Atom Calculator
Use our atom calculator to calculate atoms in an isotope: number of protons, neutrons, and electrons from atomic number, mass number, and charge. Includes how to calculate atoms, an atom formula breakdown, and an atom counting calculator walkthrough.
What Is an Atom?
An atom is made of protons and neutrons in the nucleus, with electrons surrounding the nucleus.
The atomic number (Z) tells you how many protons an element has. The mass number (A) tells you the total number of protons and neutrons.
If the atom has a charge (ion), the number of electrons changes relative to the number of protons.
Atom Formula and Calculations
Use atomic number, mass number, and charge to calculate the number of protons, neutrons, and electrons.
Atomic number directly equals the number of protons.
Mass number is protons + neutrons.
Use charge as a signed value: +2 means 2 fewer electrons; −1 means 1 extra electron.
Neutral atoms have electrons equal to protons.
A +3 ion has 3 fewer electrons than protons.
How to Use the Atom Calculator
- 1
Enter the atomic number (Z).
- 2
Enter the mass number (A).
- 3
Enter the charge (use + for cations and − for anions, or 0 for neutral).
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View the results: number of protons, neutrons, and electrons.
Frequently Asked Questions
Use Z for protons, A − Z for neutrons, and Z − charge for electrons.
Neutrons = mass number (A) − atomic number (Z).
Electrons = Z − charge. For example, a +2 charge means 2 fewer electrons than Z; a −1 charge means 1 more electron than Z.
Yes. This atom counting calculator computes counts of subatomic particles from Z, A, and charge.
No. Atom economy/atom efficiency are reaction metrics in chemistry. This calculator focuses on counting protons, neutrons, and electrons in an atom or ion.