Protons Neutrons Electrons Calculator
This protons neutrons electrons calculator works from the atomic number Z, the mass number A and the charge. For the iodide ion ¹²⁷I⁻ it gives 53 protons, 74 neutrons and 54 electrons. If you know the number of electrons instead of the charge, it finds the charge.
Example: Atomic number Z 53 · Mass number A 127 · Known: Charge · Charge -1 → Particles: ¹²⁷I⁻: 53 protons, 74 neutrons, 54 electrons. Source: OpenStax Chemistry 2e, 2.3 Atomic Structure and Symbolism – Z = protons, A = protons + neutrons, A − Z = neutrons, charge = protons − electrons; Example 2.3 iodide ¹²⁷I⁻ (53/74/54), platinum ion A = 195 with 74 electrons (78/117, 4+), Table 2.4 isotopes (read 10.10.2026). Updated: .
How it is calculated
Protons = atomic number Z, neutrons = mass number A − Z, electrons = Z − charge. OpenStax Chemistry defines the atomic number as the number of protons in the nucleus and the mass number as protons plus neutrons; the charge of an atom or ion is the number of protons minus the number of electrons. A neutral atom therefore has as many electrons as protons, a cation fewer and an anion more.
Protons, neutrons and electrons: examples
| Atom or ion | Protons | Neutrons | Electrons |
|---|---|---|---|
| ¹H | 1 | 0 | 1 |
| ²H | 1 | 1 | 1 |
| ¹²C | 6 | 6 | 6 |
| ¹⁶O²⁻ | 8 | 8 | 10 |
| ²⁴Mg²⁺ | 12 | 12 | 10 |
| ³⁵Cl⁻ | 17 | 18 | 18 |
| ³⁷Cl | 17 | 20 | 17 |
| ¹²⁷I⁻ | 53 | 74 | 54 |
| ¹⁹⁵Pt⁴⁺ | 78 | 117 | 74 |
The iodide and platinum ions are the worked examples in OpenStax: iodine has Z = 53, so ¹²⁷I⁻ has 127 − 53 = 74 neutrons and, with charge 1−, 54 electrons. A platinum ion with mass number 195 and 74 electrons has 78 protons, 117 neutrons and the charge 78 − 74 = 4+.
Isotopes and ions
- Isotopes of an element have the same number of protons but different numbers of neutrons: ³⁵Cl and ³⁷Cl both have 17 protons, but 18 and 20 neutrons.
- Hydrogen-2 (²H) is also called deuterium; it has one neutron.
- Ions differ only in the number of electrons: losing electrons gives a positive charge (cation), gaining them a negative charge (anion).
Atomic numbers and symbols are in the periodic table; the average mass of an element from its isotopes follows from the average atomic mass calculator.
Frequently asked questions
How do you find the number of neutrons?
Subtract the atomic number from the mass number: neutrons = A − Z. ¹²C has 12 − 6 = 6 neutrons.
How many electrons does an ion have?
Electrons = Z − charge. O²⁻ (Z = 8) has 8 − (−2) = 10 electrons, Mg²⁺ (Z = 12) has 12 − 2 = 10.
How many protons, neutrons and electrons are in ¹²⁷I⁻?
53 protons, 74 neutrons and 54 electrons – the OpenStax example for the iodide ion.
Where do I find the mass number?
It is the number written at the top left of the symbol (¹²⁷I) or after the name (iodine-127). The atomic mass in the periodic table is an average over the isotopes (OpenStax gives 35.453 for chlorine), so do not use it as A.
What if I know the electrons but not the charge?
Choose “Number of electrons”. The charge is then protons − electrons, e.g. 78 − 74 = 4+ for the platinum ion.
Sources and legal basis
- OpenStax Chemistry 2e, 2.3 Atomic Structure and Symbolism – Z = protons, A = protons + neutrons, A − Z = neutrons, charge = protons − electrons; Example 2.3 iodide ¹²⁷I⁻ (53/74/54), platinum ion A = 195 with 74 electrons (78/117, 4+), Table 2.4 isotopes (read 10.10.2026)
- PubChem Periodic Table (NCBI/NLM) – element symbols Z = 1–118 (read 10.10.2026)
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