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.

What decides which element an atom is?

The number of protons, its atomic number Z. Six protons is carbon whatever the neutrons and electrons, so adding a neutron to carbon-12 gives carbon-13, while adding a proton gives nitrogen. With no protons there is no element, only a set of particles, and the builder shows no name or symbol.

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?

Press Practice question for an original question: find a mass number, a charge, a neutron or electron count, or an atomic number, or build a target Z, A and charge on the drawing and press Check build. A typed 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.

Atom Builder

Add and remove protons, neutrons and electrons with buttons, drags or the keyboard, and the element, atomic number Z, mass number A, charge and nuclide symbol update with every particle, with the sums behind Z, A and the charge written out. It covers hydrogen to argon, sets original practice questions you answer or build, and saves the drawing as an SVG or prints it.

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.
What decides which element an atom is?: The number of protons, its atomic number Z. Six protons is carbon whatever the neutrons and electrons, so adding a neutron to carbon-12 gives carbon-13, while adding a proton gives nitrogen. With no protons there is no element, only a set of particles, and the builder shows no name or symbol.
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?: Press Practice question for an original question: find a mass number, a charge, a neutron or electron count, or an atomic number, or build a target Z, A and charge on the drawing and press Check build. A typed 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.

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.