By avoiding generation, use, or release to the environment of compounds with the
kinds of bonds described above as part of a program of green chemistry, the practice
of chemistry and the chemical industry can be made much safer. Green chemistry also
considers bonds that may have to be protected in chemical synthesis. Often steps must
be added to attach protecting groups to bonding groups to prevent their reacting during
a step of a synthesis. After the desired step is performed, the protecting groups must be
removed to give the final product. Materials are consumed and byproducts are generated
by these steps, so the practice of green chemistry attempts to avoid them whenever
possible.
3.7. Predicting Covalently Bound Compounds
It is often possible to predict the formulas of molecules using the Lewis symbols of
the elements in the compound with the octet rule for chemical bonding. This is shown in
Figure 3.8 for the hydrogen compounds of several elements in the second period of the
periodic table.
C H C
H H
H
HH
H H C H (CH 4 )
H
N H N H N
H H
H
H
(NH 3 )
H
Unshared pair of
electronsO H O H O
H
H
(H 2 O)
H
F H F H F
H
(HF)
H
H
H
H
H
Figure 3.8. Formation of covalent hydrogen compounds with elements in the second row of the periodic
table. All of carbon’s valence electrons are involved in bonds in CH 4 , NH 3 has 1 unshared pair of valence
electrons, H 2 O has 2 unshared pairs of valence electrons, and HF has 3 unshared pairs of valence electrons.
The unshared pairs of electrons are not shown in the structures in which the bonds are represented by
straight lines.
Chap. 3, Compounds: Safer Materials for a Safer World 67