Shells and Sub-Shells

Shell (Orbit):

• Electrons revolve in fixed circular paths called shells.
• Each shell is given a number n = 1, 2, 3, 4... and a letter K, L, M, N...
• As n increases, the shell is farther from the nucleus and has higher energy.

Sub-Shell:

• Each shell is divided into smaller parts called sub-shells.
• There are 4 types of sub-shells: s, p, d, f
• These names stand for: sharp, principal, diffused, fundamental.

Capacity of sub-shells:

• s sub-shell → 1 orbital → maximum 2 electrons
• p sub-shell → 3 orbitals → maximum 6 electrons
• d sub-shell → 5 orbitals → maximum 10 electrons
• f sub-shell → 7 orbitals → maximum 14 electrons

Shape of sub-shells:

• s → spherical shape
• p → dumbbell shape
• d and f → complex shapes

Aufbau Principle: Electrons fill sub-shells starting from the lowest energy first.
Energy order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → ...
Important: 4s is filled before 3d because 4s has lower energy than 3d.