Chemical Bonds and the Formation of Molecules

For an atom to be very stable, its outer orbital must contain the maximum number of electrons. If an atom does not have its outer orbital full, then it tends to fill its outer orbital by bonding with other atoms in order to become maximally stable.

Most atoms do not have their outer orbitals filled with the maximum number of electrons. To fill their outer orbitals, atoms can either gain electrons from, or lose electrons to, other atoms. The number of electrons that an atom must gain or lose to fill its outer orbital is its valence. This is the number of cova-lent bonds that the atom can form (see table 2.1). To gain or lose electrons, atoms bond with other atoms to form molecules. A molecule consists of two or more atoms held together by chemical bonds. The atoms that make up a molecule may be of the same or different elements. For example, H2 is a molecule of hydrogen gas formed from two atoms of hydrogen; water (H2O) is an association of two hydrogen atoms with one oxygen atom. A compound consists of two or more different elements. The molecular weight of a molecule or compound is the sum of the atomic weights of the component atoms. Thus, the molecular weight of water is 18 (1 + 1 + 16).

The chemical bonds that hold atoms together are of various types, which differ in strength. These include covalent bonds, ionic bonds, and hydrogen bonds.

Covalent Bonds

Atoms often achieve stability by sharing electrons with other atoms, thereby filling the outer orbitals of both atoms simultaneously. This sharing creates strong bonds, called covalent bonds. Carbon (C), the most important single atom in biology, is frequently involved in covalent bonding. Carbon has four electrons but requires a total of eight to fill its outer orbital. A

2.2 Chemical Bonds and the Formation of Molecules 19

hydrogen (H) atom has one electron and requires an additional one to fill its outer orbital. Thus, the C atom can fill its outer orbital by sharing electrons with four H atoms. Each H atom has its outer orbital filled by sharing two electrons with the C atom. The compound CH4 is methane, and its formation involves one carbon atom sharing eight electrons with four hydrogen atoms (figure 2.2, and see table 2.1) The outer orbitals of both C and H are filled with shared electrons. When C forms covalent bonds with C or H atoms, an organic compound is formed. Inorganic compounds do not contain C to C bonds.

Since a C atom can bond with four other atoms, it can build up a large number of different molecules, which explains why it is the key atom in all cells. A single covalent bond is designated by a dash between the two atoms sharing the electrons and is written as C—H. Sometimes two pairs of electrons are

Carbon needs four more electrons to fill its outer orbital

Carbon needs four more electrons to fill its outer orbital

Each hydrogen atom requires another electron to fill its outer orbital

Four hydrogen atoms bond with one carbon atom to form one methane molecule

Four hydrogen atoms bond with one carbon atom to form one methane molecule

Figure 2.2 Covalent Bonds The carbon atom fills its outer electron orbital by sharing a total of 8 electrons with four H atoms. Each hydrogen atom shares 2 electrons with a C atom, thereby filling its outer orbital.
20 Chapter 2 The Molecules of Life

Table 2.2 Non-Polar and Polar Covalent Bonds

Type of

Atoms Involved and Charge

Covalent Bond

Distribution

Nonpolar

C—C

C and H have equal attractions

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