Question

mass number of atom represents the number of
1 protons only 2 protons and neutrons
3 protons and electrons 4 neutrons and electrons​

Answer 1

Answer:

Experimental data showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number (represented by the letter A) is defined as the total number of protons and neutrons in an atom.

Explanation:

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Answer 2

Answer:

2. protons and neutrons