The idea of the family nucleus responds to a modern conception of the family limited to the closest family relationships (relationships between parents and children). Families may be of one or more nuclei, according to the number of these relationships.People not belonging to the nucleus are classified as isolated individuals. The following categories are considered:
Couples.
Couples, with children who have not formed their own nucleus.
Single father, with children who have not formed their own nucleus.
Single mother, with children who have not formed their own nucleus.
As well as the four types of nuclei described above we have given the term complete nuclei to those in which both members of the marriage or couple remain together, regardless of whether or not they have children.