Chemical reactions take place when elements or compounds link together by exchanging electrons. Only electrons on the outermost shell, or valence shell, may be exchanged, and the electrons on the inner shells stay put at all times.

These links, called bonds, explain why table salt dissolves in water, why most snowflakes have six angles, and why most metals are shiny. The three simplest ways of bonding are:

• ionic bonds between a metal and a nonmetal or a molecule, like in table salt
• covalent bonds between a nonmetal and another nonmetal, like in gas stove methane
• metal bonds between a metal and another metal, like in a nugget of cadmium

The more complex include:

• dipole-dipole bonds that appear between molecules that have accumulation of charge somewhere within themselves
• hydrogen bonds, a special case of dipole-dipole bonds with hydrogen involved