Atom Calculator
Quickly relate atomic number, mass number, charge, and subatomic particles.
Use this Atom Calculator to find an atom or ion’s protons, neutrons, electrons, atomic number, mass number, and charge from the values you know.
Atom Calculator Formulas
- Protons = atomic number
- Mass number = protons + neutrons
- Neutrons = mass number − protons
- Charge = protons − electrons
- Electrons = protons − charge
A neutral atom has the same number of protons and electrons. A positive ion has fewer electrons than protons, while a negative ion has more electrons than protons.
Worked Example
Suppose chlorine-35 has an atomic number of 17 and a charge of −1.
Protons = 17
Neutrons = 35 − 17 = 18
Electrons = 17 − (−1) = 18
The chlorine-35 ion therefore contains 17 protons, 18 neutrons, and 18 electrons.
Important Note
Use the isotope’s whole-number mass number, not the decimal average atomic mass shown on most periodic tables. Mass number is the total number of protons and neutrons in one isotope.
This calculator is intended for basic atoms and monatomic ions. It does not calculate average atomic mass, isotope abundance, electron configuration, polyatomic-ion composition, or radioactive decay.