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.