Metallic bonding radii for metals and covalent bonding
radii for nonmetals form a smooth, continuous series. Metallic radii
are larger than ionic radii for the corresponding positive ions
The effective radius of sodium, for example, is greater
when mutually repelling positive ions are packed together in a metal
and held by a "glue" of electrons, than when an ion which
has completely lost its outer electron is packed in a NaCl salt
crystal next to ions of opposite charge.Thus the metallic radius
of sodium in the metal is 1.57 A, whereas its ionic radius in NaCI
is only 0.97 A.
Covalent radii of nonmetals are smaller than their corresponding
ionic radii, both because the ions have added one or more electrons
to complete their outer shell, and because atoms can come closer
to one another in covalent bonds.
For example, F and Cl (covalent radii) are smaller than F- and
Cl- (ionic radii). This can be seen opposite.
The hydrogen atom has essentially no size at all when it loses
an electron and becomes a bare hydrogen nucleus, or H+ ion. In covalent
compounds of hydrogen such as methane, CH4, the atomic radius of
hydrogen is 0.37 A. In LiH, the less electronegative lithium gives
its electron to hydrogen, and the H- ion, with a filled first-electron
shell, has an ionic radius of 1.54 A.