How do I find the number of protons, neutrons and electrons?

Protons = atomic number Z. Neutrons = mass number A − Z. A neutral atom has Z electrons; a cation has Z minus its positive charge and an anion Z plus its negative charge. For ³²S²⁻: Z = 16, neutrons = 32 − 16 = 16, electrons = 16 + 2 = 18.

How do I calculate isotope abundances from the average atomic mass?

For two isotopes, let x be the abundance of the first: m₁x + m₂(1 − x) = Ar, so x = (m₂ − Ar)/(m₂ − m₁). For chlorine, 35x + 37(1 − x) = 35.5 gives x = 0.75: 75% ³⁵Cl and 25% ³⁷Cl, a 3 : 1 ratio. Copper (63/65, Ar 63.5) is also 3 : 1, and boron (10/11, Ar 10.8) is 1 : 4.

What is the difference between isotopes, allotropes, isomers and homologs?

Isotopes are atoms of one element with different neutron numbers (¹H, ²H, ³H). Allotropes are different structural forms of one element (O₂ and O₃, diamond and graphite). Isomers are compounds with the same molecular formula but different structures (ethanol and dimethyl ether). Homologs are structurally similar organic compounds differing by CH₂ units (methane and ethane). H₂ and D₂ are the same substance, and ethene and cyclopropane differ by CH₂ but are not homologs.

Which species have 10 or 18 electrons?

10 electrons (isoelectronic with Ne): Ne, HF, H₂O, NH₃, CH₄, Na⁺, Mg²⁺, Al³⁺, F⁻, O²⁻, N³⁻, OH⁻, NH₄⁺, H₃O⁺, NH₂⁻. 18 electrons (isoelectronic with Ar): Ar, HCl, H₂S, PH₃, SiH₄, K⁺, Ca²⁺, Cl⁻, S²⁻, P³⁻, HS⁻, O₂²⁻, plus F₂, H₂O₂, N₂H₄, C₂H₆, CH₃OH, CH₃F, NH₂OH and CH₃NH₂. One mole of NH₄⁺ contains 10 Nₐ electrons and 11 Nₐ protons.

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⚛️Atomic Structure Calculator - Protons, Neutrons, Electrons, Isotope Abundance

An atom has Z protons (the atomic number), N = A − Z neutrons and, as a neutral atom, Z electrons; an ion has Z minus its charge. So ²³₁₁Na⁺ has 11 protons, 12 neutrons and 10 electrons, and ³⁷₁₇Cl⁻ has 17 protons, 20 neutrons and 18 electrons. This tool reads many notations (23Na+, ²³₁₁Na⁺, Cl-37, O2-, ^{37}Cl^-, U-235, chlorine-37, D, T), draws proper nuclide notation with the mass number top left, the atomic number bottom left and the charge top right, and gives Unicode and LaTeX versions to copy. It also works backwards from proton, neutron and electron counts. The isotope section uses NIST/IUPAC isotopic masses and abundances to compute average relative atomic mass from exact masses or from textbook mass numbers: ³⁵Cl (34.969, 75.76%) and ³⁷Cl (36.966, 24.24%) give Ar ≈ 35.45. The inverse problem is solved too. With Ar = 35.5 and isotopes 35 and 37, the abundances are 75% and 25% (3 : 1), shown with the cross (alligation) method, and the three Cl₂ molecules of mass 70, 72 and 74 occur in the ratio 9 : 6 : 1. A classifier decides whether two inputs are isotopes, allotropes (O₂/O₃, diamond/graphite, red/white phosphorus), isomers (ethanol/dimethyl ether) or homologs (methane/propane). It accepts nuclides, elements, common organic names, SMILES and formulas, and explains classic traps such as H₂ vs D₂, ethene vs cyclopropane and phenol vs benzyl alcohol. Finally it counts electrons in any particle, lists 10- and 18-electron species with a quick quiz, and draws shell diagrams for Z = 1–20 and their common ions.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Atomic Structure & Average Atomic Mass

1Pick a mode: nuclide notation, isotopes and atomic mass, the four-way classifier, or electron counts and shell diagrams
2Type a nuclide, isotope data, two substances or a particle formula, or click an example
3Read the proton, neutron and electron counts, the worked average-mass or abundance calculation, the verdict with its reason, and the shell diagram

FAQ

How do I find the number of protons, neutrons and electrons?: Protons = atomic number Z. Neutrons = mass number A − Z. A neutral atom has Z electrons; a cation has Z minus its positive charge and an anion Z plus its negative charge. For ³²S²⁻: Z = 16, neutrons = 32 − 16 = 16, electrons = 16 + 2 = 18.
How do I calculate isotope abundances from the average atomic mass?: For two isotopes, let x be the abundance of the first: m₁x + m₂(1 − x) = Ar, so x = (m₂ − Ar)/(m₂ − m₁). For chlorine, 35x + 37(1 − x) = 35.5 gives x = 0.75: 75% ³⁵Cl and 25% ³⁷Cl, a 3 : 1 ratio. Copper (63/65, Ar 63.5) is also 3 : 1, and boron (10/11, Ar 10.8) is 1 : 4.
What is the difference between isotopes, allotropes, isomers and homologs?: Isotopes are atoms of one element with different neutron numbers (¹H, ²H, ³H). Allotropes are different structural forms of one element (O₂ and O₃, diamond and graphite). Isomers are compounds with the same molecular formula but different structures (ethanol and dimethyl ether). Homologs are structurally similar organic compounds differing by CH₂ units (methane and ethane). H₂ and D₂ are the same substance, and ethene and cyclopropane differ by CH₂ but are not homologs.
Which species have 10 or 18 electrons?: 10 electrons (isoelectronic with Ne): Ne, HF, H₂O, NH₃, CH₄, Na⁺, Mg²⁺, Al³⁺, F⁻, O²⁻, N³⁻, OH⁻, NH₄⁺, H₃O⁺, NH₂⁻. 18 electrons (isoelectronic with Ar): Ar, HCl, H₂S, PH₃, SiH₄, K⁺, Ca²⁺, Cl⁻, S²⁻, P³⁻, HS⁻, O₂²⁻, plus F₂, H₂O₂, N₂H₄, C₂H₆, CH₃OH, CH₃F, NH₂OH and CH₃NH₂. One mole of NH₄⁺ contains 10 Nₐ electrons and 11 Nₐ protons.

Features

Reads 23Na+, ²³₁₁Na⁺, Cl-37, ^{37}Cl^-, U-235, D, T and element names
Proper nuclide notation with copyable Unicode and LaTeX
Identifies an atom or ion from its proton, neutron and electron counts
NIST/IUPAC isotope masses and abundances; average atomic mass from exact masses or mass numbers
Two-isotope abundances from the average mass, with the cross method and X₂ molecule ratios
Isotope / allotrope / isomer / homolog classifier with explanations of common traps
Protons, electrons and neutrons in any particle and in n mol, 10- and 18-electron lists and quiz
Shell (Bohr) diagrams for elements 1–20 and their common ions

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