Re: Does an element have only one kind of atom?

The answer depends on what you mean by "only one kind of atom."

If you mean, are all atoms of an element chemically identical, the answer is 
yes. Elements are CHEMICAL elements, and if an atom had different chemistry, it 
would be a different element. What this means in practice is that all atoms of a 
given element have the same number of protons in their nuclei (hence the same 
number of electrons, which determine the chemistry).

On the other hand, different atoms of the same element can have different masses 
because they can have different numbers of neutrons. Such atoms are called 
"isotopes" and are different from other isotopes of the same element. Usually, 
though, the difference is not detectable through the atoms' chemistry. However, 
for a physicist they are certainly "different kinds of atoms," and some isotopes 
are radioactive (NOT a chemical reaction as we normally think of them!)

(Hydrogen is a special case. There are detectable chemical differences between 
the major isotope of hydrogen, hydrogen-1 with one proton and no neutrons, and 
the other, hydrogen-2 with one proton and one neutron. This is because 
hydrogen-1 weighs only half as much as hydrogen-2.)

                                                     Dan Berger