Period 3: sodium to argon

The third shell: 3s and 3p fill, 3d waits

Every atom in the row has a neon core, 1s² 2s² 2p⁶, written [Ne]. Sodium ([Ne] 3s¹) and magnesium ([Ne] 3s²) fill the 3s subshell, and aluminum through argon fill 3p, ending with [Ne] 3s² 3p⁶.

The third shell also has a 3d subshell with room for ten more electrons, but its energy is higher than that of 4s. It stays empty here and only starts filling in period 4, which is why period 3 has eight elements rather than eighteen.

How properties change from sodium to argon

First ionization energy climbs from 5.139 eV for sodium to 15.76 eV for argon, and electronegativity from 0.93 for sodium to 3.16 for chlorine. The same dips seen in period 2 return: aluminum (5.986 eV) falls below magnesium (7.646 eV), and sulfur (10.36 eV) below phosphorus (10.487 eV).

Melting points follow the type of bonding. They rise through the metals, from 370.94 K for sodium to 933.47 K for aluminum, peak at 1687 K for silicon, whose atoms form a giant covalent network, then drop for the small molecules P₄, S₈ and Cl₂, down to 83.81 K for argon. The oxides change too, from basic sodium oxide through aluminum oxide, which reacts with both acids and bases, to the acid-forming oxides of phosphorus, sulfur and chlorine.

Where you meet period 3

Table salt is sodium chloride, and chlorine compounds disinfect tap water and swimming pools. Aluminum goes into drink cans and window frames, magnesium into light alloys and bright white flares, and silicon into almost every computer chip and solar panel. Phosphorus is a vital nutrient in fertilizers, most sulfur is turned into sulfuric acid, one of the most widely made industrial chemicals, and argon shields hot metal from the air during welding.