Why do the transition elements exhibit higher enthalpies of atmoisation?

Key ideas

1. Enthalpy of atomisation ($\Delta_{a}H$)
   • Energy needed to convert 1 mole of a solid metal into 1 mole of free gaseous atoms.
2. Metallic bonding strength governs $\Delta_{a}H$. Stronger bonding → higher energy needed.
3. Origin of strong bonding in transition elements
   • They have partially-filled $(n-1)d$ orbitals.
   • Many unpaired $d$-electrons give multiple metal–metal overlaps.
   • Greater overlap increases the number of bonding interactions per atom.
4. Result: Larger cohesive (binding) energy → larger $\Delta_{a}H$ compared to s-block metals or p-block elements.

Logical chain a student follows

Step 1: Recall definition of enthalpy of atomisation.

Step 2: Link enthalpy to metallic bond strength.

Step 3: Ask, ‘What makes bonding strong in transition metals?’ ➔ unpaired $d$-electrons providing extra metal–metal bonding.

Step 4: Conclude: more bonding interactions = higher energy required to break them = higher $\Delta_{a}H$.

Why is it so hard to pull apart transition-metal atoms?

Imagine you have a bunch of kids (atoms) holding hands really tight on a playground. The tighter they hold hands, the harder it is to pull each kid away. For metals, holding hands is called metallic bonding.

Transition-metal atoms have many ‘free hands’ (unpaired $d$-electrons). More free hands = more places to grab other kids’ hands ⇒ a much stronger chain. Because the chain is so strong, you need to spend a lot of energy (heat) to separate every atom into single, free atoms in the gas state. That required heat energy is called enthalpy of atomisation ($\Delta_{a}H$), and for transition metals it is big!

Step-by-step answer

1. Definition $\Delta_{a}H = \text{Energy required to convert 1 mol of solid M to 1 mol of gaseous M atoms}$

2. Link to metallic bonding $\text{Higher metallic bond strength}\;\Rightarrow\;\text{higher }\Delta_{a}H$

3. Reason for strong metallic bonds in transition elements

   • Transition metals possess partially-filled }(n-1)d\text{ subshells}.
   • Many unpaired }d\text{ electrons can overlap with neighbouring atoms’ $d$ and $s$ orbitals.
   • This produces a large number of metal–metal bonds (strong cohesive forces).

4. Conclusion $\boxed{\text{Transition elements exhibit high enthalpies of atomisation because their many unpaired }d\text{ electrons create strong, multi-directional metallic bonding, so more energy is needed to separate the atoms.}}$