Question 1

What is the difference between Aufbau filling, Pauli's exclusion principle, and Hund's rule?

Accepted Answer

Aufbau says electrons fill orbitals successively in order of increasing energy. Pauli allows at most two electrons in one orbital, and those two must have opposite spins. Hund says degenerate orbitals of the same subshell fill singly with parallel spins before pairing occurs.

Question 2

How many electrons can s, p, d, and f subshells hold?

Accepted Answer

An s subshell has one orbital and holds 2 electrons, p has 3 orbitals and holds 6, d has 5 orbitals and holds 10, and f has 7 orbitals and holds 14. The limiting factor in each orbital is two electrons.

Question 3

Why are Na+, Mg2+, and Al3+ isoelectronic?

Accepted Answer

Their electron counts are 11 - 1 = 10, 12 - 2 = 10, and 13 - 3 = 10. Therefore all three have the Ne-type configuration 1s^2 2s^2 2p^6 and zero unpaired electrons. Their electron-cloud sizes differ because their nuclear charges differ.

Question 4

Why are the configurations of Cr and Cu exceptions to naive filling?

Accepted Answer

Cr has [Ar] 3d^5 4s^1, giving a half-filled d subshell, while Cu has [Ar] 3d^10 4s^1, giving a completely filled d subshell. NCERT attributes these configurations to the extra stability associated with half-filled and completely filled subshells.

Question 5

Which electrons are removed first when transition-metal cations form?

Accepted Answer

The 4s electrons are removed before 3d electrons. This is why neutral Fe has [Ar] 3d^6 4s^2, while Fe2+ is written as [Ar] 3d^6.

Aufbau Configuration Builder — Pauli, Hund & Exceptions

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