Family Definition in Chemistry

In chemistry, a family is a group of elements with similar chemical properties. Chemical families tend to be associated with the vertical columns on the periodic table. The term "family" is synonymous with the term "group". Because the two words have defined different sets of elements over the years, the IUPAC recommends the numerical system numbering elements from group 1 to group 18 be used over the common names of families or groups. In this context, families are distinguished by the orbital location of the outermost electron. This is because the number of valence electrons is the primary factor in predicting the types of reactions an element will participate in, the bonds it will form, its oxidation state, and many of its chemical and physical properties.

Examples: Group 18 on the periodic table is also known as the noble gas family or noble gas group. These elements have 8 electrons in the valence shell (a complete octet). Group 1 is also known as the alkali metals or the lithium group. Elements in this group have one orbital electron in the outer shell. Group 16 is also known as the oxygen group or chalcogen family.

Names of Element Families

Here is a chart that shows the IUPAC number of the element group, its trivial name, and its family name. Note that while families are generally vertical columns on the periodic table, group 1 is called the lithium family rather than the hydrogen family. The f-block elements between groups 2 and 3 (the elements found below the main body of the periodic table) may or may not be numbered. There is controversy over whether group 3 includes lutetium (Lu) and lawrencium (Lw), whether it includes lanthanum (La) and actinium (Ac), and whether it includes all of the lanthanides and actinides.

Other Ways of Identifying Element Families

Probably the best way to identify an element family is to associate it with an IUPAC group, but you'll find references to other element families in the literature. At the most basic level, sometimes the families are simply considered the metals, metalloids or semimetals, and nonmetals. Metals tend to have positive oxidation states, high melting and boiling points, high density, high hardness, high density, and be good electrical and thermal conductors. Nonmetals, on the other hand, tend to be lighter, softer, have lower melting and boiling points, and be poor conductors of heat and electricity. In the modern world, this is problematic because whether an element has metallic character or not depends on its conditions. For example, hydrogen can act as an alkali metal rather than a nonmetal. Carbon can act as a metal rather than a nonmetal.

Common families include the alkali metals, alkaline earths, transition metals (where the lanthanides or rare earts and actinides may be considered a subset or as their own groups), basic metals, metalloids or semimetals, halogens, noble gases, and other nonmetals.

Examples of other families you may encounter might be the post-transition metals (groups 13 to 16 on the periodic table), the platinum group, and the precious metals.

Element Homologs

Element homologs are members of the same element family. Because homologous elements share similar electrochemical properties, they can be used to predict the behavior of new elements. This becomes increasing helpful for the superheavy elements, of which only a few atoms have been prepared. However, predictions are not always accurate. The reason is because valence electron effects aren't quite as significant when an atom has extremely high numbers of both protons and electrons. Lighter homologs more often share common properties.

Element Family Key Takeaways

• An element family is a column of elements on the periodic table.
• Each member of a family has the same number of valence electrons.
• Family members share similar chemical and physical properties.
• An element family is also called an element group. Because of the potential for confusion, the IUPAC prefers element groups be labelled by number rather than name.
• There are 18 element families or groups.

Sources

• Fluck, E. (1988). "New Notations in the Periodic Table" (PDF). Pure Appl. Chem. IUPAC. 60 (3): 431–436. doi:10.1351/pac198860030431
• Leigh, G. J. Nomenclature of Inorganic Chemistry: Recommendations 1990. Blackwell Science, 1990. ISBN 0-632-02494-1.