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 outer shell of sodium contains one electron. If it can lose that electron it will have a stable outer shell with 8 electrons (2-8) and become an ion with a positive charge. The positive charge (+1) is due to the presence of 10 electrons and 11 protons in the sodium ion. Chlorine has 17 electrons in its outer shell and will be stable if it can gain an electron. The result will give chlorine 18 electrons (2-8-8) and convert the chlorine atom into an ion with a negative charge (18 electrons and 17 protons) called chloride. Play the animation to see how these two ions form. The moving red dot is an electron which moves from the sodium atom to the chlorine atom. Notice that at the end of the animation, the outer shell of each ion is now filled. Sodium now has 2 shells and chlorine has three.

In the animation, two hydrogen atoms share each other's electrons and form a molecule of hydrogen. The animation fails to show this, but the two electrons are actually zooming around both nuclei so fast that it is as if each hydrogen atom has 2 electrons in its outer shell. This sharing provides each atom with the required 2 electrons to fill its first shell. We can indicate this covalent bonding by writing H-H. The line connecting the two letters represents the bond. Sometimes each atom shares two electrons with its partner (a total of 4 electrons) and a double bond is formed. This is written C=C or O=O. Similarly, when each partner shares 3 of its electrons with the other, a triple bond forms.

Polar molecules can be attracted to each other much as oppositely charged ions are.http://www2.nl.edu/jste/water.gif 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.