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1

### 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.
2

### AIPMT 2007

Identify the correct order of the size of the following :
A
Ca2+ < K+ < Ar < Cl$$-$$ < S2$$-$$
B
Ar < Ca2+ < K+ < Cl$$-$$ < S2$$-$$
C
Ca2+ < Ar < K+ < Cl$$-$$ < S2$$-$$
D
Ca2+ < K+ < Ar < S2$$-$$ < Cl$$-$$

## Explanation

Among isoelectronic ions, ionic radius of anions is more than that of cations. Further size of the anion increases with increase in negative charge and size of the cation decreases with increase in positive charge.
3

### 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.
4

### AIPMT 2006

Which one of the following orders is not in accordance with the property stated against it ?
A
F2 > Cl2 > Br2 > I2 : Bond dissociation energy
B
F2 > Cl2 > Br2 > I2 : Oxidising power
C
HI > HBr > HCl > HF : Acidic property in water
D
F2 > Cl2 > Br2 > I2 : Electronegativity

## Explanation

$$\mathop {F - F}\limits_{38} \,\,\,\mathop {Cl - Cl}\limits_{57} \,\,\mathop {Br - Br}\limits_{45.5} \,\,\mathop {I - I}\limits_{35.6}$$
X - X bond dissociation energy (kcal/mol)

The lower value of bond dissociation energy of fluorine is due to the high inter-electronic repulsion between non-bonding electrons in the 2p-orbitals of fluorine. As a result F - F bond is weaker in comparison to Cl - Cl and Br - Br bonds.

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