Definition: what is a group of protons and neutrons in the center of all atoms.

Explanation:

As it is known that nucleus of an atom consists of protons and neutrons. And, these protons and neutrons are responsible for the mass of an atom.

Hence, the sum of total number of both protons and neutrons is known as atomic mass of an atom.

For example, atomic mass of helium is 4 as it contains 2 protons and 2 neutrons.

On the other hand, only the sum of total number of protons is known as the atomic number of an atom.

For example, atomic number of helium is 2.

Thus, we can conclude that a group of protons and neutrons in the center of all atoms is known as atomic mass.

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