Lowest Atomisation Enthalpy in d-Block

❓ Question

Among:

Cr, Fe, Co, Ni

Which metal has the lowest enthalpy of atomisation?

Find the number of valence electrons in that metal.

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🖼 Question Image

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✍️ Short Solution

Enthalpy of atomisation depends on:

👉 Strength of metallic bonding
👉 Number of unpaired electrons

More unpaired electrons ⇒ stronger bonding ⇒ higher enthalpy

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🔷 Step 1 — Write Electronic Configurations 💯

Cr:

$$[Ar]\, 3d^5 4s^1$$

Fe:

$$[Ar]\, 3d^6 4s^2$$

Co:

$$[Ar]\, 3d^7 4s^2$$

Ni:

$$[Ar]\, 3d^8 4s^2$$

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🔷 Step 2 — Count Unpaired Electrons

Cr → 6 unpaired
Fe → 4 unpaired
Co → 3 unpaired
Ni → 2 unpaired

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🔷 Step 3 — Relation with Atomisation Enthalpy

More unpaired electrons ⇒ stronger metallic bonding

So:

Cr > Fe > Co > Ni

👉 Lowest enthalpy = Ni

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🔷 Step 4 — Valence Electrons in Ni

For transition metals:

Valence electrons = $ns + (n-1)d$

Ni:

$$3d^8 4s^2$$

Total valence electrons:

$$= 10$$

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✅ Final Answer

$$\boxed{{\displaystyle 10\mathrm{}\mathrm{<br>}}}$$

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⭐ Golden JEE Insight

In d-block:

👉 More unpaired electrons
⇒ stronger bonding
⇒ higher atomisation enthalpy

So minimum enthalpy ⇒ minimum unpaired electrons.