try to remember that electrons are responsible for reactions and stable elements ALWAYS have the same number of protons as electrons. If you remember this you'll realise that the only thing that can vary in the number of neutrons.
This might not work for you but it really helps me :)
An isotope is another version of the original element but it just has a different number of neutrons. Same number of electrons, same number of protons just a different number of neutrons. This does not effect the charge only the mass.
Hope this helps :)
Answered Wed 9th January, 2013 @ 15:23 by Megan Cox
An element with the same number of protons but different number of neutrons is an isotope of that element