1

### AIPMT 2010 Prelims

Which of the following represents the correct order of increasing electron gain enthalpy with negative sign for the elements O, S, F and C$l$ ?
A
C$l$ < F < O < S
B
O < S < F < C$l$
C
F < S < O < C$l$
D
S < O < C$l$ < F

## Explanation

Cl atom has the highest electron affinity in the periodic table. F is a member of group 17 has highest electron gain enthalpy than S which is a group 16 element. This is turn is higher than the electron affinity of O atom. Thus,

Cl > F > S > O

2

### AIPMT 2010 Prelims

The correct orfer of the decreasing ionic radii among the following isoelectronic species is
A
Ca2+ > K+ > S2$-$ > C$l$$- B Cl$$-$ > S2$-$ > Ca2+ > K+
C
S2$-$ > Cl$-$ > K+ > Ca2+
D
K+ > Ca2+ > Cl$-$ > S2$-$

## Explanation

For isoelectronic species, ionic radius increases when there is increase in negative charge. It happens because effective nuclear charge on the atom (Zeff) decreases.

Same when ionic radius decreases with increase in positive charge as Zeff increases.
3

### AIPMT 2009

Amongst the elements with following electronic configurations, which one of them may have the highest ionisation energy ?
A
Ne [3s2 3p2]
B
Ar [3d10 4s2 4p3]
C
Ne [3s2 3p1]
D
Ne [3s2 3p3]

## Explanation

Among the given options (a), (c) and (d), the option (d) has the highest ionisation energy because of extra stability associated with half-filled 3p-orbital. In option (b), the presence of 3d10 electrons offers shielding effect, as a result the 4p3 electrons do not experience much nuclear charge and hence the electrons can be removed easily.
4

### AIPMT 2007

Which one of the following ionic species has the greatest proton affinity to form stable compound ?
A
NH$_2^ -$
B
F$-$
C
I$-$
D
HS$-$

## Explanation

Going from left to right across a period in the periodic table, the basicity decreases as the electronegativity of the atom possessing the lone pair of electron increases. Hence basicity of $NH_2^-$ is higher than F-. In moving down a group, as the atomic mass increases, basicity decreases. Hence F- is more basic than I- and HO- is more basic than HS-. Hence among the ionic species, $NH_2^-$ has maximum proton affinity.