a periodic table are called …………… (b) In going across a period (right to left) in periodic table, the atomic size of the atom …………… © On moving form right to left in the second period, the number of valency electrons …………… (d) On going down in a group in the periodic table, the metallic character of elements …………… (e) The tendency to gain an electron …………… on moving down in a group of the periodic table.

ANSWER: (a) The horizontal rows in the periodic table are called periods.

(b) On moving across a period (right to left) in the periodic table, the atomic size of an atom increases.

© On moving from right to left in the second period, the number of valence electrons decreases.

(d) On moving down in a group of the periodic table, the metallic character of elements increases.

(e) The tendency to gain an electron decreases on moving down in a group of the periodic table.

Page No 303: Question 21: Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

ANSWER: Electronic configuration of nitrogen (atomic number 7) : Shells: K L Electrons: 2 5

Electronic configuration of phosphorus (atomic number 15) : Shells: K L M Electrons: 2 8 5

The element nitrogen will be more electronegative because of the smaller size of its atom compared to phosphorus. Since, nitrogen has a smaller atomic radius than phosphorus, the attraction of its nucleus towards the incoming electron is more than phosphorus. Therefore, nitrogen accepts electrons more easily.

Page No 303: Question 22: An element X belongs to group 2 and another element Y belongs to group 15 of the periodic table:

(a) What is the number of valence electron in X? (b) What is the valency of X? © What is the number of valence electrons in Y? (d) What is the valency of Y? Explain how you have arrived at your answers.

ANSWER: (a) The number of valence electrons in element X is 2 because this element belongs to group 2.

(b) The valency of the group 2 element X is 2.

© Valence electrons of group 15: Group number = Valence electrons + 10 15 = Valence electrons + 10 Valence electrons = 15 - 10 = 5

(d) Valency of the group 15 element Y is 3, that is, (8 - 5).

Page No 303: Question 23: (a) What is a period in a periodic table? How do atomic structures (electron arrangements) change in a period with increase in atomic numbers from left to right? (b) How do the following change on going from left to right in a period of the periodic table? (i) Chemical reactivity of elements (ii) Nature of oxides of elements Give examples in support of your answer.

ANSWER: (a) A period is a horizontal row of elements in a periodic table. As the atomic number increases from left to right in a periodic table, the size of the atom decreases.

(b) (i) On moving from left to right in a period of the periodic table, the chemical reactivity of the elements first decreases and then increases. For example, in the third period elements comprising of Na, Mg, Al, Si, P, S and Cl, sodium (Na) is very reactive as it has one valence electron, and can therefore, lose its electron easily. The chemical reactivity gradually decreases as we go to aluminium (Al) and silicon (Si). This is because the number of valence electrons increases, making it difficult to lose electrons. Moving further right in the period towards non-metals, the chemical reactivity again gradually increases. Phosphorus (P) has 5 valence electrons and needs 3 electrons to complete its octet. Sulphur (S) has 6 valence electrons and needs 2 more electrons to complete its octet. Chlorine (Cl) has 7 valence electrons and needs only one more electron to complete its octet. As chlorine can easily accept an electron as compared to phosphorus and sulphur, the chemical reactivity increases from phosphorus to chlorine.

(ii) On moving from left to right in a period of the periodic table, the basic nature of oxides decreases and their acidic nature increases. For example, taking the third period elements again, the oxides of sodium are highly basic whereas those of chlorine are highly acidic.

Page No 303: Question 24: (a) How does the size of atoms (atomic size) generally vary in going from left to right in a period of the periodic table? Why does it vary this way? (b) What happens to the metallic character of the elements as we move from left to right in a period of the periodic table?

ANSWER: (a) The size of the atoms decreases on moving from left to right in a periodic table. This happens because the number of electrons and protons also increases and the nucleus of the atom becomes more positively charged. The nucleus exerts greater force of attraction on the electrons and pulls them tightly. As the nucleus pulls the outermost electrons towards it, the size of the atom (or the atomic radius) decreases.

(b) The metallic (electropositive) character of the elements decreases as we move from left to right and the non-metallic (electronegative ) character increases in a period of the periodic table.

Page No 304: Question 26: (a) How does the electropositive character of elements change on going down in a group of the periodic table? (b) State how the valency of elements varies (i) in a group, and (ii) in a period, of the periodic table.

ANSWER: (a) The electropositive character of the elements increases on going down in a group of the periodic table.

(b) (i) In a group, all the elements will have the same valency because all the electrons have the same number of electrons in their outermost shell.

(ii) In a period, different elements have different valencies. On moving from left to right in a period, the valency of the elements increases from 1 to 4 and then decreases to 0.