electron configuration

Electrons in atoms have only certain specific energy levels which are quantized (fixed). Electrons are organised according to their energy levels into shells, labelled by the principle quantum number.

Example:
The electron configuration for He (2 electrons) would be 1s² and for Al (13 electrons) would be 1s²2s²2s⁶3s²3p¹ or [Ne] 3s²3p¹ as the electron configuration for Ne is 1s²2s²2s⁶.

The electron configuration for Cl^- would be [Ne] 3s²3p⁶ while for Na⁺ it would be [He] 2s²2p⁶.