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

> Get protons, neutrons and electrons from 23Na+ or Cl-37, average atomic mass and isotope abundances, isotope/allotrope checks and shell diagrams.

- URL: https://qilingtools.com/en/chemistry/atomic-structure
- Category: Chemistry Tools (https://qilingtools.com/en/chemistry)
- Tags: [Chemistry](https://qilingtools.com/en/tags/chemistry)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

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.

## How to use

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

## 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

## 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.

## Related tools

- [Significant Figures Calculator](https://qilingtools.com/en/math/significant-figures): Free online sig fig calculator: count significant figures digit by digit, round to N sig figs or decimals with six rounding modes, and compute by sig fig rules.
- [Chemical Equation Balancer](https://qilingtools.com/en/chemistry/chemical-equation): Balance chemical equations by the algebraic method: smallest whole-number coefficients, brackets, hydrates, ionic charges, electrons, atom and charge checks.
- [Molar Mass Calculator](https://qilingtools.com/en/chemistry/molar-mass): Molar mass of any formula with IUPAC atomic weights: brackets and hydrates, mass percent of each element, grams ↔ moles ↔ particles and solution molarity.
- [Interactive Periodic Table](https://qilingtools.com/en/chemistry/periodic-table): Interactive periodic table of all 118 elements: colour by category, block, electronegativity, atomic mass, phase or melting point; search and click for details.
- [pH Calculator](https://qilingtools.com/en/chemistry/ph): Free pH calculator for strong and weak acids and bases, polyprotic acids, salts and buffers, with an exact charge-balance solution and species plots.
- [Titration Curve](https://qilingtools.com/en/chemistry/titration): Free acid-base titration curve simulator: exact curves for strong, weak and polyprotic acids and bases, equivalence-point pH, the jump and indicator picks.
- [Gas Law Calculator](https://qilingtools.com/en/chemistry/gas-law): Free gas law calculator: solve PV=nRT for any variable, combined gas laws, gas density, Dalton's partial pressures, gas over water and van der Waals Z.
- [Solution & Dilution Calculator](https://qilingtools.com/en/chemistry/solution): Free dilution calculator: solve C1V1=C2V2, convert molarity, % w/w, ppm and molality, mix solutions, plan serial dilutions and dilute stock acids.