Sodium is a silvery metal resembling lithium, but is even softer than Li because the atoms are larger and cannot be held together as strongly by the mobile electrons.

A piece of lithium metal can be cut only with difficulty even with a sharp steel knife, but sodium has the consistency of a block of pine wood or a hard cheddar cheese. The weaker bonds between atoms in sodium also are manifested by melting points: 181°C for Li, but only 98°C for Na.

The same trend (see right) is observed for beryllium and magnesium. Both are harder and higher melting than Li and Na because they have twice as many binding electrons per ion, but Mg is softer than Be because of its greater atomic size and weaker hold on its electrons. Their melting points are consistent with the trend: 1278°C for Be and only 650°C for Mg.

Boron holds its three outer electrons strongly because the B3+ ion is small, and electrons can come close to the central positive charge. Hence boron is nonmetallic.

Aluminum, below boron in the third row, is larger. Its attraction for the three outer electrons is weak enough that the electrons are lost easily, so the atom is metallic. Solid aluminum has a close-packed spherical-ion structure typical of a metal, and melts at 660°C, almost as low as magnesium.