3.4. Covalent Bonds in H 2 and Other Molecules
From their Lewis symbols it is easy to see how some atoms of elements on the left
side of the table with only one or two outer-shell electrons can lose those electrons to
form cations such as Na+ or Ca2+. It is also easily seen that atoms from groups near the
right side of the periodic table can accept one or two electrons to gain stable octets and
become anions such as Cl
-
or O^2 -. But, it is difficult to impossible to take more than two
electrons away from an atom to form cations with charges greater than +2 or to add 3 or
more electrons to form anions with charges of -3 or even more negative, although ions
such as Al3+ and N^3 - do exist. So atoms of elements in the middle of the periodic table
and the nonmetals on the right have a tendency to share electrons in covalent bonds,
rather than becoming ions.
It is readily visualized how mutually attracting ions of opposite charge are held
together in a crystalline lattice. Shared electrons in covalent bonds act to reduce the
forces of repulsion between the positively charged nuclei of the atoms that they join
together. That is most easily seen for the case of the hydrogen molecule, H 2. The nuclei
of H atoms consist of single protons, and the two H atom nuclei in the H 2 molecule
repel each other. However, their 2 shared electrons compose a cloud of negative charge
between the two H nuclei, shielding the nuclei from each other’s repelling positive charge
and enabling the molecule to exist as a covalently bound molecule (Figure 3.7).
+ +
-
-
Figure 3.7. Electrons in motion between and around mutually repelling H nuclei shield the nuclei from
each other constituting a single covalent bond that holds the H 2 molecule together.
3.5. Covalent Bonds in Compounds
Consider next some example covalent bonds between atoms of some of the lighter
elements. These are best understood in reference to Figure 2.9, the abbreviated version
of the periodic table showing the Lewis symbols (outer shell valence electrons) of the
first 20 elements. As is the case with ions, atoms that are covalently bonded in molecules
often have an arrangement of outer shell electrons like that of the noble gas with an
atomic number closest to the element in question. It was just seen that covalently bonded
H atoms in molecules of H 2 have 2 outer shell electrons like the nearby noble gas helium.
For atoms of many other elements, the tendency is to acquire 8 outer shell electrons— an
octet — in sharing electrons through covalent bonds. This tendency forms the basis of
the octet rule discussed in Section 3.2. In illustrating the application of the octet rule to
covalent bonding Section 3.7 considers first the bonding of atoms of hydrogen to atoms
of elements with atomic numbers 6 through 9 in the second period of the periodic table.
64 Green Chemistry, 2nd ed