What does a group in the periodic table represent?

A group in the periodic table represents a vertical column of elements that share similar chemical properties. This similarity arises from the elements having the same number of valence electrons, which determines their reactivity and bonding characteristics. For example, the elements in Group 1, known as alkali metals, all have one valence electron and exhibit similar chemical behaviors, such as reacting vigorously with water.

The reason the other options do not apply is that atomic mass does not define groups; elements may have varying atomic masses within the same group. There is also a mix of isotopes within any group, which means the number of neutrons can vary among elements in a group. Lastly, while some groups consist of gases (like the noble gases in Group 18), not all elements in a group are gases. Thus, the defining feature of a group is the similar chemical properties shared by its elements.