Atom Calculator

01 Set up your calculation

1. Enter the atomic number Z and the isotope’s whole-number mass number A.
2. Use a positive charge for a cation, a negative charge for an anion, or zero for a neutral atom.
3. Read the three particle counts. These inputs describe a species, not whether it is stable.

02 The relationship

For the assumptions behind these results, read the explanation below. You can also explore Atomic Mass and Average Atomic Mass.

03 Read an isotope label correctly

The upper number in an isotope label counts nucleons. Subtracting the proton count leaves the neutron count. A decimal atomic weight from a periodic table is an average over isotopes and must not be used as this integer input.

For an ion, the sign refers to net electric charge. Removing an electron increases the charge by one, so a +2 ion has two fewer electrons than its neutral atom.

04 What the counts do and do not establish

Changing the neutron number describes a different isotope of the same element. Changing the proton number describes a different element. Electron loss or gain changes the ion while leaving the nucleus unchanged.

The tool checks arithmetic consistency. It does not predict nuclear stability, isotope abundance, allowed oxidation states, or the lifetime of an unusual ion. A formally valid count need not represent a readily obtainable substance.