Biology 12

Chapter 1 The Chemistry of Life • MHR 11

electronegative element, is found near the top
right corner of the periodic table and has an
electronegativity value of 4.0. Both cesium and
francium, the least electronegative elements, are
found near the bottom left corner of the periodic
table and each has an electronegativity value of 0.7.
Elements that are most likely to form ionic bonds,
such as sodium and chlorine, are far apart in the
periodic table and have a large difference in their
electronegativity.
Elements that are close together in the
periodic table have a small difference in their
electronegativity. If two of these elements react to
form a compound, their similar abilities to attract
electrons results in the formation of a covalent
bond, in which electrons are shared. In a covalent
bond, atoms share two valence electrons. An
example of this is the covalent bonding of two
chlorine atoms, as shown in Figure 1.5, top. Double
covalent bonds involve the sharing of two pairs
of shared valence electrons. The two oxygen atoms
in an oxygen molecule are joined by a double
covalent bond, as shown in Figure 1.5, middle.
The shared electrons in covalent bonds belong
exclusively to neither one nor the other atom.
However, by sharing these valence electrons, both
atoms appear to have the same number of valence
electrons as a noble gas atom. In a covalent bond
formed by two atoms of the same element, the
electronegativity difference is zero. Therefore, the
electrons in the bond are shared equally between
the two atoms. This type of bond is described as
non-polar covalent. Examples of non-polar
covalent bonds are found in chlorine and carbon
dioxide molecules.
A covalent bond is said to be polar covalentwhen
the electronegativity difference between the atoms
is not zero and the electrons are therefore shared
unequally. In a water molecule (see Figure 1.5,
bottom), oxygen is more electronegative than is
hydrogen. The shared electrons spend more of
their time near the oxygen nucleus than near the
hydrogen nucleus. As a result, the oxygen atom
gains a slight negative charge and the hydrogen
atoms become slightly positively charged.
Chemists represent molecules formed through
covalent bonds with various formulas, such as
those in Figure 1.6. Electron-dot and structural
formulas are simplified ways of showing what
electrons are being shared.

Figure 1.6In an electron-dot formula, only the electrons in

the valence shell are shown. In a structural formula, each

line represents a pair of electrons shared by two atoms. A

molecular formula shows only the number of each type of

atom in a molecule.

Hydrogen Bonds and the

Properties of Water

Some molecules with polar covalent bonds are

known as polar molecules. A polar moleculehas

an unequal distribution of charge as a result of its

polar bonds and its shape. More information about

polar molecules is provided in Appendix 6. Water

is a common example of a polar molecule. In a

water molecule, as shown in Figure 1.7, the

slightly negative end of each bond can be labelled

δ−and the slightly positive end can be labelled δ+.

These two ends, with slightly different charges, are

sometimes referred to as “poles.” Because a water

molecule is polar, it can attract other water

molecules, due to the attraction between negative

poles and positive poles (see Figure 1.7). The

attractions between water molecules are called

hydrogen bonds. Hydrogen bonds can also be

found between other molecules that contain

hydrogen atoms bonded covalently to atoms of a

much more electronegative element. Examples

include ammonia (NH 3 ) and hydrogen fluoride

(HF) in their liquid states.

To learn more about ionic and covalent bonding, go to your

Electronic Learning Partner now.

ELECTRONIC LEARNING PARTNER

Electron-Dot

formula Structural formula Molecular formula

carbon dioxide

ammonia

water

carbon dioxide

ammonia

water

carbon dioxide

ammonia

water

OCO

N

H

H

H

O

H

H

HO 2

CO 2

NH 3

O C O

H

H

NH

H

OH