Help me out here I don’t understand please :(, ppl srsly answer my questions a week later :/

2. For any given isotope, the sum of the numbers of protons and neutrons in the nucleus is called the mass number.

3. in fact, electron mass is so small that it is not counted in an atom's mass

Explanation:

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

1 a

2 d

3 a

4 c

explanation: