An atom's mass number is determined by the number of (1 Point)nucleons in its nucleusneutrons in its nucleusprotons in its nucleuselectrons in its nucleus

The mass number of an atom is determined by the total number of nucleons in its nucleus. Nucleons are the particles that reside in the nucleus and consist of both protons and neutrons.

Explanation:

1. Nucleons: The term "nucleons" encompasses both protons and neutrons. Protons are positively charged particles, while neutrons have no charge. The sum of these two types of particles gives the mass number.

2. Mass Number Calculation: The mass number (denoted as $A$) is calculated using the formula: $A = Z + N$ where $Z$ is the number of protons and $N$ is the number of neutrons present in the nucleus.

3. Role of Electrons: Electrons, which are negatively charged and orbit the nucleus, do not contribute to the mass number as they have a significantly smaller mass compared to protons and neutrons.

Conclusion:

Therefore, the correct response to the question is that an atom's mass number is determined by the number of nucleons in its nucleus, which includes both protons and neutrons.