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You do have a printed periodic table, right? You can get the information at http://www.chemicalelements.com/index.html but you really need your own printed copy to carry with you. It will be a primary reference for everything you do.
Elements are arranged in the order of how many protons in the nucleus. Only certain combinations of protons and neutrons are stable. That is called "islands of stability" and nobody has any idea why it is so or how to predict what combinations are possible. So they just catalog the combinations they find, and those are called isotopes. But it's the number of protons that determine the element. The mass of a neutron is pretty close to the mass of a proton but slightly higher. The atomic mass indicated on the periodic chart is the average of naturally occurring isotopes.