2.3: Atoms can bond together to form molecules or compounds.

When you eat a meal, it is like filling your car’s tank with gasoline; you have a source of energy that can be used to fuel activities like running, thinking, building muscle, and maintaining the machinery of life. That energy initially comes from the sun and is captured and stored by plants. When we eat plants or eat other animals that eat plants, we ingest the energy stored in the plant material. But how exactly is energy stored in plants?

Groups of atoms held together by bonds are called molecules. It takes a certain amount of energy to break a bond between two atoms. The amount of this energy, called the bond energy, depends on the atoms involved. When chemical reactions occur—such as when animals (including humans) consume and digest molecules in their diet—some existing bonds are usually broken and some new bonds are usually formed. If the bond energy of the new bonds formed is less than the bond energy of the starting materials, the excess energy is released—and can be used to fuel the body’s activities. In a sense, molecules are created as a short-term storage of energy that can be harnessed later.

Before looking at specific types of bonds that hold molecules together, let’s look at how molecules are illustrated. In this book you will most often see molecular structures represented by the ball-and-stick and space-filling models.

There are three principal types of bonds that hold atoms together. The type of bonding that any atom is likely to take part in depends almost entirely on the number of electrons in its outermost shell.

Covalent bonds When two atoms share electrons, a strong covalent bond is formed. The simplest example of a covalent bond is the bonding of two hydrogen atoms to form a hydrogen molecule, H₂ (FIGURE 2-9), the simplest of all molecules. A hydrogen atom has an atomic number of 1: it has a single proton in its nucleus and a single electron circling around the nucleus in the first shell. Because the atom is most stable when the first shell has two electrons, two hydrogen atoms can each achieve a complete outermost shell by sharing electrons. The nuclei come close together (but not too close, because they are positively charged and their mutual repulsion would destabilize the molecule), and the two electrons circle around both of the nuclei, almost in a figure 8. The new H₂ molecule is very stable because, now that both atoms have two electrons in their outermost shell, they are no longer likely to bond with other atoms. The sharing of two electrons between two atoms forms a single covalent bond.

Oxygen, with an atomic number of 8, has two electrons in its innermost shell and six in its outermost shell. Consequently it needs to gain two electrons to fill its outermost shell. Sometimes two oxygen atoms join together to form O₂. Each oxygen atom shares a pair of electrons with the other, filling the outermost shell of both. The sharing of two pairs of electrons between two atoms is called a double bond (see Figure 2-9). O₂ is the most common form in which we find oxygen in the world.

Carbon is a particularly “extroverted” atom. It has an atomic number of 6, meaning that two electrons fill its first shell and four remain to occupy the second shell. Because four electron vacancies are left, carbon can, and frequently does, form four covalent bonds, joining up with other atoms in a wide variety of molecules. Methane, the chief component of natural gas, is formed when one atom of carbon covalently bonds with four atoms of hydrogen (see Figure 2-9). It has the chemical formula CH₄.

Ionic bonds Atoms can also bond together without sharing electrons. When one atom transfers one or more of its electrons completely to another, each atom becomes an ion, since each has an unequal number of protons and electrons. The atom gaining electrons becomes negatively charged, while the atom losing electrons becomes positively charged. An ionic bond occurs when the two oppositely charged ions attract each other. Unlike covalent bonds, in ionic bonds each electron circles around a single nucleus. Ions of two or more elements linked by ionic bonds form an ionic compound. Because the ions attracted to each other are of equal and opposite charge, the compound is neutral—that is, it has no charge (FIGURE 2-10). A common ionic compound you are familiar with is table salt (NaCl), composed of sodium and chloride ions.

Hydrogen bonds Ionic and covalent bonds link two or more atoms together within an ionic compound or a molecule. Hydrogen bonds, on the other hand, are important in bonding molecules together. A hydrogen bond is formed between a hydrogen atom in one molecule and another atom, often an oxygen or nitrogen atom, in another molecule (or even in another part of the same molecule). This bond is based on the attraction between positive and negative charges.

The atoms taking part in hydrogen bonds are not necessarily ions, so where do the electrical charges come from? The hydrogen atom is already covalently bonded to another atom in the same molecule and shares its electron. That electron circles both the hydrogen nucleus and the nucleus of the other atom, but the electron is not shared equally. It’s as if the nuclei of the atoms are playing tug-of-war with the electrons. Because the other atom always has more than the one proton found in the hydrogen nucleus, it is more positively charged. As a result, the electron contributed by hydrogen spends more of its time near the other, more positively charged nucleus than near the hydrogen nucleus. Having an extra electron nearby causes the larger atom to be slightly negatively charged, while the hydrogen atom becomes slightly positively charged (FIGURE 2-11).

In a sense, molecules with slightly charged atoms become like a magnet, with distinct positive and negative sides (see Figure 2-11), and are said to be “polar.” Polar molecules are attracted to other polar molecules, lining up in particular orientations such that the positive regions of one molecule are near the negative regions of another. A hydrogen atom with a slight positive charge can become attracted to an atom in the neighboring molecule with a slight negative charge; these attractions are called hydrogen bonds. Although hydrogen bonds are only about one-thirtieth as strong as covalent or ionic bonds, a multitude of hydrogen bonds form in water and are responsible for many of the unique characteristics that make water one of the most important molecules for life on earth. See FIGURE 2-12 for a review of the three types of bonds discussed in this section.