The mass number of an atom is equal to the:

Question

What is the molar mass of sulfur dioxide (SO$_2)$?

Answer 1

Use the standard notation for atoms to see this clearly.

An atom is written as $^{A}_{Z}X$, where $Z$ is the atomic number, protons, and $A$ is the mass number.
By definition, $A = Z + N$, where $N$ is the number of neutrons.
Protons and neutrons are collectively called nucleons because they reside in the nucleus, so $A$ is simply the total nucleon count.
Checking the other options: protons alone give $Z$, not $A$; protons plus electrons only reflects charge neutrality; neutrons alone give only $N$, not the full mass number.
So the mass number equals the number of nucleons, option C.

Answer 2

Use the standard notation for atoms to see this clearly.

An atom is written as $^{A}_{Z}X$, where $Z$ is the atomic number, protons, and $A$ is the mass number.
By definition, $A = Z + N$, where $N$ is the number of neutrons.
Protons and neutrons are collectively called nucleons because they reside in the nucleus, so $A$ is simply the total nucleon count.
Checking the other options: protons alone give $Z$, not $A$; protons plus electrons only reflects charge neutrality; neutrons alone give only $N$, not the full mass number.
So the mass number equals the number of nucleons, option C.

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