Mendeleev noticed that if he organized the elements by atomic weight certain patterns emerged. for instance, elements in the same group (family) appeared to have the same a) valence. b) melting point. c) isotopic ratio. d) number of neutrons.

I've had a similar question on an assignment myself, so I'm going to assume it had the same idea.

When you organize them in such a way, the pattern that emerges tells you elements organized by atomic weight have the same number of valence electrons.

Hope this helps!