Final answer:
Elements in the same group or family on the Periodic Table have similar chemical properties due to having the same number of valence electrons, as seen with the alkali metals and halogens.
Step-by-step explanation:
Elements found in the same group, or family, in the Periodic Table all have similar chemical properties because they all have the same number of valence electrons. The vertical columns on the periodic table are known as groups or families, and they showcase elements with similar chemical behaviors. For instance, all of the group 1A elements, also known as alkali metals, have one valence electron, which contributes to their high reactivity and the ability to form compounds with similar properties. Similarly, the group 17 elements, known as halogens, each have seven valence electrons, hence they exhibit comparable chemical reactivity. This common feature explains why Mendeleev arranged the periodic table to group these elements together based on their chemical properties.