About the Protons, Neutrons & Electrons Calculator
This calculator gives the number of protons, neutrons and electrons in any atom, isotope or ion. Type an element symbol or name (Fe, iron, 26), an isotope (C-14, carbon-14, 14C, U-235) or a monatomic ion (Fe3+, O2-, Cl^-) and it applies the three rules every chemistry student learns: protons = atomic number, neutrons = mass number − atomic number, electrons = protons − charge.
If you do not give a mass number, the calculator uses the most abundant naturally occurring isotope from the NIST isotopic composition tables (for example Cu-63, not the rounded atomic weight 64), or a long-lived isotope for radioactive elements. That avoids a classic textbook error: rounding the atomic weight does not always give a real isotope.
It is useful for homework on isotopes and ions, checking nuclear notation such as ⁵⁶Fe³⁺, and quick lookups in the lab. Results are for single atoms or monatomic ions; for molecules add up the counts for each atom.
How to use the protons, neutrons & electrons calculator
- 1Type an element symbol, name or atomic number.
- 2Add an isotope mass number (C-14) or enter it in the mass number box.
- 3For ions, type the charge after the symbol (Fe3+, O2-) or use the charge box.
- 4Read the proton, neutron and electron counts and the nuclear notation.
Formula and method
The atomic number Z, fixed for each element, is the number of protons in the nucleus. The mass number A of a particular isotope is the total number of protons and neutrons, so neutrons = A − Z. A neutral atom has as many electrons as protons; a positive ion (cation) has lost electrons and a negative ion (anion) has gained them, so electrons = Z − charge.
When no isotope is specified, the most abundant stable isotope from the NIST isotopic composition tables is used. Rounding the average atomic weight is a common shortcut but can be wrong: copper averages 63.546 yet its most common isotope is Cu-63, and bromine averages 79.904 although Br-80 does not occur naturally.
- Z
- Atomic number (protons)
- A
- Mass number (protons + neutrons)
- charge
- Ion charge (+2 for Fe²⁺, −1 for Cl⁻)
Worked examples
Iron (most common isotope)
Iron has Z = 26. Its most abundant isotope is Fe-56 (about 92% of natural iron), so it has 56 − 26 = 30 neutrons, and a neutral atom has 26 electrons.
Carbon-14
Carbon always has 6 protons. Carbon-14 has a mass number of 14, so it carries 14 − 6 = 8 neutrons — two more than ordinary carbon-12, which makes it radioactive.
Iron(III) ion, Fe3+
The 3+ charge means three electrons have been lost: 26 − 3 = 23 electrons. Protons and neutrons are unchanged.
Oxide ion, O2-
Oxygen-16 has 8 protons and 8 neutrons. A 2− charge means two extra electrons, giving 10 — the same electron count as neon.
Frequently asked questions
How do you find the number of neutrons?+
Subtract the atomic number from the mass number: neutrons = A − Z. For chlorine-37, 37 − 17 = 20 neutrons. If only the average atomic weight is given, use the mass number of a specific isotope rather than rounding the average.
How many electrons does an ion have?+
Start with the number of protons and subtract the charge. Na⁺ has 11 − 1 = 10 electrons, while Cl⁻ has 17 + 1 = 18. Positive ions have lost electrons and negative ions have gained them.
Do isotopes have different numbers of protons?+
No. All isotopes of an element have the same number of protons (that defines the element) and, when neutral, the same number of electrons. They differ only in their number of neutrons and therefore in mass.
What is the difference between mass number and atomic mass?+
Mass number is a whole-number count of protons plus neutrons in one isotope. Atomic mass (atomic weight) is the average mass of an element’s natural isotopes in daltons, such as 35.45 for chlorine, and is rarely a whole number.
How do I write nuclear notation?+
Write the mass number as a left superscript, the atomic number optionally as a left subscript, and any charge as a right superscript, for example ⁵⁶Fe³⁺ or ¹⁴C. The element symbol already implies the atomic number.