American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

To determine the number of neutrons in an atom, you start with two key pieces of information: the atomic number and the mass number. The atomic number represents the number of protons in the nucleus of an atom, while the mass number is the total number of protons and neutrons combined.

By subtracting the atomic number (which tells you how many protons there are) from the mass number (the total count of protons and neutrons), you are left with the number of neutrons. This is because the mass number is composed of both protons and neutrons. Thus, the formula can be expressed as:

Number of Neutrons = Mass Number - Atomic Number

This method provides an accurate count of neutrons, essential for understanding isotopes and various properties of elements. The other choices, while involving the atomic and mass numbers, do not yield the correct count of neutrons in an atom as they either combine the quantities incorrectly or do not relate to the neutrons directly.