Period 4: potassium to krypton

4s first, then 3d, then 4p

Potassium ([Ar] 4s¹) and calcium ([Ar] 4s²) open the fourth shell. The ten elements from scandium to zinc then fill the 3d subshell, which belongs to the third shell but is lower in energy than 4p. Gallium through krypton finish with 4p, ending at [Ar] 3d¹⁰ 4s² 4p⁶.

Two transition metals bend the pattern. Chromium is [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s², and copper is [Ar] 3d¹⁰ 4s¹, because a half-filled or completely filled 3d subshell is especially favorable.

Trends across period 4

At the two ends, the row behaves like periods 2 and 3. Electronegativity rises from 0.82 for potassium to 2.96 for bromine, and first ionization energy from 4.341 eV for potassium to 14.0 eV for krypton. Among the transition metals the changes are gentler, because each new electron goes into the inner 3d subshell: electronegativity only drifts between 1.36 for scandium and 1.91 for nickel.

Melting point and density peak in the middle. Vanadium melts at 2183 K, the highest in the row, and copper is the densest at 8.96 g/cm³, compared with 0.862 g/cm³ for potassium. Near the right end, gallium melts at 302.91 K, just above room temperature, and bromine is a red-brown liquid that boils at 332 K.

Where you meet period 4

Iron, the main ingredient of steel, is the most widely used metal, and copper carries electricity through almost every wire. Zinc coats steel to keep it from rusting, titanium makes strong, light aircraft parts, and chromium gives stainless steel its resistance to corrosion. Calcium builds bones and teeth, potassium helps nerves send signals, and your body also needs small amounts of iron, zinc, cobalt and selenium.