What term describes vertical rows in the periodic table?

The term that describes the vertical rows in the periodic table is "groups." Groups are the columns of the periodic table, and elements within the same group typically share similar chemical properties and exhibit similar behavior in chemical reactions due to having the same number of valence electrons.

For example, the elements in Group 1, known as the alkali metals, all have one electron in their outermost shell, which makes them highly reactive and gives them similar properties. Understanding the grouping of elements helps predict how they will interact with one another.

In contrast, the term "periods" refers to the horizontal rows in the periodic table, which indicate the number of electron shells an atom possesses. The term "categories" is not a standard term used to describe any part of the periodic table. Lastly, "series" is a term that can refer to groups of elements but is more commonly used in other contexts, such as transition metals in a particular series rather than denoting their arrangement in the periodic table. Thus, "groups" is the most accurate term for the vertical columns.