Final answer:
An element's family (group) on the periodic table is determined based on chemical properties and characteristics such as valence electrons and reactivity. Elements in the same group, like the alkali metals, share similar properties and react in analogous ways.
Step-by-step explanation:
To determine what family (group) an element belongs to, we look at its characteristics and properties as mentioned in the provided texts. According to the information, the elements in group 1A have one valence electron, are very reactive, and share similar chemical properties, which include forming compounds in the same ratios and with similar properties with other elements. These elements are known as alkali metals and are characterized by being soft and easily cut, which differentiates them from other metals.
For example, lithium (Li), as mentioned in Example 2.5.1, is an alkali metal located in period two of the periodic table. Similarly, barium (Ba) is an alkaline earth metal in group 2 and krypton (Kr) would be a noble gas in group 18, based on their respective properties and their place within the periodic table.