Chemistry, Third edition

COVALENT BONDING 55

BOX 4.4

Families of carbon compounds

Carbon forms many, many covalent compounds in which the carbon atoms are covalently

bonded to each other and to other atoms, such as hydrogen. Carbon has four outer shell

electrons to share with other atoms and, in these examples, achieves the stable electronic

structure of neon by sharing electrons with hydrogen or another carbon atom. Carbon atoms

can share one, two or even three electrons with each other. Some examples are:

BOX 4.5

Electron density map for hydrogen (H 2 )

The results of quantum mechanical

calculations can be used to plot the electron

density distribution in a hydrogen molecule.

The contour lines join regions of the same

electron density. Notice that there is a region

of high electron density between the nuclei,

which corresponds to the covalent bond in

the Lewis structure. This electron density

attracts the positive nuclei and prevents

repulsion between the two positive nuclei

from driving the atoms apart – it ‘glues’ the

atoms together. A region of high electron

density, holding atoms together in this way, is

a chracteristic feature of covalent molecules

(Fig. 4.6).

Fig. 4.6An electron density map for

hydrogen (H 2 ). Numbers represent electrons

per nm^3.

Properties of covalent compounds

Covalent compounds have the following characteristic properties:

1. Low melting points and boiling points (they are volatile)

They are often liquids or gases at room temperature (think of O 2 , H 2 O, N 2 and

CO 2 ). The molecules are not attracted towards each other by strong electrostatic

forces – so they are pulled apart at relatively low temperatures. The forces that attract

molecules together (the intermolecularforces) are called van der Waals’ forcesand

are relatively weak.