Atoms, Molecules, Ions, and Bonds
An atom consists of a nucleus of positively charged protons and neutrally charged neutrons. Negatively charged electrons
are arranged outside the nucleus. Molecules are groups of two or more atoms held together by chemical bonds.
Chemical bonds between atoms form because of the interaction of their electrons. The electronegativity of an atom, or
the ability of an atom to attract electrons, plays a large part in determining the kind of bond that forms. There are three
kinds of bonds, as follows:
1. Ionic bonds form between two atoms when one or more electrons are transferred from one atom to the other. This
bond occurs when the electronegativities of the atoms are very different and one atom has a much stronger pull on the
electrons (high electronegativity) than the other atom in the bond. The atom that gains electrons has an overall negative
charge, and the atom that loses electrons has an overall positive charge. Because of their positive or negative
charges, these atoms are ions. The attraction of the positive ion to the negative ion constitutes the ionic bond. Sodium
and chlorine form ions (Na+ and Cl–), and the bond formed in a molecule of sodium chloride (NaCl) is an ionic bond.
2. Covalent bonds form when electrons between atoms are shared, which means that neither atom completely retains
possession of the electrons (as happens with atoms that form strong ionic bonds). Covalent bonds occur
when the electronegativities of the atoms are similar.
Nonpolar covalent bonds form when electrons are shared equally. When the two atoms sharing electrons are
identical, such as in oxygen gas (O2), the electronegativities are identical, and both atoms pull equally on the
Polar covalent bonds form when electrons are shared unequally. Atoms in this kind of bond have electronegativities
that are different, and an unequal distribution of the electrons results. The electrons forming the bond are
closer to the atom with the greater electronegativity and produce a negative charge, or pole, near that atom. The
area around the atom with the weaker pull on the electrons produces a positive pole. In a molecule of water (H2O),
for example, electrons are shared between the oxygen atom and each hydrogen atom. Oxygen, with a greater electronegativity,
exerts a stronger pull on the shared electrons than does each hydrogen atom. This unequal distribution
of electrons creates a negative pole near the oxygen atom and positive poles near each hydrogen atom.
Single covalent, double covalent, and triple covalent bonds form when two, four, and six electrons are shared,
3. Hydrogen bonds are weak bonds between molecules. They form when a positively charged hydrogen atom in one
covalently bonded molecule is attracted to a negatively charged area of another covalently bonded molecule. In
water, the positive pole around a hydrogen atom forms a hydrogen bond to the negative pole around the oxygen
atom of another water molecule.
Properties of Water
The hydrogen bonds among water molecules contribute to some very special properties for water.
1. Water is an excellent solvent. Ionic substances are soluble (they dissolve) in water because the poles of the polar
water molecules interact with the ionic substances and separate them into ions. Substances with polar covalent
bonds are similarly soluble because of the interaction of their poles with those of water. Substances that dissolve
in water are called hydrophilic (“water loving”). Because they lack charged poles, nonpolar covalent substances
do not dissolve in water and are called hydrophobic (“water fearing”).
2. Water has a high heat capacity. Heat capacity is the degree to which a substance changes temperature in response
to a gain or loss of heat. Water has a high heat capacity, changing temperature very slowly with changes in its heat
content. Thus, the temperatures of large bodies of water are very stable in response to the temperature changes of
the surrounding air. You must add a relatively large amount of energy to warm (and boil) water or remove a relatively
large amount of energy to cool (and freeze) water. When sweat evaporates from your skin, a large amount of
heat is taken with it and you are cooled.
3. Ice floats. Unlike most substances that contract and become more dense when they freeze, water expands as it
freezes, becomes less dense than its liquid form, and, as a result, floats in liquid water. Hydrogen bonds are typically
weak, constantly breaking and reforming, allowing molecules to periodically approach one another. In the
solid state of water, the weak hydrogen bonds between water molecules become rigid and form a crystal that
keeps the molecules separated and less dense than its liquid form. If ice did not float, it would sink and remain
frozen due to the insulating protection of the overlaying water.
4. Water has strong cohesion and high surface tension. Cohesion, or the attraction between like substances, occurs
in water because of the hydrogen bonding between water molecules. The strong cohesion between water molecules
produces a high surface tension, creating a water surface that is firm enough to allow many insects to walk
upon it without sinking.
5. Water has strong adhesion. Adhesion is the attraction of unlike substances. If you wet your finger, you can easily
pick up a straight pin by touching it because the water on your finger adheres to both your skin and the pin.
