The electronic configuration of Eu (Atomic no. 63), Gd (Atomic no. 64) and Tb (Atomic no. 65) are (NEET - Phase II) (a) [Xe] 4f$^6$ 5d$^1$ 6s$^2$, [Xe] 4f$^7$ 5d$^1$ 6s$^2$ and [Xe] 4f$^8$ 5d$^1$ 6s$^2$ (b) [Xe] 4f$^7$ 6s$^2$, [Xe] 4f$^7$ 5d$^1$ 6s$^2$ and [Xe] 4f$^9$ 6s$^2$ (c) [Xe] 4f$^7$ 6s$^2$, [Xe] 4f$^8$ 6s$^2$ and [Xe] 4f$^8$ 5d$^1$ 6s$^2$ (d) [Xe] 4f$^6$ 5d$^1$ 6s$^2$, [Xe] 4f$^7$ 5d$^1$ 6s$^2$ and [Xe] 4f$^9$ 6s$^2$

The electronic configuration of Eu (Atomic no. 63) – Class 11 Chemistry

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Q. The electronic configuration of Eu (Atomic no. 63), Gd (Atomic no. 64) and Tb (Atomic no. 65) are (NEET - Phase II)

(a) [Xe] 4f

^6

^1

^2

, [Xe] 4f

^7

^1

^2

and [Xe] 4f

^8

^1

^2

(b) [Xe] 4f

^7

^2

, [Xe] 4f

^7

^1

^2

and [Xe] 4f

^9

^2

^7

^2

, [Xe] 4f

^8

^2

and [Xe] 4f

^8

^1

^2

(d) [Xe] 4f

^6

^1

^2

, [Xe] 4f

^7

^1

^2

and [Xe] 4f

^9

^2

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Step 1. A straightforward Aufbau-only prediction for the lanthanides Eu (63), Gd (64) and Tb (65) would suggest filling 4f progressively - but as with Cr and Cu, a half-filled or fully-filled f-subshell carries extra stability, which alters some of these configurations.

Step 2. Europium (Z = 63) achieves the extra-stable, exactly half-filled $4f^7$ subshell: its real configuration is $[\text{Xe}]4f^7\,6s^2$ (not $4f^6\,5d^1\,6s^2$ ).

Step 3. Gadolinium (Z = 64) keeps that stable $4f^7$ core and adds one more electron into 5d rather than continuing to fill 4f (since an 8th f electron would break the stable half-filled arrangement): $[\text{Xe}]4f^7\,5d^1\,6s^2$ . …

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Q.The electronic configuration of Eu (Atomic no. 63), Gd (Atomic no. 64) and Tb (Atomic no. 65) are (NEET - Phase II)

Concept understanding — Stability of Half-filled and Fully-filled Orbitals

Q.The electronic configuration of Eu (Atomic no. 63), Gd (Atomic no. 64) and Tb (Atomic no. 65) are (NEET - Phase II)

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