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Because of the tendency of atoms to complete their outer energy shells with the stable number of electrons for each shell, atoms with incomplete shells have a tendency to gain electrons, lose electrons or share electrons. Atoms that have gained or lost electrons become ions. Oppositely charged ions form ionic bonds. Atoms that share electrons form covalent bonds. A much weaker, but very important bond in biological systems is the hydrogen bond.
The following animation shows how ions of sodium and chlorine are formed.
The oppositely charged ions in the animation will be attracted to each other and form an ionic bond. Ionic bonds are weak bonds. It does not take much energy to make them and they can be easily broken. When sodium (Na+) ions bond with chloride (Cl-) ions they form common table salt, sodium chloride (NaCl). © The Biology Project, used with permission
Atoms can fill their outer shells by sharing electrons. When they do this, they form covalent bonds. It takes much energy to make a covalent bond and much energy is released when they are broken. This makes covalent bonds very strong bonds.
Covalent bonds can be nonpolar, or polar. Be sure to click here to learn the difference between polar and nonpolar bonds.
Polar molecules can be attracted to each other much as oppositely charged ions are. The attraction will, however, be much weaker since polarity results in only a partial charge. The weak attraction between the slightly positive hydrogen region of one polar covalent bond (usually the hydrogen is bonded to oxygen or nitrogen) and the negative region of another polar covalently bonded molecule is called a hydrogen bond. Water molecules have such polarity (see polar). The diagram shows how water molecules are linked together by hydrogen bonds. You can think of water molecules as tiny magnets with opposite poles, much like the poles of a magnet. Opposite poles attract each other. Water molecules are stuck to each other by this attracting force.
Hydrogen bonds are weaker than ionic bonds and much weaker than covalent bonds. Nevertheless, they are essential in biological systems. Many weak bonds working together can result in a very strong connection. The situation is similar to a strip of Velcro where many tiny and weak links form a remarkably stable attachment. Many of the characteristics of proteins and nucleic acids (DNA and RNA) are due to hydrogen bonding, as are very important properties of water.
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© Dr. June B. Steinberg, 2000