Arrange the following in Increasing order of enthalpy of atomization:…

The correct answer is Option (4) → (A), (C), (B), (D)

The enthalpy of atomisation is the energy needed to break a metal into free gaseous atoms, so it measures the strength of metallic bonding. In transition metals both the ns and the (n−1)d electrons take part in this bonding: in general, the greater the number of valence electrons, the stronger the bonding. A completely filled d subshell takes no part, which is why zinc has the lowest value of the series.

• (A) Zn (Z = 30) is [Ar]3d¹⁰4s²: the 3d subshell is full, so only the two 4s electrons bond. Lowest, 126 kJ mol⁻¹.

• (C) Sc (Z = 21) is [Ar]3d¹4s²: 3 valence electrons, 326 kJ mol⁻¹.

• (B) Cr (Z = 24) is [Ar]3d⁵4s¹: 6 valence electrons, 397 kJ mol⁻¹.

• (D) Co (Z = 27) is [Ar]3d⁷4s²: 9 valence electrons, 425 kJ mol⁻¹.

    Zn < Sc < Cr < Co

Shortcut: Zinc (A) has the lowest value and cobalt (D) the highest of the four. Options (3) and (4) both start with (A) and end with (D); since Sc (326) is below Cr (397), only Option (4) remains.

NCERT (2026-27 edition):
General Properties of the Transition Elements (d-Block) – …