1

### 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.
2

### AIPMT 2010 Mains

Among the elements Ca, Mg, P and Cl, the order of increasing atomic radii is
A
Mg < Ca < Cl < P
B
Cl < P < Mg < Ca
C
P < Cl < Ca < Mg
D
Ca < Mg < P < Cl

## Explanation

Atomic radius decreases on moving from left to right in a period. so order of sizes for Cl, P and Mg is Cl < P < Mg. Down the group size increases. So overall order is : Cl < P < Mg < Ca.
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

With which of the following electronic configuration an atom has the lowest ionisation enthalpy ?
A
1s2 2s2 2p3
B
1s2 2s2 2p5 3s1
C
1s2 2s2 2p6
D
1s2 2s2 2p5

## Explanation

The larger the atomic size, smaller is the value of the ionisation enthalpy. Again higher the screening effect, lesser is the value of ionisation potential, So option(b) has lowest ionisation enthalpy.