Basics of Biology

Basic Chemistry For Biology

How does the mass number of an element differ from the atomic number?

The mass number is the sum of the number of protons and neutrons in the nucleus of an element. For example, the mass number of helium is 4, because it has two protons and two neutrons in its nucleus. Since it has only two protons, the atomic number of helium is 2. When the atomic number changes (for example, the number of protons change), the result is a different element.


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