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The sub-shell with lowest value of (n + l) is filled up first. When two or more sub-shells have the same (n + l) value, the sub-shell with lowest value of 'n' is filled up first. Therefore, the correct order is:

1. 4s 3d 4p 5s 4d
2. 4+0 3+2 4+1 5+0 4+2
3. 4 5 5 5 6
4. d = l = 3 means f-subshell. Maximum no. of electrons = 4l + 2 = 4 × 3 + 2 = 14

Correct answer: 4s 3d 4p 5s 4d

Solution

The correct order of filling sub-shells is determined by the (n + l) rule. Among sub-shells with the same (n + l) value, the one with the lower 'n' is filled first. Option A correctly represents the filling order: 4s (n+l=4), 3d (n+l=5), 4p (n+l=5), 5s (n+l=5), 4d (n+l=6).

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