Define mass number / nucleon number as the total number of protons and neutrons in the nucleus of an atom

Atoms, Elements and Compounds – Atomic Structure & the Periodic Table

Mass Number (Nucleon Number)

⚛️ The mass number (also called the nucleon number) is the total count of the two main particles inside an atom’s nucleus: protons and neutrons. It tells you how heavy the nucleus is, but not its charge.

Mathematically we write it as:

\$A = Z + N\$

Where:

• \$A\$ = mass number (total nucleons)
• \$Z\$ = atomic number (number of protons)
• \$N\$ = number of neutrons

Analogy: The Atom as a Tiny City

Imagine an atom as a small city. The protons are the city council members who decide the city’s charge (like the mayor’s badge). The neutrons are the citizens who help build the city but don’t vote. The mass number is simply the total population of the city – council members plus citizens.

Quick Example: Carbon

Carbon has an atomic number of 6, meaning every carbon atom has 6 protons. The most common isotope, Carbon‑12, has 6 neutrons. So:

\$A = 6\;(\text{protons}) + 6\;(\text{neutrons}) = 12\$

That’s why we write it as C‑12.

How to Find the Mass Number

1. Count the number of protons (\$Z\$) – this is the element’s atomic number.
2. Count the number of neutrons (\$N\$) – usually given in the isotope’s name.
3. Add them together: \$A = Z + N\$.

Isotopes of Carbon – A Quick Table

💡 Remember: The mass number is always a whole number because you’re counting whole particles. It’s a quick way to identify which isotope you’re looking at and to compare how heavy different atoms are.