Modern inorganic chemistry

STRUCTURE AND BONDING 37

covalent bonds with four hydrogen atoms, each element thus

attaining a noble gas configuration :

+ — - H : c ; H

x •

4H- H

Although the electrons from hydrogen and carbon are given •

and x signs, these are used only for convenience and there is, of

course, no difference between them. Each pair of electrons x

constitutes a single bond (a sigma bond) and is more conveniently

represented in graphical formulae by a single line, for esample

H H

H, or better (to illustrate shape) C^\H

H

Compounds formed by the sharing of electrons are said to be

covalent.

THE SHAPE OF COVALENTLY BONDED MOLECULES

AND IONS

Unlike the forces between ions which are electrostatic and without
direction, covalent bonds are directed in space. For a simple
molecule or covalently bonded ion made up of typical elements the
shape is nearly always decided by the number of bonding electron
pairs and the number of lone pairs (pairs of electrons not involved in
bonding) around the central metal atom, which arrange themselves
so as to be as far apart as possible because of electrostatic repulsion
between the electron pairs. Table 2.8 shows the essential shape
assumed by simple molecules or ions with one central atom X.
Carbon is able to form a great many covalently bonded compounds
in which there are chains of carbon atoms linked by single covalent
bonds. In each case where the carbon atoms are joined to four
other atoms the essential orientation around each carbon atom is
tetrahedral.
The shapes indicated in Table 2.8 are only exact in cases in which
all the electron pairs are equivalent, i.e. they are all bonding pairs.