Most elements on group 18 are the Noble Gases. They already have a complete last level with 8 electrones. Actually they can form compounds but only on the lab and they will not even last half a second.
Their weights could be different.
Their volumes could be different.
Their densities could be different.
The volume for an ounce of lead is much different than an ounce of aluminum.
the weight of a cubic meter of balsa wood is much different (and much lighter) than a cubic meter of water. That's why the ancients used balsa for their rafts.
Answer:
the types of chemical reaction are combination, decomposition, single replacement, double replacement, combustion