Protons, Neutrons and Electrons Calculator for Any Atom, Isotope or Ion
Find the protons, neutrons and electrons in any atom, isotope or ion from the element, mass number and charge, or identify an atom from its particle counts.
How it works
Protons p = atomic number Z · Neutrons n = mass number A − Z · Electrons e = Z − charge · and in reverse: Z = p, A = p + n, charge = p − e
The atomic number Z is the number of protons, and it alone decides which element an atom is: every carbon atom has 6 protons and every iron atom has 26. The mass number A counts the protons and neutrons in the nucleus, so the neutrons are A − Z. Isotopes of an element share Z but differ in A, which is why carbon-12 has 6 neutrons and carbon-14 has 8. In a neutral atom the electrons equal the protons; an ion has lost electrons (a cation, positive charge) or gained them (an anion, negative charge), so electrons = Z − charge. Choose an element and the calculator uses its most abundant natural isotope (or the longest-lived one for radioactive elements), or choose “Isotope” and enter any mass number. In the third mode you enter the three particle counts and it names the element, the isotope and the charge, the reverse of the usual homework question. The results include the nuclide symbol with mass number, atomic number and charge, the neutron-to-proton ratio and the total particle mass. Unlike the average atomic mass and atoms-to-moles calculators, which work with average masses and amounts, this tool counts the particles in one atom or ion. Standard atomic weights are the IUPAC values shown on the 2024 NIST periodic table.
Worked example
Iron’s most common isotope is iron-56. Iron has atomic number 26, so it has 26 protons. Neutrons = 56 − 26 = 30. A neutral atom has as many electrons as protons, 26. The nuclide symbol is ⁵⁶₂₆Fe and the neutron-to-proton ratio is 30 ÷ 26 = 1.154. The particles add up to 26 × 1.007276 + 30 × 1.008665 + 26 × 0.000549 = 56.463 u, a little more than the real mass because of nuclear binding energy. For the Fe³⁺ ion, set the charge to +3: the protons and neutrons stay the same and the electrons drop to 26 − 3 = 23.
Sources: Omni Calculator: Atom calculator (Na-23 and S²⁻ examples) · NIST: Periodic Table of the Elements, 2024 (standard atomic weights)
Frequently asked questions
How do you find the number of neutrons?
Subtract the atomic number from the mass number: neutrons = A − Z. Chlorine-35 has 35 − 17 = 18 neutrons. If you only have the atomic mass from the periodic table, round it to the nearest whole number to estimate the most common isotope’s mass number.
How many electrons does an ion have?
Electrons = protons − charge. A 2+ cation has two fewer electrons than protons: Mg²⁺ has 12 − 2 = 10. A 2− anion has two more: O²⁻ has 8 + 2 = 10. Protons and neutrons do not change when an ion forms.
What is the difference between atomic number and mass number?
The atomic number Z is the number of protons and identifies the element. The mass number A is protons plus neutrons and identifies the isotope. Carbon-12 and carbon-14 both have Z = 6 but A = 12 and 14.
Why is the atomic mass on the periodic table not a whole number?
It is the average mass of the element’s natural isotopes weighted by their abundance, and each isotope’s mass is slightly less than the sum of its particles because of nuclear binding energy. Chlorine is 35.45 because it is about 76% chlorine-35 and 24% chlorine-37.
How do I identify an atom from its protons, neutrons and electrons?
The proton count gives the element, protons + neutrons give the mass number and protons − electrons give the charge. 16 protons, 16 neutrons and 18 electrons is sulfur-32 with a 2− charge, the sulfide ion S²⁻. Choose “Particle counts” to do this.
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This calculator is for educational purposes. Double-check important results before acting on them.