Similarly, some people wet their fingers to help them turn pages. When water adheres to the walls of narrow
tubing or to absorbent solids like paper, it demonstrates capillary action by rising up the tubing or creeping
through the paper.
Organic molecules are those that have carbon atoms. In living systems, large organic molecules, called macromolecules,
may consist of hundreds or thousands of atoms. Most macromolecules are polymers, molecules that consist of a single
unit (monomer) repeated many times. Four of carbon’s six electrons are available to form bonds with other atoms. Thus, you will always see four lines connecting a carbon atom to other atoms, each line representing a pair of shared electrons (one electron from carbon and
one from another atom). Complex molecules can be formed by stringing carbon atoms together in a straight line or by
connecting carbons together to form rings. The presence of nitrogen, oxygen, and other atoms adds additional variety to
these carbon molecules. Many organic molecules share similar properties because they have similar clusters of atoms, called functional groups.
Each functional group gives the molecule a particular property, such as acidity or polarity. Four important classes of organic molecules—carbohydrates, lipids, proteins, and nucleic acids—are discussed below.
Carbohydrates are classified into three groups according to the number of sugar (or saccharide) molecules present.
1. A monosaccharide is the simplest kind of carbohydrate. It consists of a single sugar molecule, such as fructose
or glucose. (Note that the symbol C for carbon may be omitted in ring structures; a carbon exists
wherever four bond lines meet.) Sugar molecules have the formula (CH2O)n, where n is any number from 3 to 8.
For glucose, n is 6, and its formula is C6H12O6. The formula for fructose is also C6H12O6, but the placement of the carbon atoms is different.
2. A disaccharide consists of two sugar molecules joined by a glycosidic linkage. During the process of joining, a
water molecule is lost. Thus, when glucose and fructose link to form sucrose, the formula is C12H22O11 (not
C12H24O12). This type of chemical reaction, where a simple molecule is lost, is generally called a condensation
reaction (or specifically, a dehydration reaction, if the lost molecule is water). Some common disaccharides
• glucose + fructose = sucrose (common table sugar)
• glucose + galactose = lactose (the sugar in milk)
• glucose + glucose = maltose
3. A polysaccharide consists of a series of connected monosaccharides. Thus, a polysaccharide is a polymer because it
consists of repeating units of a monosaccharide. The following examples of polysaccharides may contain thousands
of glucose monomers:
• Starch is a polymer of α-glucose molecules. It is the principal energy storage molecule in plant cells.
• Glycogen is a polymer of α-glucose molecules. It differs from starch by its pattern of polymer branching. It is
a major energy storage molecule in animal cells.• Cellulose is a polymer of β-glucose molecules. It serves as a structural molecule in the walls of plant cells and is the major component of wood.
• Chitin is a polymer similar to cellulose, but each β-glucose molecule has a nitrogen-containing group attached
to the ring. Chitin serves as a structural molecule in the walls of fungus cells and in the exoskeletons of insects,
other arthropods, and mollusks.
Lipids are a class of substances that are insoluble in water (and other polar solvents) but are soluble in nonpolar substances
(like ether or chloroform). There are three major groups of lipids:
1. Triglycerides (triacylglycerols) include fats and oils. They consist of three fatty acids attached to a glycerol
molecule. Fatty acids are hydrocarbons (chains of covalently bonded carbons and hydrogens) with a
carboxyl group (–COOH) at one end of the chain. Fatty acids vary in structure by the number of carbons and by
the placement of single and double covalent bonds between the carbons.
• A saturated fatty acid has a single covalent bond between each pair of carbon atoms, and each carbon has two
hydrogens bonded to it (three hydrogens bonded to the last carbon). You can remember this by thinking that
each carbon is “saturated” with hydrogen.
• A monounsaturated fatty acid has one double covalent bond and each of the two carbons in this bond has
only one hydrogen atom bonded to it.
• A polyunsaturated fatty acid is like a monounsaturated fatty acid except that there are two or more double
2. A phospholipid looks just like a lipid except that one of the fatty acid chains is replaced by a phosphate group.
The two fatty acid “tails” of the phospholipid are nonpolar and hydrophobic and the phosphate “head” is polar and hydrophilic.
A phospholipid is termed an amphipathic molecule because it has both polar (hydrophilic) and nonpolar (hydrophobic) regions. Phospholipids are often found oriented in sandwichlike
formations with the hydrophobic tails grouped together on the inside of the sandwich and the hydrophilic
heads oriented toward the outside and facing an aqueous environment. Such formations of phospholipids provide
the structural foundation of cell membranes.