An atom's mass is the total mass of all of the little parts inside the atom. You should know that one neutron has a mass about as much as one proton. Electrons have a very small mass compared to all of the other parts in an atom. If you want to figure out the number of neutrons in an atom, you have to do a little subtraction. In Biology atoms are used in every reaction and process, but they don't usually act alone.
Atoms connect to each other to form compounds.
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