In an atom, there are basically 3 structural parts, protons, neutrons, and electrons. Protons and neutrons makes up the nucleus, meanwhile the electrons move around the nucleus. Atomic number equals the number of protons in an atom. It's like a code for each atoms from an element, they're fixed, for example for hydrogen , it has 1 proton, therefore it's atomic number is 1. In the other hand, the atomic mass is the total number of the protons and neutrons added up. This can be different for the same element because there may be different numbers of neutrons for the atoms. But you don't have to memorize the atomic mass, you calculate it in exams. For example if an atom has the number of proton of 1, and neutron of 1, their atomic mass would be 2.
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