ANS:D - both(a)&(b)

The mass number of an atom is a fundamental concept in atomic physics and chemistry. It represents the total number of nucleons (particles found in the nucleus) in an atom. Nucleons include both protons and neutrons. Here's why the mass number is the sum of protons and neutrons:

1. Protons: Protons are positively charged subatomic particles found in the nucleus of an atom. Each proton carries a single positive unit of electric charge. The number of protons in an atom determines its atomic number, which in turn identifies the element. For example, if an atom has six protons, it is carbon; if it has eight protons, it is oxygen.
2. Neutrons: Neutrons are neutral subatomic particles found in the nucleus alongside protons. Unlike protons, they have no electrical charge. The presence of neutrons in the nucleus contributes to the stability of the nucleus by balancing the repulsive forces between positively charged protons. Neutrons also contribute to the mass of the nucleus.

When you sum the number of protons and neutrons in an atom, you obtain the mass number. Mathematically, it can be represented as: Mass Number=Number of Protons+Number of NeutronsMass Number=Number of Protons+Number of Neutrons For example, consider carbon, which has an atomic number of 6. This means it has 6 protons in its nucleus. Most carbon atoms also contain 6 neutrons, making their mass number 12 (6 protons + 6 neutrons). However, some carbon atoms may have a different number of neutrons, leading to different isotopes of carbon, such as carbon-13 (6 protons + 7 neutrons) and carbon-14 (6 protons + 8 neutrons). So, the mass number of an atom indeed corresponds to the sum of the numbers of neutrons and protons, as stated in option (a).