Electrons occupy energy levels in a set order, and the outer electrons determine bonding.

• Shells fill in order: 2, 8, 8 for the first twenty elements.
• Aufbau notation fills sub-levels by increasing energy: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
• Valence electrons are those in the outermost energy level.
• Atoms react to reach a full outer shell (the octet rule).

• Valence electrons — outer-shell electrons involved in bonding.
• Aufbau principle — fill the lowest energy sub-level first.