How to Find Protons, Neutrons, and Electrons
By the Periodixy Editorial Team · Last reviewed July 10, 2026
Given any atom or ion, three questions come up constantly: how many protons, neutrons and electrons does it have? All three answers come straight from two numbers — the atomic number and the mass number — plus the charge, if any.
The three rules
| Particle | How to find it | Example: ³⁵Cl |
|---|---|---|
| Protons | = atomic number (Z) | 17 |
| Neutrons | = mass number (A) − atomic number | 35 − 17 = 18 |
| Electrons | = protons − charge | 17 (neutral) |
The atomic number is printed on every periodic table tile. The mass number comes from the isotope name (carbon-14 → A = 14) or the superscript in symbols like ¹⁴C.
Neutral atoms
Oxygen-16
Find p, n and e for a neutral oxygen-16 atom.
- Oxygen's atomic number is 8 → 8 protons.
- Neutrons = 16 − 8 = 8.
- Neutral → electrons = protons = 8.
Answer: 8 protons, 8 neutrons, 8 electrons.
Ions: adjust the electrons only
Charge changes electrons, never protons. A positive charge means electrons were lost; a negative charge means electrons were gained.
The Ca²⁺ ion
Find p, n and e for ⁴⁰Ca²⁺.
- Calcium: Z = 20 → 20 protons.
- Neutrons = 40 − 20 = 20.
- Charge +2 → lost 2 electrons → 20 − 2 = 18 electrons.
Answer: 20 protons, 20 neutrons, 18 electrons.
Why the mass number isn't on the table
The decimal on the periodic table (like 35.45 for chlorine) is the average atomic mass across isotopes, not a mass number. For particle counting you need a specific isotope's mass number, which the question must supply. More on this in Atomic Number vs Atomic Mass.
Summary
- Protons = atomic number — always.
- Neutrons = mass number − atomic number.
- Electrons = protons in neutral atoms; subtract positive charge or add negative charge for ions.
- Charges change electrons only — the nucleus is untouched by chemistry.