1

### AIPMT 2005

Which one of the following arrangements represents the correct order of electron gain enthalpy (with negative sign) of the given atomic species ?
A
S < O < Cl < F
B
Cl < F < S < O
C
F < Cl < O < S
D
O < S < F < Cl

## Explanation

The molar enthalpy change accompanying the addition of an electron to an atom (or ion) is known as electron gain enthalpy.
Generally in increases on moving from left to right in a period and in a group it decreases as the size increases.
Exception: Because of the small size of F, electron-electron repulsion present in its relatively compact 2p-subshell, do not easily allow the addition of an extra electron. On the other hand, Cl because of its comparatively bigger size than F, allows the addition of an extra electron more easily.
$\mathop O\limits_{ - 1.48} < \mathop S\limits_{ - 2.0} < \mathop F\limits_{ - 3.6} < \mathop {Cl}\limits_{ - 3.8}$
2

### AIPMT 2004

A
inversely proportional to effective nuclear charge
B
inversely proportional to square of effective nuclear charge
C
directly proportional to effective nuclear charge
D
directly proportional to square of effective nuclear charge.

## Explanation

Ionic radius in the nth orbit is given by

rn = ${{{n^2}{a_0}} \over {{Z^*}}}$ or, ${r_n} \propto {1 \over {{Z^*}}}$

When n is the principal quantum number, a0 the Bohr's radius of H-atom and Z*, the effective nuclear charge.
3

### AIPMT 2003

The ions O2$-$, F$-$, Na+, Mg2+ and Al3+ are isoelectronic. Their ionic radii show
A
a significant increase from O2$-$ to Al3+
B
a significant decrease from O2$-$ to Al3+
C
an increase from O2$-$ to F$-$ and then decrease from Na+ to Al3+
D
a decrease from O2$-$ to F$-$ and then increase from Na+ to Al3+.

## Explanation

Amongst isoelectronic ions, ionic radii of anions is more than that of cations. Further size of the anion increases with increase in -ve charge and size of cation decreases with increase in +ve charge. Hence, the correct order is

O2- > F- > Na+ > Mg2+ > Al3+
4

### AIPMT 2000

Which statement is wrong ?
A
Bond energy of F2 > Cl2
B
Electronegativity of F > Cl
C
F is more oxidising than Cl
D
Electron affinity of Cl > F

## Explanation

Bond energy generally decreases on moving from top to bottom along a group. It happens due to fact that size increases on moving down the group and thus, the two nuclei are far apart and less capable of holding the two atoms together.

But in case of F2 molecule due to more repulsion in between non-bonding electron pair (2p) of two fluorines (due to small size of F-atom) in comparison to non-bonding electron pair (3p) in chlorine, the bond energy of F2 is less than Cl2.

BE(F2) = 158.5 kJ/mole and

BE(Cl2)= 242.6 kJ/mole