Atomic Number and Atomic Mass

The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1. All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8. The atomic number of an element never changes.

Atomic masses of elements, however, depend on the number of neutrons as well as protons in an atom and differ from their atomic numbers. Atomic mass is indicated in the periodic table of elements as Atomic Mass Units (AMU) and is a larger value than the atomic number.

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