The mass number of an atom is determined by adding the number of neutrons and number of protons.
Option 1: This option suggests adding the number of neutrons and number of electrons to determine the mass number. However, electrons do not contribute significantly to the mass of an atom because they have negligible mass compared to protons and neutrons. Therefore, this option is not correct.
Option 2: This option suggests adding the number of protons and number of electrons to determine the mass number. As explained before, electrons have negligible mass, so adding them to the number of protons would not accurately determine the mass number. Hence, this option is also incorrect.
Option 3: This option states that the mass number is determined by the number of protons only. However, the mass number includes the sum of protons and neutrons and not just the protons. Therefore, this option is not correct.
Option 4: This is the correct answer. It correctly states that the mass number of an atom is determined by adding the number of neutrons and number of protons. Protons contribute positively to the mass number, while neutrons also have a significant mass and contribute to the overall mass of an atom.
In summary, the mass number of an