Humber Admissions Practice Test 2025 - Free Practice Questions and Study Guide for Humber College Admissions

Question: 1 / 400

What is the mass number of an atom?

The number of electrons

The number of protons only

The sum of protons and neutrons

The mass number of an atom is defined as the sum of its protons and neutrons. This total count represents the overall mass of the nucleus since protons and neutrons are substantially more massive than electrons. Understanding the composition of the nucleus is vital when analyzing isotopes or calculating atomic mass, as different isotopes of an element have the same number of protons but a varying number of neutrons.

The concept of the mass number is central to various disciplines in chemistry and physics, including understanding atomic stability and reactions. In contrast, the mass number does not consider electrons, which contribute negligibly to the mass of the atom when compared to the nucleus. Therefore, while the total number of subatomic particles could include electrons, protons, and neutrons, only protons and neutrons are relevant for calculating the mass number of an atom.

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The total number of subatomic particles

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