Protons, Neutrons and Electrons Calculator
The subatomic particle count for any element, isotope or ion.
Enter a symbol, a name or an atomic number.
Leave blank to use the most common isotope.
Use 2 for a 2+ ion, -1 for a 1− ion.
Iron (Fe), mass number 56
26
protons
30
neutrons
26
electrons
No mass number given, so the most common isotope is assumed: 56. A different isotope changes the neutron count and nothing else.
Working
- Protons = atomic number = 26. This is what makes the atom iron.
- Neutrons = mass number − protons = 56 − 26 = 30
- Electrons = protons = 26 (a neutral atom)
Full data for Iron: Iron on the periodic table
Reading isotope notation
Written as ¹⁴C or carbon-14, the number is the mass number: protons plus neutrons. The element name already fixes the proton count, so the two together give the neutrons by subtraction. Ionic charge is written separately, as a superscript after the symbol, and affects only the electrons.
Why the three counts matter
Protons set the element and the chemistry. Electrons set the bonding, the ion charge and the reactivity. Neutrons set the mass and the nuclear stability — too few or too many and the nucleus decays, which is what makes some isotopes radioactive.
Related: molar mass calculator, chemical equation balancer, periodic table.
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Frequently Asked Questions
How do you find the number of protons?
The number of protons is the atomic number, the number printed above each symbol on the periodic table. It never changes for a given element — change the protons and you have changed the element.
How do you find the number of neutrons?
Subtract the atomic number from the mass number: neutrons = A − Z. Iron-56 has 56 − 26 = 30 neutrons. Different isotopes of the same element differ only in this number.
How do you find the number of electrons?
A neutral atom has as many electrons as protons. For an ion, subtract the charge: a 2+ ion has lost two electrons, a 1− ion has gained one. So Ca²⁺ has 20 protons and 18 electrons.
What is the difference between mass number and atomic mass?
The mass number is a whole-number count of protons plus neutrons in one specific atom. Atomic mass is the weighted average over all naturally occurring isotopes, which is why the periodic table shows 35.45 for chlorine rather than 35 or 37.
Why does the neutron count change but not the proton count?
Because protons define identity and neutrons do not. Carbon with six neutrons and carbon with eight neutrons are both carbon — carbon-12 and carbon-14 — and behave the same chemically. Only the nuclear stability differs.