There are several ways of classifying the elements on the periodic table. One is by the periods, or the horizontal rows, in each of which the properties of the elements change going from left to right. Another common classification is by groups, or vertical columns on the table. All elements in the same group are expected to have similar properties, which is the periodic property, originally noted in the law of the octaves, after which the table is named.

Yet another classification is by blocks, meaning the elements are classified by the type of orbital in which the highest energy electrons reside (see below). The logic behind this type of classification is that the type of orbital in which the highest energy electron resides strongly influences the reactivity of the element, thus elements in the same block usually have similar properties. There are even more ways still of clustering the elements on the table, but these three are the most commonly used.