Question

2.9 Arrange each of the following sets in order of increasing atomic/ionic radius; that is, A...

2.9 Arrange each of the following sets in order of increasing atomic/ionic radius; that is,
A < B < C in size.
(a) Li, C, F
(b) Co, Ni, Fe
(c) N, P, O
(d) O2–, F–, S2–
(e) K+, Br–, Rb+
(f) Cd2+, Rb+ , Ag+
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Answer #1

Before solving questions these types of questions,we must remember these two points-

  • Point 1 : Going down from top to bottom in a group atomic radius increases. This is because number of shells get added which are ultimately further away from the nucleus, thus increasing the size of atom.
  • Point 2 : Going left to right across a period atomic radius decreases. This happens because more protons are added to the nucleus but the electrons which are being added are not being added in a new shell. So the effective nuclear charge increases and the atom contracts.

a) The order is F < C < Li . Here they all lie in period 2 and in accordance with point 2 mentioned above the effective nuclear charge on Li is least, so its size is maximum.

b) The order is Ni < Co < Fe. Here they all lie in period 4 and in accordance with point 2 mentioned above the effective nuclear charge on Fe is least, so its size is maximum. The radius here are comparable though and they dont have major difference.

c) Out of the three P has maximum radius because it lies in period 3 and radius increasing on going down from top to bottom. Now N, O both lie in period 2 but N has less effective nuclear charge so its radius is more. So, the order is O < N < P

d) O-2 AND F-1 are isoelectronic. Now when number of electrons are same we compare the charge. Since O-2 is divalent anion it is bigger than F-1 which is a monovalent anion. Now, S-2 has more shells than both O-2 and F-1 as it is in period 3 so its size is maximum. So the order is F-1 < O-2 < S-2

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