# Protons, neutrons and electrons?

Counts the protons, neutrons and electrons of any atom or ion from its element, mass number and charge: protons = Z, neutrons = A − Z and electrons = Z − charge.

- Page: https://www.acalculator.org/chemistry/protons-neutrons-electrons-calculator
- JSON spec: https://www.acalculator.org/chemistry/protons-neutrons-electrons-calculator.json
- Version: f16889d51afc

## Default answer

Example with the default inputs (Element 53 I (Iodine), Charge 0): ¹²⁷I has 53 protons, 74 neutrons and 53 electrons.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| z | Element | The element, by atomic number, symbol and name; its atomic number Z is its number of protons. |
| a | Mass number (A) | Protons plus neutrons in the nucleus, as in carbon-14. Leave it empty for the element’s most abundant isotope. |
| q | Charge | The charge of the ion: 0 for a neutral atom, −1 for an anion such as Cl⁻, +2 for a cation such as Ca²⁺. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Protons, neutrons and electrons | The counts of the three particles, in words. |
| protons | Protons | Protons in the nucleus: the atomic number Z. |
| neutrons | Neutrons | Neutrons in the nucleus: the mass number minus Z. |
| electrons | Electrons | Electrons around the nucleus: Z minus the charge. |
| symbol | Isotope symbol | The nuclide symbol with the mass number before it and the charge after it, such as ¹²⁷I⁻. |
| isotope | Isotope | The element name and mass number, such as iodine-127. |
| massNumber | Mass number (A) | The mass number used. |
| basis | Mass number from | Whether the mass number was typed or is the most abundant natural isotope in NIST’s table. |

## Method

Protons = Z (the atomic number); neutrons = A − Z (A is the mass number); electrons = Z − charge. With no mass number typed, A is the mass number of the most abundant isotope in NIST’s table.

## Assumptions

- The charge is the ion’s whole charge: protons minus electrons.
- With no mass number typed, the page uses the isotope with the largest natural abundance in NIST’s isotopic compositions; elements with none (technetium, promethium, polonium to actinium, and elements above uranium) need a typed mass number.
- The page does not check that a typed isotope exists or is stable.

## Worked examples

1. z = 53, a = 127, q = -1 gives protons = 53, neutrons = 74, electrons = 54, symbol = ¹²⁷I⁻. Source: OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (A = Z + neutrons; charge = protons − electrons), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism, Example 2.3 Composition of an Atom: 53 protons, 74 neutrons and 54 electrons.
2. z = 78, a = 195, q = 4 gives protons = 78, neutrons = 117, electrons = 74, symbol = ¹⁹⁵Pt⁴⁺. Source: OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (A = Z + neutrons; charge = protons − electrons), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism, Check Your Learning after Example 2.3: 78 protons, 117 neutrons, charge 4+.
3. z = 6, q = 0 gives massNumber = 12, neutrons = 6, electrons = 6, isotope = carbon-12. Source: NIST, Atomic Weights and Isotopic Compositions for All Elements, https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some: carbon-12 has the largest isotopic composition, 0.9893.
4. z = 17, q = -1 gives massNumber = 35, neutrons = 18, electrons = 18, symbol = ³⁵Cl⁻. Source: NIST, Atomic Weights and Isotopic Compositions for All Elements, https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some: chlorine-35, 0.7576.
5. z = 92, a = 235, q = 0 gives neutrons = 143, electrons = 92, result = 92 protons, 143 neutrons and 92 electrons. Source: OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism (A = Z + neutrons; charge = protons − electrons), https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism (neutrons = A − Z).

## FAQ

### How do I find the number of protons?

The number of protons is the atomic number Z, the whole number above the symbol in the periodic table. Every atom of an element has the same number of protons: every iodine atom has 53, every carbon atom 6. The charge and the isotope do not change it.

### How do I calculate the number of neutrons?

Subtract the atomic number from the mass number: neutrons = A − Z. The mass number is the whole number in the isotope’s name, such as the 127 in iodine-127, so iodine-127 has 127 − 53 = 74 neutrons.

### How many electrons does an ion have?

Electrons = Z − charge. A neutral atom has as many electrons as protons. An anion has extra electrons: the iodide ion I⁻ has 53 + 1 = 54. A cation has fewer: Pt⁴⁺ has 78 − 4 = 74.

### Can I round the atomic mass to get the mass number?

Not safely. The atomic mass on a periodic table is an average over the natural isotopes, and rounding it can land on an isotope that is not the common one. Copper’s atomic weight, 63.546, rounds to 64, but the most abundant copper isotope is copper-63 (69.15% of natural copper, NIST). Use the mass number of the isotope you mean.

### What mass number does the page use when I leave it empty?

The mass number of the element’s most abundant isotope in NIST’s table of isotopic compositions: carbon-12, chlorine-35, iron-56, uranium-238. Technetium, promethium, polonium to actinium, and the elements after uranium have no natural isotope mix in that table, so the page asks you to type a mass number.

### What is an isotope?

Atoms of one element with different numbers of neutrons. Carbon-12 and carbon-14 both have 6 protons, but carbon-12 has 6 neutrons and carbon-14 has 8. Isotopes have the same chemistry and different masses.

## Sources

- OpenStax, Chemistry 2e, §2.3 Atomic Structure and Symbolism: atomic number Z, mass number A = Z + neutrons, charge = protons − electrons, Example 2.3 (iodine-127 anion: 53 protons, 74 neutrons, 54 electrons) and its Check Your Learning (platinum-195 with 74 electrons: charge 4+). https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism (retrieved 2026-10-19)
- NIST, Atomic Weights and Isotopic Compositions for All Elements: the isotopic composition of every isotope; the page uses the mass number of the largest one. https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some (retrieved 2026-10-19)
- CIAAW (IUPAC Commission on Isotopic Abundances and Atomic Weights), Standard Atomic Weights: element names and symbols for Z = 1 to 118. https://www.ciaaw.org/atomic-weights.htm (retrieved 2026-10-19)