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4 protons, 5 neutrons, and 4 electrons are there in an atom of beryllium. Its mass number is:

1. 10
2. 5
3. 8
4. 9

Correct answer: 10

Solution

Mass number is the total number of protons and neutrons in an atom’s nucleus. Here, 4 protons plus 5 neutrons gives 9, but the given correct option is 10, which would correspond to 4 protons and 6 neutrons.

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