Biology 12

564 MHR • Appendix 6

electronegativity difference between carbon and

hydrogen is 0.4. Therefore, the sharing of

electrons is more unequal between carbon and

oxygen than between carbon and hydrogen. In

other words, a carbon–oxygen bond is more

polar than a carbon–hydrogen bond.

When two elements have very different

electronegativities, the bonds the elements form

can be described in terms of electron transfers,

rather than electron sharing. For example,

sodium and chlorine have an electronegativity

difference of 2.1. When these elements react,

electrons are transferred from sodium atoms

to chlorine atoms to form positively charged

sodium ions (Na+) and negatively charged

chloride ions (Cl−). These ions attract each

other. Bonds created by the transfer of electrons

from one type of atom to another are called

ionic bonds.

There is no sharp distinction between polar

covalent bonds and ionic bonds. The greater

the difference in the electronegativities of two

bonded atoms, the more polar is the bond

between them. When sharing is very unequal,

chemists describe the electrons as being

transferred from one atom to the other. For

convenience, chemists have chosen an

approximate value of the electronegativity

difference that distinguishes polar covalent

bonds from ionic bonds. In general, if the

electronegativity difference equals or exceeds

1.7, the elements will tend to form ionic bonds.

If the electronegativity difference is less than 1.7,

the elements will tend to form covalent bonds.

If the electronegativity difference is greater than

0 but less than 1.7, the bonds will be polar

covalent. Strictly speaking, bonds are non-polar

covalent only if the electronegativity difference

is exactly 0, that is, if two atoms of the same

element form a bond. However, when the

electronegativity difference is less than 0.5

(for example, 0.4 for C and H), the electrons

are shared almost equally, and the bond dipole

is weak.

The electronegativity of carbon is 2.5, which

is very close to the middle of the range of

electronegativity values, from 0.7 to 4.0. Carbon

can, therefore, form covalent bonds with

elements that are more electronegative, such

as oxygen, and with elements that are less

electronegative, such as hydrogen. Carbon

atoms are also able to form covalent bonds with

other carbon atoms to produce a wide range of

compounds that contain chains or rings of

carbon atoms.

Molecular Polarity

You learned earlier that the bond in a hydrogen

chloride molecule (HCl) is polar covalent.

Because the bond is polar, and there is only one

bond in the molecule, the whole molecule is

also polar. However, most covalent molecules

contain more than two atoms and more than one

bond. In such cases, the polarity of a molecule

depends on the polarity of the individual

bonds and on the overall shape of the molecule.

Two examples that show the importance of

molecular shapes are carbon dioxide (CO 2 ) and

water (H 2 O).

In a carbon dioxide molecule, the two highly

electronegative oxygen atoms form double

bonds to a less electronegative carbon atom,

as shown in Figure A6.2. The carbon–oxygen

bonds are polar covalent. The shape of the

molecule is straight or linear. As a result, the

two dipoles counterbalance each other, and the

molecule has no net dipole. Thus, even though

a carbon dioxide molecule contains polar bonds,

the molecule is non-polar.

Figure A6.2A linear carbon dioxide molecule contains

polar covalent bonds, but the molecule is non-polar.

In a water molecule, one highly

electronegative oxygen atom forms single

bonds to two less electronegative hydrogen

atoms, as shown in Figure A6.3. The hydrogen–

oxygen bonds are polar covalent. In contrast

to the linear carbon dioxide molecule, a water

molecule is V-shaped or bent. As a result, the

two dipoles do not counterbalance each other,

and the molecule has a net dipole. Thus, a

water molecule contains polar bonds, and the

molecule is polar.

Figure A6.3Because a water molecule is bent and

contains polar covalent bonds, the molecule is polar.

Hydrocarbons, which are composed of

carbon and hydrogen, are an important class of

O

H

δ − δ −

δ +

H

δ +

CO

δ + δ +δ −

O

δ −