Electron Subshells in the n=2 Shell

Quantum Mechanics: Subshells of the Second Principal Energy Level

The principal quantum number n = 2 corresponds to the second electron shell. It contains two subshells, not three:

Subshells for n = 2

• 2s subshell (ℓ = 0): 1 orbital → holds 2 electrons
• 2p subshell (ℓ = 1): 3 orbitals (pₓ, pᵧ, p₂) → holds 6 electrons

Why No 2d Subshell?

The azimuthal quantum number ℓ ranges from 0 to n−1. For n = 2, ℓ can only be 0 (s) or 1 (p). The d subshell (ℓ = 2) first appears at n = 3 (3d). The original answer stating "s, p, and d" for n=2 is incorrect.

Electron Capacity

• 2s² 2p⁶ = 8 electrons maximum in the n=2 shell
• This fills the second period of the periodic table (Li through Ne)

Visual Summary

Key Takeaway

The n=2 shell has exactly two subshells: 2s and 2p. The d subshell begins at n=3. This quantum rule (ℓ ≤ n−1) is fundamental to atomic structure and the periodic table's shape.