What determines the number of electrons in an atom?

The number of electrons in an atom is determined by the atomic number. The atomic number is defined as the number of protons found in the nucleus of an atom, and in a neutral atom, the number of electrons equals the number of protons. Thus, the atomic number directly correlates to the number of electrons, establishing the identity of the element and its chemical behavior.

While other options relate to different aspects of atomic structure, such as mass number, which indicates the total number of protons and neutrons and not specifically electrons, or isotope count, which pertains to variations of elements with a different number of neutrons, they do not determine the number of electrons. Electron configuration, while essential for understanding how electrons are arranged in an atom, depends on the atomic number to identify the total number of electrons present.