Atom Calculator
Enter the atomic number Z, the mass number A, and the ionic charge to see how many protons, neutrons, and electrons the atom or ion has. Use a positive charge for a cation (electrons removed) and a negative charge for an anion (electrons added).
Last updated: September 26, 2026
Frequently asked questions
How do I find protons, neutrons, and electrons?
The number of protons equals the atomic number Z. The number of neutrons equals the mass number A minus Z. The number of electrons equals Z minus the ionic charge.
How does the ionic charge work?
A positive charge means electrons were removed, so a +1 ion has one fewer electron than protons. A negative charge means electrons were added. A neutral atom has a charge of 0.
What is the difference between mass number and atomic mass?
Mass number is a whole-number count of protons and neutrons. Atomic mass is a measured mass in unified atomic mass units and is not a whole number, and average atomic mass weights it over the natural isotopes of an element.
Can the mass number be smaller than the atomic number?
No. The mass number includes all protons, so it must be at least the atomic number. The calculator rejects smaller values.
Why does the calculator reject some charges?
A charge larger than the atomic number would remove more electrons than the atom has. The number of electrons cannot be negative.
Where can I get the average atomic mass of an element?
Use the Average Atomic Mass Calculator with isotope masses and abundances. To turn a formula into molar mass, use the Molecular Weight Calculator.