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Atom Calculator – Protons, Neutrons & Electrons

Every atom is defined by three numbers: atomic number (protons), mass number (protons + neutrons), and charge (which determines electrons). Knowing how to derive each subatomic particle count from the others is a foundational chemistry skill used in nuclear science, chemical bonding theory, and spectroscopy.

Enter an element's atomic number or symbol and mass number to instantly get the proton, neutron, and electron count. For ions, entering the charge adjusts the electron count accordingly.

How to use the Atom Calculator

  1. Enter the element symbol or atomic number.
  2. Enter the mass number (protons + neutrons).
  3. For ions, enter the charge (e.g., +2 or −1).
  4. View proton, neutron, and electron count.
Subatomic Particle Count for Common Elements
ElementSymbolAtomic Number (Protons)Common Mass NumberNeutronsElectrons (neutral)
HydrogenH1101
CarbonC61266
NitrogenN71477
OxygenO81688
IronFe26563026
GoldAu7919711879

Atom Calculator FAQ

What is the difference between atomic number and mass number?
Atomic number = number of protons (defines the element). Mass number = protons + neutrons (defines the specific isotope). Carbon-12 and Carbon-14 are both carbon (6 protons) but have different mass numbers due to different neutron counts.
How do you calculate neutrons?
Neutrons = mass number − atomic number. For Carbon-12: 12 − 6 = 6 neutrons. For Carbon-14 (radioactive isotope used in dating): 14 − 6 = 8 neutrons.
How does ion charge affect electron count?
In a neutral atom, electrons = protons. Losing electrons creates a positive ion (cation): Na+ has 11 protons but only 10 electrons. Gaining electrons creates a negative ion (anion): Cl⁻ has 17 protons and 18 electrons.
What is an isotope?
Atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers. Some isotopes are stable; others are radioactive.