If we take a purely mechanistic view of life, we can consider each living cell as an intricate web of chemical reactions. Atoms combine with other atoms to form molecules and then recombine again during other chemical reactions to form different molecules. These chemical reactions are the basis of a cell's structure and metabolism.
In the accompanying animation, we examine chemical bonds, which are the attractive forces between atoms. We focus, in particular, on the role of electrons in chemical bonds. The behavior of electrons explains most aspects of how atoms interact with each other in the living and nonliving world.
The chemical reactivity of an atom is based on the number of electrons the atom contains in its outermost electron shell. Electron shells are depicted here as circles around the nuclei of the atoms. Each shell contains electrons with particular energy levels.
Let's examine the electron shells of the element argon. The first electron shell holds just two electrons. These are electrons at the lowest energy level, the shell that electrons of an atom fill first. The next two shells can each hold eight electrons. Argon's third and outermost shell is already filled to capacity, making this element extremely unreactive, or inert.
Chlorine also has electrons in three electron shells, but its third shell contains only seven electrons. In chemical reactions, chlorine has a tendency to gain an electron, filling this shell with eight electrons. This tendency to fill the outermost electron shell with eight electrons is known as the octet rule.
Sodium has electrons in three electron shells, but its outermost shell contains just one electron. Sodium cannot receive seven additional electrons to complete this shell—this would require a huge amount of energy. Instead, sodium has a tendency to lose its outermost electron, leaving this atom with two filled inner shells.
In general, elements with filled outer shells are inert, but atoms of all the other elements are chemically reactive and can form chemical bonds with other atoms. A chemical bond is an attractive force between atoms. Bond formation relies on the ability of atoms to share, donate, or receive electrons from other atoms.
Let's examine the nature of a chemical bond, using hydrogen atoms as examples. Two atoms are depicted with their electron shells. Two others are depicted more realistically, showing a region of space around the nucleus where the electron has a high probability of being found. This region is an electron's orbital.
If two hydrogen atoms approach each other, the positively charged nucleus of one is attracted to the negatively charged electron of the other. The orbitals merge, and the electrons are then equally attracted to and shared by the two nuclei. The electron-rich region between the nuclei binds the atoms together.
A covalent bond results when two atoms share electrons. In the case of two hydrogen atoms, each shares its single electron with the other. This sharing allows each to fill its electron shell with two electrons. The pair of shared electrons constitutes a covalent single bond.
Let's now consider oxygen, an atom with eight electrons. Two electrons fill the innermost shell, and the other six electrons reside in the next shell. This outer shell needs two more electrons to complete it (the octet rule). Two oxygen atoms form a covalent double bond by sharing two electron pairs from their outer shells.
Carbon is perhaps the most versatile element on Earth, in large part because it contains only four electrons in a shell that can hold eight. To fill its outer shell, carbon forms four covalent bonds with up to four other atoms.
In a molecule of methane, carbon shares electrons with hydrogen atoms, forming four covalent single bonds. Although this molecule is relatively simple, carbon often forms the backbone of large, complex molecules. With each carbon atom able to bond to four other atoms, carbon-based molecules are incredibly diverse.
Triple bonds are rare, but nitrogen gas molecules (the most abundant molecule in the air we breathe) form triple bonds. The two nitrogen atoms share three pairs of electrons, allowing each to have eight electrons in its outermost electron shell.
In a covalent bond, two atoms may share electrons equally or unequally. In cases in which both atoms are of the same element, such as in molecular hydrogen or molecular oxygen, the electrons are shared equally by each atom. Sharing may be unequal between atoms of different elements, such as in the carbon and hydrogen atoms of ethane or the hydrogen and oxygen atoms of ethanol.
For these molecules, we need to consider a concept called electronegativity, the ability of an atom to attract electrons. Oxygen has a high electronegativity, meaning that it strongly attracts electrons. Hydrogen has a much lower electronegativity. The shared electrons are drawn closer to the oxygen atom of ethanol.
Because oxygen draws the negatively charged electrons closer to itself, it becomes slightly negative in charge, indicated by δ-. Hydrogen, which doesn't hold the electrons as closely, becomes slightly positive in charge, indicated by δ+. A covalent bond in which electrons are not equally shared is a polar covalent bond.
Now let's examine the covalent bonds in ethane. All of the bonds are formed between carbon and hydrogen, which have similar, low electronegativities and attract shared electrons nearly equally. No atom in ethane takes on a significant partial charge. The bonds between the atoms are nonpolar covalent bonds.
The polar nature of the bond between oxygen and hydrogen in ethanol results in a polar molecule that can interact with other polar molecules. The hydrogen with the partial positive charge is attracted to an atom with a partial negative charge on another polar molecule, such as water. This kind of attraction is a hydrogen bond.
The structures of many molecules, as well as their ability to dissolve in water, are based on the ability to form hydrogen bonds. A DNA molecule, for example, contains two strands of atoms. The bonds within each strand are covalent, but the two strands associate with each other through hydrogen bonds.
The ability of an atom to attract electrons—its electronegativity—might be so strong that the atom does not share electrons with other atoms, but instead strips electrons away from other atoms. Chlorine has a high electronegativity, whereas sodium has a very low electronegativity.
Because sodium has a low electronegativity, it holds its outer electron very loosely and easily loses it to chlorine. When this happens, chlorine fills its outer shell with eight electrons, becoming negatively charged. Sodium loses its electron and becomes positively charged.
Atoms that have lost or acquired electrons are called ions. Positive ions (cations) and negative ions (anions) form ionic bonds by an electrical attraction. In solid form, the ionic bonds between sodium and chloride ions are very strong, because the ions are so close together. These ions make up table salt.
Ionic bonds can be broken down in water. The partial charges on water molecules are attracted to the full ionic charges on the ions. When the solid dissolves, it breaks down into separate ions, each surrounded by a sphere of water molecules.
Two main properties influence how or whether bonds form between atoms. One, the state of an atom's outermost electron shell, determines whether an atom is reactive. Helium and neon have outer shells that are completely filled with electrons, and, for this reason, these atoms are inert. If an atom has a partially filled outer shell, however, it has a tendency to form bonds with other atoms. The second main property is an atom's electronegativity, which influences the kinds of bonds an atom forms. If two atoms have similar electronegativities, they tend to share electrons equally in nonpolar covalent bonds. However, if electronegativities differ, then the atoms tend to form either polar covalent bonds or ionic bonds.
Bonds between atoms in molecules also influence how these molecules interact with other molecules. For example, if a molecule contains polar covalent bonds, then the molecule itself is likely polar. A polar molecule can form hydrogen bonds with water molecules and therefore dissolve in the watery milieu of a cell. Once dissolved, it can more easily find and interact with other molecules and thereby participate in the cell's metabolism. Hydrogen bonds are also important because they hold together regions of large molecules, such as DNA or proteins, maintaining the shapes and functions of these important biomolecules.