Protons, neutrons and electrons?
Pick an element and, if you know them, the mass number and charge. The calculator counts the protons, neutrons and electrons, and writes the isotope symbol. Leave the mass number empty to use the most abundant isotope in nature.
- Protons, neutrons and electrons
- 53 protons, 74 neutrons and 53 electrons
¹²⁷I has 53 protons, 74 neutrons and 53 electrons.
- Protons
- 53
- Neutrons
- 74
- Electrons
- 53
- Isotope symbol
- ¹²⁷I
- Isotope
- iodine-127
- Mass number (A)
- 127
- Mass number from
- The most abundant natural isotope (NIST)
Protons, neutrons and electrons: 53 protons, 74 neutrons and 53 electrons. ¹²⁷I has 53 protons, 74 neutrons and 53 electrons.
How to calculate
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.
Example with the default inputs (Element 53 I (Iodine), Charge 0): ¹²⁷I has 53 protons, 74 neutrons and 53 electrons.
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.
- 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
Each example is checked against the calculator on every build.
- Element 53 I (Iodine), Mass number (A) 127, Charge -1 gives Protons 53, Neutrons 74, Electrons 54, Isotope 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
- Element 78 Pt (Platinum), Mass number (A) 195, Charge 4 gives Protons 78, Neutrons 117, Electrons 74, Isotope 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+
- Element 6 C (Carbon), Charge 0 gives Mass number (A) 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
- Element 17 Cl (Chlorine), Charge -1 gives Mass number (A) 35, Neutrons 18, Electrons 18, Isotope 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
- Element 92 U (Uranium), Mass number (A) 235, Charge 0 gives Neutrons 143, Electrons 92, Protons, neutrons and electrons 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)
How it works
An atom of element number Z (the atomic number) has:
- Protons = Z.
- Neutrons = A − Z, where A is the mass number: protons plus neutrons in the nucleus.
- Electrons = Z − q, where q is the charge (0 for a neutral atom, negative for an anion, positive for a cation).
Mass number. Type it if you know the isotope (iodine-127: A = 127). Leave it empty and the page uses the mass number of the element’s most abundant isotope: the isotope with the largest isotopic composition in NIST’s table (chlorine-35 at 0.7576, copper-63 at 0.6915, bromine-79 at 0.5069). NIST lists no isotopic composition for technetium (43), promethium (61), polonium to actinium (84 to 89) or any element above uranium (92); for those elements the mass number must be typed. The mass numbers the page uses:
| Z | Element and mass number of its most abundant isotope |
|---|---|
| 1–10 | H 1, He 4, Li 7, Be 9, B 11, C 12, N 14, O 16, F 19, Ne 20 |
| 11–20 | Na 23, Mg 24, Al 27, Si 28, P 31, S 32, Cl 35, Ar 40, K 39, Ca 40 |
| 21–30 | Sc 45, Ti 48, V 51, Cr 52, Mn 55, Fe 56, Co 59, Ni 58, Cu 63, Zn 64 |
| 31–40 | Ga 69, Ge 74, As 75, Se 80, Br 79, Kr 84, Rb 85, Sr 88, Y 89, Zr 90 |
| 41–50 | Nb 93, Mo 98, Tc —, Ru 102, Rh 103, Pd 106, Ag 107, Cd 114, In 115, Sn 120 |
| 51–60 | Sb 121, Te 130, I 127, Xe 132, Cs 133, Ba 138, La 139, Ce 140, Pr 141, Nd 142 |
| 61–70 | Pm —, Sm 152, Eu 153, Gd 158, Tb 159, Dy 164, Ho 165, Er 166, Tm 169, Yb 174 |
| 71–80 | Lu 175, Hf 180, Ta 181, W 184, Re 187, Os 192, Ir 193, Pt 195, Au 197, Hg 202 |
| 81–90 | Tl 205, Pb 208, Bi 209, Po —, At —, Rn —, Fr —, Ra —, Ac —, Th 232 |
| 91–92 | Pa 231, U 238 (and no value for Z = 93 to 118) |
Rules the page checks.
- The mass number must be at least Z (neutrons cannot be negative). It is a whole number from 1 to 300.
- The charge is a whole number from −10 to +118, and at most +Z: an ion cannot lose more electrons than the atom has.
- The page does not check that a typed isotope exists.
How the result is written. The counts in words (“1 proton” in the singular), the isotope symbol with the mass number as a superscript before the element symbol and the charge as a superscript after it (¹²⁷I⁻, ¹⁹⁵Pt⁴⁺, a charge of ±1 written as ⁺ or ⁻ alone), and the isotope name (iodine-127). Element names are CIAAW’s IUPAC names (aluminium, caesium).
Worked examples by hand
Iodide ion, iodine-127, charge 1− (OpenStax Example 2.3). Z = 53, so 53 protons. Neutrons = 127 − 53 = 74. Electrons = 53 − (−1) = 54. Symbol ¹²⁷I⁻.
Platinum-195 with 74 electrons (OpenStax Check Your Learning). Z = 78. Neutrons = 195 − 78 = 117. Charge = 78 − 74 = +4, so with q = 4 the page gives 74 electrons. Symbol ¹⁹⁵Pt⁴⁺.
Carbon, no mass number typed. NIST’s largest isotopic composition for carbon is carbon-12 (0.9893), so A = 12: 6 protons, 6 neutrons and 6 electrons.
Chloride ion, no mass number typed. Chlorine-35 (0.7576) is the most abundant isotope, so A = 35. Neutrons = 35 − 17 = 18; electrons = 17 − (−1) = 18. Symbol ³⁵Cl⁻.
Uranium-235, neutral. Neutrons = 235 − 92 = 143; electrons = 92.
Other questions people ask
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.