Chapter 5: Periodic Classification of Elements

Q27Written answer

The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as? Give one example of such a set of elements.

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Worked solution

Such an arrangement of elements are called Triads. Ex: Lithium Sodium and Potassium make a triad. Their atomic masses are 6.9, 23.0 and 39.0 respectively. An average mass of Li and K is approximately equal to the atomic mass of Na.

Q28Written answer

Elements have been arranged in the following sequence on the basis of their increasing atomic masses. F, Na, Mg, Al, Si, P, S, Cl, Ar, K (a) Pick two sets of elements which have similar properties. (b) The given sequence represents which law of classification of elements?

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Worked solution

Na, Mg, Al and K are metals. F and Cl are Halogens. These make two sets of elements
Given set represent Mendeleev’s law of periodicity.

Q29Written answer

Can the following groups of elements be classified as Dobereiner’s triad?

(a) Na, Si, Cl

(b) Be, Mg, Ca

Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40 Explain by giving reason.

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Worked solution

Though atomic mass of Silicon is the average of atomic masses of Sodium and Chlorine they do not have similar properties hence they cannot be classified as Dobereiner’s triad

Q30Written answer

In Mendeleev’s Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with an atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give a reason for the same.

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Worked solution

In Mendeleev ′s Periodic Table there are instances where elements with higher atomic mass is placed before the element with lower atomic mass. This was done to ensure that elements with similar properties were included in the same group. Hence Cobalt was placed before Nickel despite of higher atomic number of Cobalt than Nickel.

Q31Written answer

“Hydrogen occupies a unique position in Modern Periodic Table”. Justify the statement.

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Worked solution

Hydrogen occupies a unique position in the Modern Periodic Table because of the following reasons.

Hydrogen and alkali metals have similar outer electronic configuration as both have one electron in their outermost shell.
Properties of Hydrogen are similar to properties of Halogens as electronic configurations of halogens and Hydrogen are same.

Q32Written answer

Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeleev.

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Worked solution

Chlorides of Eka-silicon: ECl₄

Chlorides of Eka-Aluminium: ECL₃

Q33Written answer

Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.

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Worked solution

A Belongs to Group 13, B Belongs to Group 14 and C Belongs to Group 2. Valency of A is3, B is 4 and C is 2.

Q34Written answer

If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?

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If an element X is placed in group 14 it has 4 electrons in its outermost orbit. Formula of its Chloride is ECl_4.Here element can form compound by sharing electrons hence it will be a chemical bonding.

Q35Written answer

Compare the radii of two species X and Y. Give reasons for your answer.

(a) X has 12 protons and 12 electrons

(b) Y has 12 protons and 10 electrons

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Worked solution

From electron arrangement

X -2.8.2

Y -2.8

Since Y has fewer energy levels it’s atomic radius will be smaller than that of X.

Q36Written answer

Arrange the following elements in increasing order of their atomic radii.

(a) Li, Be, F, N

(b) Cl, At, Br I

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Worked solution

Given elements are in the same group and are arranged from left to right. So Increasing order of their atomic radii

Li>Be>F>N
Cl>Br>I>At

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