Families of Element

For those families of elements found among the main-group elements, that is, elements in groups 1, 2, and 13 through 18 of the periodic table, each member of a given family has the same number of valence electrons. A detailed examination of the electron configurations of the elements in these families reveals that each family has its own characteristic arrangement of electrons. For example, each element in group 1, the alkali metals, has its valence electron in an s sublevel. As a result, all the elements in this family have an electron configuration which, when written in linear form, terminates with ns¹, where n is an integer representing the principal quantum number of the valence shell. Thus, the electron configuration of lithium is 1s² 2s²;, that of sodium is 1s² 2s² 2p⁶ 3s¹, potassium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹, and so on. In a similar way, the elements in group 2, the alkaline earth metals, each have two valence electrons and electron configurations that terminate in ns². For example, beryllium is 1s² 2s², magnesium is 1s² 2s² 2p⁶ 3s², calcium is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². Because the s sub-level can only accommodate a maximum of 2 electrons, the members of group 13, which have 3 valence electrons, all have electron configurations terminating in ns² np¹; for example, aluminum is 1s² 2s² 2p⁶ 3s² 3p¹. The remaining main-group families, group 14 (the carbon family), group 15 (the pnicogens), group 16 (the chalcogens), group 17 (the halogens), and group 18 (the rare gases) have 4, 5, 6, 7, and 8 valence electrons, respectively. Of these valence electrons, two occupy an s sublevel and the remainder occupy the p sub-level having the same principal quantum number.

The similarity in electron configurations within a given main-group family results in the members of the family having similar properties. For example, the alkali metals are all soft, highly reactive elements with a silvery appearance. None of these elements is found uncombined in nature, and they are all willing to give up their single valence electron in order to form an ion with a charge of +1. Each alkali metal will react with water to give hydrogen gas and a solution of the metal hydroxide.

Characteristic patterns of behavior can also be identified for other main-group families; for example, the members of the carbon family all form chlorides of the type ECl₄ and hydrides of the type EH₄, and have a tendency towards catenation, that is, for identical atoms to join together to form long chains or rings. Similarly, although little is known about the heaviest, radioactive halogen, astatine, its congeners all normally exist as diatomic molecules, X₂, and show a remarkable similarity and predictability in their properties. All the members of this family are quite reactive-fluorine, the most reactive, combines directly with all the known elements except helium, neon and argon-and they all readily form ions having a charge of -1.

The family of elements at the far right of the periodic table, the rare gases, consists of a group of colorless, odorless gases that are noted for their lack of reactivity. Also known as noble gasses, the first compounds of these elements were not prepared until 1962. Even today there are only a limited number of krypton compounds known and still no known compounds of helium, neon, or argon.