First ionisation enthalpy values of first four group 15 elements are given below. Choose the correct value for the element that is a main component of apatite family: (1) 1012 kJ mol⁻¹$1012\text{ kJ mol}^{-1}$ (2) 1402 kJ mol⁻¹$1402\text{ kJ mol}^{-1}$ (3) 834 kJ mol⁻¹$834\text{ kJ mol}^{-1}$ (4) 947 kJ mol⁻¹$947\text{ kJ mol}^{-1}$

Core Logic

Let us check the trends for the provided main group elements (Al, In, Tl$\mathrm{Al, In, Tl}$ from Group 13 and Ge, Sn, Pb$\mathrm{Ge, Sn, Pb}$ from Group 14):

• Highest First Ionization Enthalpy (IE₁$\mathrm{IE}_1$): Out of these options, Germanium (Ge$\mathrm{Ge}$) sits highest and further right along its period layout, demonstrating the highest IE₁$\mathrm{IE}_1$ value among this set. Its most stable oxidation state is +4$+4$.
• Lowest First Ionization Enthalpy (IE₁$\mathrm{IE}_1$): Indium (In$\mathrm{In}$) lies lowest leftward among these relative coordinates, maintaining the lowest IE₁$\mathrm{IE}_1$. Its most stable group oxidation state is +3$+3$ (as the inert pair effect is much more pronounced for the heavier element Tl$\mathrm{Tl}$ which prefers +1$+1$).

Pattern Recognition

Sees: IE₁$\mathrm{IE}_1$ extrema vs stable oxidation state profiles. Trap: Forgetting that inert pair shifts display max stability values at +1$+1$ for Tl$\mathrm{Tl}$ and +2$+2$ for Pb$\mathrm{Pb}$, while lighter counterparts like In$\mathrm{In}$ favor +3$+3$ and Ge$\mathrm{Ge}$ favors +4$+4$.

Chapter Mix

Class 11 Chemistry: The p-Block Elements Class 12 Chemistry: The p-Block Elements

Related Formula

Core Logic

The first ionisation enthalpy trend for Group 13 elements is irregular due to poor shielding by d and f electrons:

• Boron (B) is the smallest atom in the group with no d-orbital shielding issues, so it has the highest ionisation enthalpy.
• As we move down, poor shielding by 3d$3d$ electrons causes a slight increase at Gallium (Ga$Ga$). Poor shielding by 4f$4f$ electrons causes a sharp increase at Thallium (Tl$Tl$).
• Indium (In) ends up with the weakest effective attraction for its outermost valence electron, giving it the lowest first ionisation enthalpy.

Therefore, the highest and lowest elements are B & In respectively.

Pattern Recognition

Group 13 does not follow a linear downward trend. Remember the characteristic 'W' shape or zig-zag pattern of its ionisation energies. Indium sits at the absolute minimum point of this curve.

Chapter Mix

Class 11 Chemistry: The p-Block Elements

Core Logic

• Statement I is correct: Moving left-to-right across a period from Group 13 to Group 14 increases the effective nuclear charge (Zeff$Z_{\text{eff}}$) and decreases the atomic radius. Consequently, more energy is required to extract an electron, so Group 14 elements exhibit higher first ionisation enthalpies.
• Statement II is incorrect: Group 14 elements (like Carbon, Silicon, Germanium) build robust, highly stable three-dimensional covalent network crystal structures. As a result, the melting and boiling points of Group 14 elements are generally much higher than those of the corresponding Group 13 elements.

Pattern Recognition

Covalent network solids (Group 14) create huge upward steps in phase transition energy compared to Group 13 frameworks (e.g., Gallium, which melts at nearly room temperature).

Chapter Mix

Class 11 Chemistry: The p-Block Elements

Core Logic

• Statement I is true: Nitrogen has a small atomic size and high electronegativity, allowing it to form strong multiple pπ - pπ$p\pi - p\pi$ bonds with oxygen atoms. This enables stable oxide forms across a wide range of oxidation numbers spanning +1$+1$ to +5$+5$ (e.g., N₂O₅$N_2O_5$).
• Statement II is true: Nitrogen is a second-period element with a valence shell configuration of 2s² 2p³$2s^2 2p^3$. It completely lacks vacant 2d$2d$ orbitals. As a result, it cannot expand its octet beyond a covalency of 4, preventing the synthesis of pentahalides like NX₅$NX_5$.

Pattern Recognition

Second-period elements are structurally bounded by a maximum covalency of 4 due to the complete absence of d-orbitals. This explains why NF₅$NF_5$ is unstable/non-existent while PF₅$PF_5$ is easily synthesized.

Chapter Mix

Class 12 Chemistry: The p-Block Elements