Final answer:
Elements within the same group on the periodic table have the same number of valence electrons in their outermost shell, which determines their chemical properties and reactivity.
Step-by-step explanation:
The elements of a group have the same number of valence electrons in their outermost shell. This is because the periodic table is arranged in such a way that elements are organized by their atomic number, which dictates the number of protons and electrons they have.
As part of the periodic trend, all elements in a group have the same number of valence electrons equal to the first digit of their group number.
For example, group 1 elements, which include hydrogen and the alkali metals, have one valence electron; group 2, the alkaline earth metals have two; and group 17, the halogens, have seven. These valence electrons determine the chemical properties of the elements and how they react with other elements.