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1.3 Atoms combine to form
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molecules linked by chemical F
bonds

Atoms combine to make a great diversity of molecules in cells, N + C F C F
which in turn leads to the diversity of life. Atoms bond with
other atoms to form molecules, which are groups of two or 4 Fluorine 1 Carbon
more atoms bonded together that act as a single unit. An exam- atoms (F) atom (C) F

ple of a molecule is hydrogen gas (H ) , made when two atoms

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of hydrogen bond, as you’ll see below. Note that a chemical 1 Carbon
formula is written as the letter abbreviation for each element, tetra uoride
followed by a subscript giving the number of that type of molecule (CF )
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atom in the molecule. When molecules form, the individual FIGURE 1.6 Covalent bonds

atoms interact through what is called a chemical bond , a Covalent bonds form when atoms share a pair, or pairs, of electrons in
type of attraction between atoms that holds them together. their outermost energy levels. Four individual F atoms, each containing
For example, joining one atom of carbon with four atoms of seven electrons in its outermost energy level, form covalent bonds with a

hydrogen creates the compound methane (CH) , which is single C atom. The C atom shares four electrons, one with every F atom,
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also known as natural gas and is used in cooking, heating, and while each individual F atom shares a single electron with the C atom. By

sharing electrons, each F atom and the C atom fill their outer levels.
industry. There are several ways in which atoms can interact
with one another, forming different types of chemical bonds.
In this section, we will look at these chemical bonds. sharing of electrons, allowing each atom to fill its outer shell
with electrons. The resulting compound, carbon tetrafluo-
Covalent Bonds ride (CF) , consists of one C atom covalently bound to four

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The ability of atoms to combine with other atoms is deter- F atoms.
mined in large part by the electrons furthest from the When two atoms share two electrons in a covalent

nucleus, the valence electrons. When atoms combine with bond, a single bond is formed. When two atoms share two

other atoms to form a molecule, the atoms share valence pairs of electrons covalently, a double bond is formed. In
electrons with each other. Specifically, when the outermost practice, the word “covalent” is often omitted, as it is under-
energy levels of two atoms come in close proximity, the shells stood that these bonds arise through a sharing of electrons
may overlap with one another so that electrons are shared and are thus covalent. A double bond is denoted by a dou-
between the two atoms. When a pair of electrons are shared ble line connecting the two chemical symbols for the atoms.

between the two atoms, a covalent bond is formed. FIGURE 1.7 shows how double bonds are represented in
Molecules tend to be the most stable when the two ethylene, a molecule that is made of two carbon atoms and
atoms forming a bond share enough electrons to com- four hydrogen atoms. The two carbon atoms share four

pletely fill the outermost energy level. Chemical stability electrons with each other, creating a PCC double bond. By
occurs when an atom or molecule has the lowest possible contrast, each carbon atom shares only two electrons with a

energy state. Stable molecules are less likely to react because single H atom, creating a OCH single bond.
they require significant inputs of energy to change bonding
partners, which is what happens during chemical reactions.
Very little energy is required, for example, to get dynamite
to react. A stable molecule, such as nitrogen gas, requires a H C C H
large amount of energy to react. H H

FIGURE 1.6 shows an example of the formation of cova- FIGURE 1.7 The double covalent bond of carbon atoms

lent bond between one carbon (C) atom and four fluorine in ethylene
(F) atoms. Each fluorine atom (F) has seven valence electrons. The double line represents four electrons shared between the two
When each of these F atoms forms a covalent bond with carbon atoms; each line denotes two shared electrons. The single lines
the carbon (C) atom, the outermost shells overlap due to the connecting carbon and hydrogen represent single covalent bonds.
MODULE 1 ELEMENTS OF LIFE 33

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