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The definition of an atomic number is the number of protons in the nucleus of an atom of an element. Three for lithium and 20 for calcium are each an example of an atomic number.
The atomic number (number at the top) is the amount of protons and the amount of electrons. So if an element has an atomic number of 5, you know that it has 5 protons and 5 electrons. The atomic mass (number at the bottom) is the amount of protons and neutrons added together.