Answer by
fellwanderer (146)
Different atoms have different numbers of protons and neutrons. If they have the same number of protons then they are different isotopes of an element. There are different groups of elements, determined by their chemical properties: I alkali metals, II alkali-earth metals, VII halogens, VII noble-gas atoms. There are non-metals, and other metals/metalloids. There are also transition metals, lanthanides, actinides.