Biology (Holt)

Chemical Bonding

Atoms can join with other atoms to form stable substances. A force
that joins atoms is called a chemical bond. A is a sub-
stance made of the joined atoms of two or more different elements.
For example, when sodium atoms, Na, bond with chlorine atoms, Cl,
the compound sodium chloride (table salt) forms. Every compound is
represented by a chemical formula that identifies the elements in the
compound and their proportions. The formula for sodium chloride,
NaCl, shows that there is one sodium atom for every chlorine atom in
the compound.

Covalent Bonds

Covalent bonds form when two or more atoms share electrons to
form a molecule. A ,such as the water molecule shown in
Figure 2,is a group of atoms held together by covalent bonds. Like the
rivets and welds that connect steel girders in a skyscraper, covalent
bonds join the atoms in molecules. Because the number of protons is
equal to the number of electrons in a molecule, the molecule has no
net electrical charge. Other examples of molecules include carbon
dioxide, CO 2 , and oxygen gas, O 2.
The arrangement of their electrons determines how atoms bond
together. Electrons are grouped into different levels. The levels clos-
est to the nucleus have less energy than the levels farther from the
nucleus. Electron levels can hold a limited number of electrons. The
outer electron levels of hydrogen and helium can hold up to two
electrons. All other atoms, however, have outer electron levels that
can hold up to eight electrons. An atom becomes stable when its
outer electron level is full. If the outer electron level is not full, an
atom will react readily with atoms that can provide electrons to fill
its outer level. As Figure 2 shows, water, H 2 O, forms when an oxygen
atom, which has six outer electrons, combines with two hydrogen
atoms, which have one outer electron each.

Hydrogen Bonds

The electrons in a water molecule are shared
by oxygen and hydrogen atoms. However, the
shared electrons are attracted more strongly
by the oxygen nucleus than by the hydrogen
nuclei. The water molecule therefore has par-
tially positive and negative ends, or poles. As
shown in Figure 3,the partially positive end
of one water molecule is attracted to the neg-
ative end of another water molecule.
Molecules with an unequal distribution of
electrical charge, such as water molecules,
are called polar molecules. This attraction
between two water molecules is an example
of a hydrogen bond—a weak chemical attrac-
tion between polar molecules.

molecule

compound

SECTION 1Nature of Matter 29

Figure 2 Water molecule.

Each water molecule is held

together by covalent bonds

between two hydrogen atoms

and one oxygen atom.

Hydrogen bonds

HH

+ +

O

Water molecules are attracted to each other by

hydrogen bonds.

Figure 3 Hydrogen bonds in water