Is only one kind of atom in a element?

Answer 1

Yes and no. All elements are composed of the same basic "stuff"--- protons, neutrons and electrons. But when the number of protons, neutrons and electrons change, a completely different element appears. A given element will always have the same number of protons but can have several numbers of neutrons. Substances with identical numbers of protons but different numbers of neutrons are called isotopes. Take for example hydrogen. One proton bound in an atom with no neutrons is what we know as protium, the most abundant isotope of hydrogen. One proton bound in an atom with one neutron is called deuterium while one proton bound in an atom with two neutrons is called tritium. These are all isotopes of hydrogen because they contain only one proton, but differ in the their number of neutrons.