A chemical bond is any force of attraction that holds two atoms or ions together. In most cases, that force of attraction is between one or more negatively charged electrons held by one of the atoms and the positively charged nucleus of the second atom. Chemical bonds vary widely in their strength, ranging from relatively strong covalent bonds (in which electrons are shared between atoms) to very weak hydrogen bonds. The term chemical bond also refers to the symbolism used to represent the force of attraction between two atoms or ions. For example, in the chemical formula H—O—H, the short dashed lines are known as chemical bonds.
Theories of chemical bonds go back a long time. One of the first was developed by Roman poet Lucretius (c. 95–c. 55 B.C. ), author of De Rerum Natura (title means "on the nature of things"). In this poem, Lucretius described atoms as tiny spheres with fishhook-like arms. Atoms combined with each other, according to Lucretius, when the hooked arms of two atoms became entangled with each other.
Covalent bond: A chemical bond formed when two atoms share one or more pairs of electrons with each other.
Double bond: A covalent bond consisting of two pairs of electrons.
Electronegativity: A numerical method for indicating the relative tendency of an atom to attract the electrons that make up a covalent bond.
Hydrogen bond: A chemical bond formed between two atoms or ions with opposite charges.
Ionic bond: A chemical bond formed when one atom gains and a second atom loses electrons. An ion is a molecule or atom that has lost one or more electrons and is, therefore, electrically charged.
Multiple bond: A double or triple bond.
Polar bond: A covalent bond in which one end of the bond is more positive than the other end.
Triple bond: A covalent bond consisting of three pairs of electrons.
Such theories were pure imagination, however, for many centuries, since scientists had no true understanding of an atom's structure until the beginning of the twentieth century. It was not until then that anything approaching a modern theory of chemical bonding developed.
Today, it is widely accepted that most examples of chemical bonding represent a kind of battle between two atoms for one or more electrons. Imagine an instance, for example, in which two hydrogen atoms are placed next to each other. Each atom has a positively charged nucleus and one electron spinning around its nucleus. If the atoms are close enough to each other, then the electrons of both atoms will be attracted by both nuclei. Which one wins this battle?
The answer may be obvious. Both atoms are exactly identical. Their nuclei will pull with equal strength on both electrons. The only possible result, overall, is that the two atoms will share the two electrons with each other equally. A chemical bond in which two electrons are shared between two atoms is known as a covalent bond.
Consider now a more difficult situation, one in which two different atoms compete for electrons. One example would be the case involving a sodium atom and a chlorine atom. If these two atoms come close enough to each other, both nuclei pull on all electrons of both atoms. In this case, however, a very different result occurs. The chlorine nucleus has a much larger charge than does the sodium nucleus. It can pull on sodium's electrons much more efficiently than the sodium nucleus can pull on the chlorine electrons. In this case, there is a winner in the battle: chlorine is able to pull one of sodium's electrons away. It adds that electron to its own collection of electrons. In a situation in which one atom is able to completely remove an electron from a second atom, the force of attraction between the two particles is known as an ionic bond.
Most cases of chemical bonding are not nearly as clear-cut as the hydrogen and the sodium/chlorine examples given above. The reason for this is that most atoms are more nearly matched in their ability to pull electrons than are sodium and chlorine, although not as nearly matched as two identical atoms (such as two hydrogen atoms).
A method for expressing the pulling ability of two atoms was first suggested by American chemist Linus Pauling (1901–1994). Pauling proposed the name "electronegativity" for this property of atoms. Two atoms with the same or similar electronegativities will end up sharing electrons between them in a covalent bond. Two atoms with very different electronegativities will form ionic bonds.
In fact, most chemical bonds do not fall into the pure covalent or pure ionic bond category. The major exception occurs when two atoms of the same kind—such as two hydrogen atoms—combine with each other. Since the two atoms have the same electronegativities, they must share electrons equally between them.
Consider the situation in which aluminum and nitrogen form a chemical bond. The electronegativity difference between these two atoms is about 1.5. (For comparison's sake, the electronegativity difference between sodium and chlorine is 2.1 and between hydrogen and hydrogen is 0.0.) A chemical bond formed between aluminum and nitrogen, then, is a covalent bond, but electrons are not shared equally between them. Instead, electrons that make up the bond spend more of their time with nitrogen (which pulls more strongly on electrons) than with aluminum (which pulls less strongly). A covalent bond in which electrons spend more time with one atom than with the other is called a polar covalent bond. In contrast, a bond in which electrons are shared equally (as in the case of hydrogen) is called a nonpolar covalent bond.
All covalent bonds, polar and nonpolar, always consist of two electrons. In some cases, both electrons come from one of the two atoms. In most cases, however, one electron comes from each of the two atoms joined by the bond.
In some cases, atoms may share more than two electrons. If so, however, they still share pairs only: two pairs or three pairs, for example. A bond consisting of two pairs of (that is, four) electrons is called a double bond. One containing three pairs of electrons is called a triple bond.
Other types of chemical bonds also exist. The atoms that make up a metal, for example, are held together by a metallic bond. A metallic bond is one in which all of the metal atoms share with each other a cloud of electrons. The electrons that make up that cloud originate from the outermost energy levels of the atoms.
A hydrogen bond is a weak force of attraction that exists between two atoms or ions with opposite charges. For example, the hydrogen-oxygen bonds in water are polar bonds. The hydrogen end of these bonds are slightly positive, and the oxygen ends are slightly negative. Two molecules of water placed next to each other will feel a force of attraction because the oxygen end of one molecule feels an electrical force of attraction to the hydrogen end of the other molecule. Hydrogen bonds are very common and extremely important in biological systems. They are strong enough to hold substances together but weak enough to break apart and allow chemical changes to take place within the system.
Van der Waals forces are yet another type of chemical bond. They are named in honor of the Dutch physicist Johannes Diderik van der Waals (1837–1923), who investigated the weak nonchemical bond forces between molecules. Such forces exist between particles that appear to be electrically neutral. The electrons in such particles shift back and forth very rapidly. That shifting of electrons means that some parts of the particle are momentarily charged, either positively or negatively. For this reason, very weak, short-term forces of attraction can develop between particles that are actually neutral.