## Classification of Elements and Periodicity in Properties Questions and Answers Part-9

1. Following statements regarding the periodic trends of chemical reactivity of the alkali metals and the halogens are given. Which of these statements gives the correct picture?
a) Chemical reactivity increases with increase in atomic number down the group in both the alkali metals and halogens
b) In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group
c) The reactivity decreases in the alkali metals but increases in the halogens with increase in atomic number down the group
d) In both the alkali metals and the halogens the chemical reactivity decreases with increase in atomic number down the group

Explanation: The alkali metals are highly reactive because their first ionisation potential is very low and hence they have great tendency to loses electron to form unipositive ion. On moving down group- I from Li to Cs ionisation enthalpy decreases hence the reactivity increases. The halogens are most reactive elements due to their low bond dissociation energy, high electron affinity and high enthalpy of hydration of halide ion. However their reactivity decreases with increase in atomic number

2. The charge/size ratio of a cation determines its polarizing power. Which one of the following sequences represents the increasing order of the polarizing power of the cationic species,$K^{+},Ca^{2+},Mg^{2+},Be^{2+}$
a) $Ca^{2+} < Mg^{2+} < Be^{+} < K^{+}$
b) $Mg^{2+} < Be^{2+} < K^{+} < Ca^{2+}$
c) $Be^{2+} < K^{+} < Ca^{2+} < Mg^{2+}$
d) $K^{+} < Ca^{2+} < Mg^{2+} < Be^{2+}$

Explanation: Smaller the size and higher the charge more will be polarising power of cation. Since the order of the size of cation is $K^{+}>Ca^{++}>Mg^{++}>Be^{++}$    . So the correct order of polarising power is $K^{+}<Ca^{2+}<Mg^{2+}<Be^{2+}$

3. In which of the following arrangements, the sequence is not strictly according to the property written against it?
a) HF < HCl < HBr < HI : increasing acid strength
b) $NH_{3} < PH_{3} < AsH_{3} < SbH_{3}$        : increasing basic strength
c) B < C < O < N : increasing first ionization enthalpy
d) $CO_{2}<SiO_{2}<SnO_{2}<PbO_{2}$        : increasing oxidising power

Explanation: In hydrides of 15th group elements, basic character decreases on descending the group i.e. $NH_{3} < PH_{3} < AsH_{3} < SbH_{3}$

4. The correct sequence which shows decreasing order of the ionic radii of the elements is
a) $AI^{3+}>Mg^{2+}>Na^{+}>F^{-}>O^{2-}$
b) $Na^{+}>Mg^{2+}>AI^{3+}>O^{2-}>F^{-}$
c) $Na^{+}>F^{-}>Mg^{2+}>O^{2-}>AI^{3+}$
d) $O^{2-}>F^{-}>Na^{+}>Mg^{2+}>AI^{3+}$

Explanation: All the given species contains 10 e- each i.e. isoelectronic. For isoelectronic species anion having high negative charge is largest in size and the cation having high positive charge is smallest.

5. The correct order of electron gain enthalpy with negative sign of F, Cl, Br and I, having atomic number 9, 17, 35 and 53 respectively, is :
a) F > Cl > Br > I
b) Cl > F > Br > I
c) Br > Cl > I > F
d) I > Br > Cl > F

Explanation: As we move down in a group electron gain enthalpy becomes less negative because the size of the atom increases and the distance of added electron from the nucleus increases. Negative electron gain enthalpy of F is less than Cl. This is due to the fact that when an electron is added to F, the added electron goes to the smaller n = 2 energy level and experiences significant repulsion from the other electrons present in this level. In Cl, the electron goes to the larger n = 3 energy level and consequently occupies a larger region of space leading to much less electron–electron repulsion. So the correct order is Cl > F > Br > I.

6. The increasing order of the ionic radii of the given isoelectronic species is :
a) $Cl^{-},Ca^{2+},K^{+},S^{2-}$
b) $S^{2-},Cl^{-},Ca^{2+},K^{+}$
c) $Ca^{2+},K^{+},Cl^{-},S^{2-}$
d) $K^{+},S^{2-},Ca^{2+},Cl^{-}$

Explanation: $Ca^{2+},K^{+},Cl^{-},S^{2-}$

7. Which of the following represents the correct order of increasing first ionization enthalpy for Ca, Ba, S, Se and Ar ?
a) Ca < S < Ba < Se < Ar
b) S < Se < Ca < Ba < Ar
c) Ba < Ca < Se < S < Ar
d) Ca < Ba < S < Se < Ar

Explanation: On moving along a period from left to right I.E. increases and on moving down a group I.E. decreases. hence correct order is : Ba < Ca < Se < S < Ar

8. An element X occurs in short period having configuration $ns^{2}np^{1}$. The formula and nature of its oxide is
a) $XO_{3}$  , basic
b) $XO_{3}$  , acidic
c) $X_{2}O_{3}$  , amphoteric
d) $X_{2}O_{3}$  , basic

Explanation: ns2np1 is the electronic configuration of III A period. Al2O3 is amphoteric oxide

9. The law of triads is applicable to a group of
a) Cl, Br, I
b) C, N, O
c) Na, K, Rb
d) H, O, N

Explanation: According to the law of triads the atomic wt of the middle element is arithmatic mean of I and III. At wt of Br = $\frac{At. wt of Cl + At. wt of I}{2}$