What is mass number of an atom?

number of protons + number of neutrons 

The correct answer is option 2. number of protons + number of neutrons.

The mass number of an atom, denoted by the symbol \(A\), represents the total number of nucleons (protons and neutrons) present in the nucleus of the atom. It is a fundamental property that defines the mass of the atom.

The nucleus of an atom contains positively charged protons and neutral neutrons. The number of protons in the nucleus is equal to the atomic number (\(Z\)) of the atom, which determines its chemical identity. The number of neutrons, on the other hand, can vary among atoms of the same element, resulting in different isotopes.

The mass number is determined by adding together the number of protons and neutrons in the nucleus. Mathematically, it is represented as:

\[ A = \text{Number of protons} + \text{Number of neutrons} \]

For example, consider the most common isotope of carbon, carbon-12 (\(^{12}C\)). It has 6 protons and 6 neutrons in its nucleus, so its mass number is:

\[ A = 6 \text{ protons} + 6 \text{ neutrons} = 12 \]

Similarly, carbon-14 (\(^{14}C\)) has 6 protons and 8 neutrons, so its mass number is 14.

In summary, the mass number of an atom is the sum of the number of protons and neutrons in its nucleus, which determines the overall mass of the atom.