Atomic Number vs Atomic Mass
By the Periodixy Editorial Team · Last reviewed July 10, 2026
Two numbers appear on every periodic table tile, and mixing them up is one of the most common early chemistry errors. The atomic number counts protons. The atomic mass is a weighted average of isotope masses. They answer completely different questions.
Atomic number: the element's identity
The atomic number (symbol Z) is the number of protons in the nucleus. It is always a whole number, and it defines the element: every atom with 8 protons is oxygen, no exceptions. In a neutral atom it also equals the number of electrons.
Mass number: one specific atom
The mass number (symbol A) is protons + neutrons for one particular atom. Carbon-12 has 6 protons and 6 neutrons (A = 12); carbon-14 has 6 protons and 8 neutrons (A = 14). Mass numbers are whole numbers because they count particles.
Neutron count
neutrons = mass number − atomic number
Atomic mass: the natural average
The atomic mass on the periodic table (e.g. 35.45 for chlorine) is the weighted average of all natural isotopes. Chlorine is about 76% chlorine-35 and 24% chlorine-37, so the average lands at 35.45 — a number that belongs to no single atom, like an average family of 2.3 children.
Worked example
Why is chlorine's atomic mass 35.45 and not 36?
- Cl-35 contributes 34.97 × 0.7576 ≈ 26.50
- Cl-37 contributes 36.97 × 0.2424 ≈ 8.96
- Add: 26.50 + 8.96 ≈ 35.45
Answer: The natural mix of isotopes averages to 35.45 amu — try it in the Isotope Abundance Calculator.
Side-by-side comparison
| Atomic number (Z) | Mass number (A) | Atomic mass | |
|---|---|---|---|
| Counts | protons | protons + neutrons | weighted isotope average |
| Whole number? | always | always | almost never |
| Applies to | every atom of the element | one specific isotope | the natural mixture |
| Example (Cl) | 17 | 35 or 37 | 35.45 amu |
Summary
- Atomic number = protons = the element's identity.
- Mass number = protons + neutrons for one specific isotope.
- Atomic mass = weighted average of natural isotopes — the decimal on the periodic table.
- Neutrons = mass number − atomic number.