Of the three major subatomic particles, the negatively charged electron was first discovered by the English physicist Joseph Thomson in 1897.

As the structure of atoms was probed, it was realized that these low‐mass particles occurred in a large “cloud” around the tiny nucleus, which contained almost all the mass of the atom. A neutral atom has precisely equal numbers of protons (+) and electrons (–). Atoms with a charge imbalance are called ions. A positive ion has lost one or more electrons, whereas a negative ion has gained one or more electrons. Table 1 shows three different charge states for copper:

The chemical behavior of the various elements is influenced more by the charge of their ion than by any other intrinsic property. In the periodic table, elements in a column form analogous compounds because they have the same charge on their ions.

The halogens all tend to gain one electron, giving their ions a characteristic charge of –1. The alkali metals, on the other side of the periodic table, all readily lose one electron, so their ions possess a charge of +1. Charges are balanced in a compound with the number of alkali metal ions equal to the number of halogen ions; NaCl is such a compound. By contrast, consider the alkaline earths in the second column on the left side of the table. These elements are metals that form oxides that have an earthy texture and yield alkaline solutions. The alkaline earths have ions with a charge of +2. For one of them to form a compound with a halogen, twice as many halogen ions are needed to balance the +2 charge on the metal ions. Consequently, the correct chemical formula for strontium fluoride is SrF 2.

Figure 1. The alkali metals form ions with a +1 charge; the alkaline earths, +2; and the halogens, –1.