What does the group number in the periodic table indicate?

The correct answer highlights an important aspect of the periodic table. The group number indicates the number of valence electrons in the atoms of that column. Elements found in the same vertical column, or group, typically exhibit similar chemical properties because they have the same number of electrons in their outermost shell.

For example, Group 1 elements, known as the alkali metals, all have one valence electron, which contributes to their high reactivity and similar behavior in chemical reactions. On the other hand, Group 18 elements, the noble gases, have a full complement of valence electrons, making them generally unreactive. This concept of valence electrons is crucial for understanding an element's reactivity, bonding behavior, and the formation of compounds.

Other options do not accurately describe the significance of the group number. The atomic mass varies among elements in the same group and does not reflect their placement in the periodic table. Neutrons are not directly indicated by the group number either, as atomic structure varies independently of group membership. While the reactivity of elements may correlate with their placement in a group, the group number itself specifically points to the number of valence electrons, which is a more direct and fundamental characteristic in determining chemical behavior.