Protons, Neutrons and Electrons Calculator
Type an element symbol, name or atomic number and get its protons, neutrons and electrons. Add a mass number for a particular isotope, or a charge for an ion.
- Symbol, name or number
- Isotopes and ions
- All 118 elements
- Free
Find the particle counts
A symbol, a full name or an atomic number. Capital letters do not matter here.
Protons + neutrons for the isotope you want. Leave it empty and the neutron count is left unanswered.
Write it as 2+, +2, 1− or −1. Empty means a neutral atom.
Show the working
How to find protons, neutrons and electrons
Protons = atomic number
The atomic number IS the proton count, and it is what makes the element that element. Iron is 26, so every iron atom has 26 protons.
Neutrons = mass number − atomic number
Iron-56 has 56 − 26 = 30 neutrons. You need to know which isotope you mean, because that is what the mass number names.
Electrons = protons − charge
A neutral atom has as many electrons as protons. Fe²⁺ has lost two, so 24. A 1− ion has gained one.
Note the direction of the charge: a positive ion has lost electrons, so you subtract. It catches people out because "adding" a plus sign means taking electrons away.
Questions people ask
How do you find the number of protons?
It is the atomic number, the number above the symbol on the periodic table. Carbon is 6, so a carbon atom has 6 protons. Change that number and you have a different element.
How do you find the number of neutrons?
Subtract the atomic number from the mass number. Carbon-14 is 14 − 6 = 8 neutrons. You need the mass number of the isotope you mean; the averaged mass on the periodic table is not it.
How do you find the number of electrons?
A neutral atom has the same number of electrons as protons. For an ion, subtract the charge: Fe²⁺ has 26 − 2 = 24 electrons, and Cl⁻ has 17 + 1 = 18.
Why does this calculator not fill in the mass number for me?
Because the mass on the periodic table is an average across isotopes, and rounding it does not reliably name the commonest one. Copper averages 63.546, which rounds to 64, but the most common copper isotope in nature is copper-63. There is a button that fills in the rounded value, which is the answer most textbooks expect, and the result says plainly that it is an estimate.
What is the difference between mass number and atomic mass?
The mass number is a whole-number count of protons plus neutrons in a single isotope. The atomic mass on the periodic table is the average mass across all the isotopes as they occur in nature, weighted by how common each one is, which is why it has decimals.
Does an ion have a different number of protons?
No. Ions form by gaining or losing electrons, never protons. The proton count is fixed for an element, which is why a charge only ever changes the electron count here.
Can the mass number be smaller than the atomic number?
No. The mass number counts protons plus neutrons, so it can never be less than the proton count on its own. If you enter one that is, the calculator refuses it rather than reporting a negative number of neutrons.
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