3. Lithium Through Neon   Previous PageNext Page
     How Big is an Atom?

The inverse of this is the volume per atom: v = 22 atom. 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 2.8 , 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 and 1.10 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 = 2.20 apart. No bonding radii are given for the noble gases helium and neon because they do not form chemical bonds.

The Fluorine molecule.
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