How do you build an atom in this atom builder?

Use the minus and plus buttons on each row, type a count, or drag from a plus button onto the drawing; drag a drawn particle off the drawing to remove it. With the drawing focused, P, N and E add a proton, neutron or electron and Shift with the same letter removes one; on a count, the arrow keys step it. The builder holds 0 to 18 protons, 0 to 30 neutrons and 0 to 18 electrons.

How can I identify a nucleus with the interactive atom builder?

Set the proton and neutron counts and read Element, Atomic number Z and Mass number A. Protons identify the element; protons plus neutrons identify its mass number. Repeat with each nucleus within the builder's ranges. Changing electrons changes charge, not which nucleus you built. With no protons there is no element. The tool does not identify nuclei from an uploaded image.

How do you find the mass number and charge?

The mass number A is protons plus neutrons, and the charge is protons minus electrons. Eight protons, eight neutrons and ten electrons give oxygen with A = 16 and a charge of −2, written ¹⁶₈O²⁻. The builder shows each sum with its numbers, such as A = p + n = 8 + 8 = 16, so you can follow the working.

What does nuclide notation show?

The mass number sits top left, the atomic number bottom left, the element symbol in the middle and any charge top right. The hyphen form, such as C-12, gives the symbol and mass number only. A single proton on its own reads ¹₁H⁺, a hydrogen ion with charge +1.

Are the electron rings real orbitals?

No. Electrons are placed in rings of 2, 8 and 8 so they are easy to count. They are not quantum orbitals or a checked electron configuration for every ion, and the drawing is not to scale.

Does the atom builder tell me whether an isotope exists or is stable?

No. It does the bookkeeping for any counts in range, so hydrogen with 30 neutrons shows as H-31 without any comment on whether that nucleus exists. It says nothing about stability, half-life or safety.

What kind of practice questions does it set?

Use Practice question for an original atom-building game or particle-count exercise: find a mass number, charge, neutron or electron count, or atomic number. Some questions ask you to build a target Z, A and charge, then press Check build. A checked answer, right or wrong, comes back with its working.

Where do the element names come from?

Names and symbols for elements 1 to 18 come from a Wikidata snapshot dated 2026-09-21, released under CC0, and are bundled with the page, so nothing is fetched while you build. They are Wikidata's English labels, so element 13 reads aluminium.

Is this the PhET atom builder?

This is an independent atom builder. Its drawing and practice questions are original to this page; it does not supply PhET's simulation, worksheets or worksheet answer keys.

Atom Builder

Build atoms and ions from hydrogen to argon one particle at a time by adding protons, neutrons and electrons, and watch the element, charge and nuclide symbol change as you go. Read the element, mass number, charge and nuclide notation as you edit, try an original practice question, or save the schematic drawing as SVG and print it. Add or remove particles with buttons, drags or the keyboard; the sums behind atomic number Z, mass number A and charge are written out.

particlesreset clears all three

protons0 to 18

neutrons0 to 30

electrons0 to 18

click, drag or use the keyboard

nucleus 0 p + 0 n = 0 particles rings none in slots of 2, 8, 8, drawn for counting only

emptyZ 0, A 0, charge 0

Z= p = 0A= p + n = 0 + 0 = 0charge= p − e = 0 − 0 = 0elementnone yet: add a protonnuclidenone without a protonNothing built yet: one proton makes hydrogenpractice

Optional: a particle-count question to answer or build.

Element: None
Atomic number Z: 0
Mass number A: 0
Charge: 0
Copy

6 protons, 6 neutrons, 6 electronsZ = 6, A = 6 + 6
carbon, C-12, charge 0
8 protons, 8 neutrons, 10 electronscharge 8 − 10
oxygen, A 16, charge −2
Carbon-12, add a neutron, then a proton6 p 7 n 6 e, then 7 p 7 n 6 e
C-13, still carbon; then nitrogen, N-14, charge +1

Sources

Wikidata snapshot 2026-09-21. Element names and symbols for numbers 1 to 18 come from Wikidata, released under CC0 1.0, and are bundled with this page, so it makes no request for them. Names are Wikidata's English labels, so element 13 reads aluminium. The drawing, the formulas and the practice questions are original to this page.

Source files, served from this site: element facts (JSON), query result (JSON), the query, licence notice.

Common questions

How do you build an atom in this atom builder?: Use the minus and plus buttons on each row, type a count, or drag from a plus button onto the drawing; drag a drawn particle off the drawing to remove it. With the drawing focused, P, N and E add a proton, neutron or electron and Shift with the same letter removes one; on a count, the arrow keys step it. The builder holds 0 to 18 protons, 0 to 30 neutrons and 0 to 18 electrons.
How can I identify a nucleus with the interactive atom builder?: Set the proton and neutron counts and read Element, Atomic number Z and Mass number A. Protons identify the element; protons plus neutrons identify its mass number. Repeat with each nucleus within the builder's ranges. Changing electrons changes charge, not which nucleus you built. With no protons there is no element. The tool does not identify nuclei from an uploaded image.
How do you find the mass number and charge?: The mass number A is protons plus neutrons, and the charge is protons minus electrons. Eight protons, eight neutrons and ten electrons give oxygen with A = 16 and a charge of −2, written ¹⁶₈O²⁻. The builder shows each sum with its numbers, such as A = p + n = 8 + 8 = 16, so you can follow the working.
What does nuclide notation show?: The mass number sits top left, the atomic number bottom left, the element symbol in the middle and any charge top right. The hyphen form, such as C-12, gives the symbol and mass number only. A single proton on its own reads ¹₁H⁺, a hydrogen ion with charge +1.
Are the electron rings real orbitals?: No. Electrons are placed in rings of 2, 8 and 8 so they are easy to count. They are not quantum orbitals or a checked electron configuration for every ion, and the drawing is not to scale.
Does the atom builder tell me whether an isotope exists or is stable?: No. It does the bookkeeping for any counts in range, so hydrogen with 30 neutrons shows as H-31 without any comment on whether that nucleus exists. It says nothing about stability, half-life or safety.
What kind of practice questions does it set?: Use Practice question for an original atom-building game or particle-count exercise: find a mass number, charge, neutron or electron count, or atomic number. Some questions ask you to build a target Z, A and charge, then press Check build. A checked answer, right or wrong, comes back with its working.
Where do the element names come from?: Names and symbols for elements 1 to 18 come from a Wikidata snapshot dated 2026-09-21, released under CC0, and are bundled with the page, so nothing is fetched while you build. They are Wikidata's English labels, so element 13 reads aluminium.
Is this the PhET atom builder?: This is an independent atom builder. Its drawing and practice questions are original to this page; it does not supply PhET's simulation, worksheets or worksheet answer keys.

Exact particle bookkeeping for the counts you set: Z is the number of protons, A is protons plus neutrons and the charge is protons minus electrons, with names and symbols for hydrogen to argon from a Wikidata snapshot dated 2026-09-21 (CC0). Electrons are drawn in rings of 2, 8 and 8 for counting, not as real orbitals, and the drawing is not to scale; the tool does not say whether an isotope or ion exists, is stable or is safe. With no protons there is no element, only a set of particles.