Chemical Bonds

When two or more atoms share or exchange valence electrons with each other, the resulting electromagnetic forces cause the atoms to stick together. This is called a chemical bond. There are three types of chemical bonds: ionic bonds, covalent bonds, and metallic bonds.

Metals are a diverse group of elements. Not all of them are solid, tough, and shiny like the ones most familiar in everyday experiences. Many are soft enough to cut with a knife. Some are so highly reactive that they burst into flame when exposed to air or explode when exposed to water. And some are just downright cool. My personal favorite is gallium (atomic number 31), which has a melting point of 29.76 °C (about 86 °F). That’s higher than room temperature, but lower than the temperature of the human body. For this reason, gallium will melt in your hand!

Substances comprised of two or more elements bonded together are called chemical compounds (or simply compounds). If a substance contains different elements that are not bonded together, it is called a mixture. Metal alloys are regarded as mixtures rather than compounds, even though metallic bonds form between the various types of metal atoms.

In all three types of bonds, atoms are held together by electromagnetic forces. In an ionic bond, the ions stick together because they have opposite electromagnetic charges, and hence attract each other. (Remember Coulomb’s law.) The sharing of electrons in metallic and covalent bonds works similarly. Think of it this way: the electrons spend part of the time orbiting one atom, and part of the time orbiting another. So, sometimes one atom will have more than its “fair share” of electrons, and sometimes it will have less. Either way, its charge will be opposite the charge of the other atoms, so it will be attracted to them.

Chemical compounds are represented by chemical formulas. There are different types of chemical formulas:

I won’t be using structural formulas or empirical formulas in this book, so in what follows I’ll simply use the word “formula” to mean molecular formula.

Elements closer to the left side of the periodic table are usually written first in chemical formulas. For example, hydrogen peroxide is written H2O2 rather than O2H2, because hydrogen is closer to the left side of the table than oxygen is. There are exceptions to this rule. The order is reversed for some polyatomic ions—charged clusters of atoms that form parts of larger molecules. For instance, the formula for sodium hydroxide is NaOH. The negatively-charged hydroxide ion is written “OH,” even though H appears to the left of O in the periodic table. Similarly, the formula for sodium chlorite is NaClO2. The chlorite ion is written “ClO2,” even though Cl appears to the right of O in the periodic table.