Atom Calculator
Count the subatomic particles in any atom or ion. Enter an element symbol (like Fe) or an atomic number, the mass number, and the charge (0 for a neutral atom) to get the protons, neutrons and electrons.
Example: with Element symbol or atomic number Fe · Mass number (A) 56 · Charge (0 = neutral) 0 → Protons: 26 protons (Fe, Z=26).
- Neutrons30 neutrons (A 56 − Z 26)
- Electrons26 electrons (neutral atom)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Protons equal the atomic number Z. Neutrons equal the mass number A minus Z. Electrons equal Z minus the charge, so a +2 ion has two fewer electrons than protons.
Three simple counts
Every atom is defined by three numbers, and each comes from an easy rule. The proton count is the atomic number Z — it never changes for a given element, and it is what makes iron iron. The neutron count is the mass number A minus Z, since the mass number totals protons and neutrons. The electron count is Z minus the charge: a neutral atom has as many electrons as protons, a cation has fewer, and an anion has more.
An oxide ion, O²⁻, shows all three at once. Oxygen's Z is 8, so it has 8 protons. Oxygen-16 has A = 16, giving 16 − 8 = 8 neutrons. The 2− charge means two extra electrons, so 8 − (−2) = 10 electrons. Changing the neutron count would make a different isotope; changing the electron count makes an ion; only changing the proton count makes a different element.
How it’s calculated
Protons = Z (atomic number). Neutrons = A − Z, using the entered mass number A rounded to a whole number. Electrons = Z − charge, so a positive charge removes electrons and a negative charge adds them. Elements can be entered by symbol (case-insensitive) or by atomic number 1–118.
The mass number is a whole count of protons plus neutrons, not the averaged atomic weight from the periodic table. A negative neutron count flags a mass number smaller than the proton count.
Particle counts for sample isotopes
| Isotope | Protons | Neutrons | Electrons (neutral) |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 6 |
| Oxygen-16 | 8 | 8 | 8 |
| Chlorine-37 | 17 | 20 | 17 |
| Iron-56 | 26 | 30 | 26 |
| Uranium-238 | 92 | 146 | 92 |
Protons = Z, neutrons = A − Z, electrons = Z for the neutral atom.
Common mistakes
- Using the periodic-table atomic weight as the mass number; use the whole-number isotope mass instead.
- Subtracting the charge the wrong way — a +2 ion has fewer electrons, a −2 ion has more.
- Assuming neutrons equal protons; they only match for a few light isotopes.
- Thinking changing neutrons changes the element — it changes the isotope, not the element.
Frequently asked questions
How do you find protons, neutrons and electrons?
Protons equal the atomic number Z. Neutrons equal the mass number minus Z. Electrons equal Z minus the charge, so a neutral atom has as many electrons as protons.
How do I count neutrons?
Subtract the atomic number from the mass number: neutrons = A − Z. Iron-56 has 56 − 26 = 30 neutrons.
How does charge change the electron count?
A positive charge means electrons were lost, so subtract it from Z; a negative charge means electrons were gained, so it adds them. Na+ has 11 protons but only 10 electrons.
What is the difference between mass number and atomic mass?
Mass number is the whole-number count of protons plus neutrons in one isotope. Atomic mass is the abundance-weighted average of all isotopes shown on the periodic table.