Sometimes, when the elements form more complex substances, their atoms lose or gain electrons. Before this change, the atoms have an equal number of protons and electrons, and because protons and electrons have an equal but opposite charge, these atoms are initially uncharged overall. When an uncharged atom gains or loses one or more electrons, it forms a charged particle, called an ion. For example, when an atom loses one or more electrons, it will have more protons than electrons and more plus charge than minus charge. Thus, it will have an overall positive charge. An atom that becomes a positively charged ion is called a cation. For example, uncharged sodium atoms have 11 protons and 11 electrons. They commonly lose one of these electrons to form +1 cations. A sodium cation’s overall charge is +1 because its 11 protons have a charge of +11, and its remaining 10 electrons have a charge of -10. The sum of +11 and -10 is +1 (Figure 1). The symbol for a specific cation is written with the charge as a superscript on the right side of the element symbol. If the charge is +1, the convention is to write + (without the 1), so the symbol for the +1 sodium cation is Na+. Aluminum atoms commonly lose 3 of their electrons to form +3 cations. The cations are +3 because each aluminum cation has a charge of +13 from its 13 protons and a charge of -10 from its 10 remaining electrons. The sum is +3. The symbol for this cation is Al3+. (Notice that the 3 comes before the +.)
Some atoms can gain electrons. When an atom gains one or more electrons, it will have more electrons than protons and more minus charge than plus charge. An atom that becomes negatively charged due to an excess of electrons is called an anion, a negatively charged ion. For example, uncharged chlorine atoms have 17 protons and 17 electrons. They commonly gain 1 electron to form -1 anions. The anions are -1 because their 17 protons have a charge of +17, and their 18 electrons have a charge of -18, giving a sum of -1. The anion’s symbol is Cl-, again without the 1. As illustrated in Figure 1, oxygen atoms commonly form anions with a -2 charge, O2-, by gaining 2 electrons and therefore changing from eight protons and 8 electrons to 8 protons (+8) and ten electrons (-10).
- This article is an excerpt from the preparatory chemistry text An Introduction to Chemistry by Mark Bishop.