How Electrons Fill Atomic Orbitals

Electrons fill atomic orbitals in an order determined by their energy levels, following the Aufbau principle, the Pauli exclusion principle, and Hund's rule. The configuration is written as a sequence of subshells (e.g., 1s² 2s² 2p⁶).

• The arrangement affects chemical reactivity and properties.
• Valence electrons (in the outer shell) are especially important.

Periodic Trends

The periodic table is organized to reflect recurring trends:

• Atomic radius decreases across a period, increases down a group.
• Ionization energy increases across a period, decreases down a group.
• Electronegativity (tendency to attract electrons) increases across a period, decreases down a group.

Why It Matters

These trends explain why sodium is reactive and neon is stable, or why chlorine readily forms negative ions.