Atom Calculator (element, mass number and charge from protons, neutrons and electrons)

Understanding the atom and ion calculator — concept and principle

This calculator identifies an atom or ion from its numbers of protons, neutrons and electrons. It gives the element, mass number, net charge, isotope name, nuclide symbol and neutron-to-proton ratio.

How it is calculated

• Element = determined by the number of protons (atomic number Z).
• Mass number A = Z + N (protons + neutrons).
• Net charge = protons − electrons.
• Particle type: positive ion (cation) if charge > 0, negative ion (anion) if charge < 0, neutral atom if 0.
• Nuclide symbol: mass number at top-left, charge at top-right; N/Z ratio = neutrons ÷ protons.

The principle

The number of protons alone defines an element: every atom with 26 protons is iron. Adding neutrons changes the mass but not the chemistry, giving different isotopes such as iron-54 and iron-56. Electrons determine the charge: losing electrons leaves a positive ion, gaining them makes a negative ion. Stable light nuclei have roughly equal numbers of neutrons and protons; heavier stable nuclei need relatively more neutrons, reaching about 1.5 neutrons per proton for lead.

Worked example

Default inputs: 26 protons, 30 neutrons, 23 electrons.

1. Z = 26: the element is iron (Fe).
2. Mass number A = 26 + 30 = 56; isotope name Fe-56.
3. Net charge = 26 − 23 = +3, so it is a cation (positive ion).
4. Nuclide symbol = ⁵⁶Fe³⁺.
5. N/Z = 30 ÷ 26 ≈ 1.154.

How to read the result

Fe³⁺ is the iron(III) ion found in rust and many minerals. Iron-56 is the most abundant natural iron isotope. The calculator does not check whether a combination exists or is stable; an unusual N/Z ratio may point to a radioactive or non-existent nucleus.

Common mistakes

• Thinking neutrons change the element.
• Getting the charge sign backwards (fewer electrons means positive).
• Confusing mass number (a count) with atomic mass (in u, not an integer).
• Writing the charge at the bottom instead of the top-right of the symbol.

Frequently asked questions

What determines which element an atom is?

The number of protons, the atomic number Z.

How do you find the number of neutrons?

Subtract the atomic number from the mass number: N = A − Z.

What is the difference between an isotope and an ion?

Isotopes differ in neutrons; ions differ in electrons.

Sources: IUPAC, Compendium of Chemical Terminology (Gold Book): nuclide, isotope, ion · Brookhaven National Laboratory, National Nuclear Data Center, NuDat (chart of nuclides)