The inverse of this is the volume per atom: v = 22
Each atom in a block of lithium metal occupies a volume of approximately
22 cubic angstroms. Without worrying about the geometry of the way
spheres pack together, we can take the cube root of this volume
and say that lithium atoms should have a diameter of approximately
, or a radius of 1.4
The true radius of a spherical atom can be defined as half the distance
from its center to the center of a like atom when they are touching.
As shown at the left, two atoms that share an electron pair in a
bond can come closer than two atoms that merely pack against one
another but are not bonded. Hence we must define two measures of
atomic size: bonding radii and packing radii, with the latter usually
about twice as large as the former.
Bonding and packing radii for the first- and second-shell atoms
are shown on page 8.
The values of 0.37
for hydrogen tell us that the centers of two bonded atoms in a
molecule are separated by 2 x 0.37
= 0.74 ,
but that the centers of hydrogen atoms in different
molecules in closely packed solid hydrogen (at very low temperatures)
are 2 x 1.10
apart. No bonding radii are given for the noble gases helium and
neon because they do not form chemical bonds.