The whole lab works on a phone: build the atom, read the chart and take the quiz. A bigger screen shows the atom and its readouts side by side.

Lessons / Chemistry

Build an Atom

Add protons, neutrons and electrons, and watch what you build change. The protons decide the element. The neutrons decide the isotope and whether the nucleus is stable. The electrons decide the charge.

Free Lesson High School Chemistry

Free

Atom builder

Add or remove particles with the buttons under the atom

Carbon-12 Nucleus: stable Charge: 0, neutral atom

Protons

Neutrons

Electrons

Element

6Nonmetal

C Carbon Atomic weight 12.011

Period2 Group14 State at 25 °Csolid Electrons, neutral atom[He] 2s² 2p²

Chart of nuclides

stable
β−
β+ / EC
α
other
N = Z
you

Each square is a nucleus that has been observed, coloured by how it decays. Stable ones form a band that bends toward extra neutrons. Other means fission or a proton or neutron flying off. Tap a square to build it.

Atom info

What you built

ElementCarbon (C) Protonsatomic number, Z6 NeutronsN6 Mass numberA = Z + N12 Electrons6 Chargeprotons − electrons0 Electrons per shell2, 4 Outer-shell electrons4

Stable nucleus

Neutral atom

Build challenge

1 of 12

Built: 0

Try these atoms

Three numbers tell the story

Protons decide the element. This count is the atomic number, Z. Change it and you have a new element.
Protons + neutrons give the mass number, A. Atoms of one element with different neutron counts are isotopes.
Protons − electrons give the charge. If they don't match, the atom is an ion.

Key ideas

What the lab is showing you

Electron shells

ShellHolds up toRule 122 × 1² 282 × 2² 3182 × 3² 4322 × 4²

Shell n can hold at most 2n² electrons. No known atom puts more than 32 electrons in one shell.
For the first 20 elements, shells fill in order: 2, then 8, then 8. Calcium is 2, 8, 8, 2.
From scandium (21) on, electrons go back to finish shell 3 after shell 4 has started, so the simple 2, 8, 8 rule stops working. The lab always shows the real count in each shell.

Isotopes and stability

Isotopes have the same protons but different neutrons: carbon-12, carbon-13 and carbon-14 are all carbon.
Protons push each other apart with electric force. The strong force pulls all protons and neutrons together. Neutrons add pull without adding push.
Light nuclei are stable with about as many neutrons as protons. Heavy ones need extra neutrons: lead-208 has 126 neutrons for 82 protons.
A nucleus with too many or too few neutrons is radioactive. It changes, one decay at a time, until it becomes stable.
Lead (82) is the heaviest element with stable isotopes. Technetium (43) and promethium (61) have none either.
Nuclei with 2, 8, 20, 28, 50 or 82 protons, or those numbers of neutrons (plus 126), are extra stable. They are called magic numbers, and you can see them as lines of stable squares on the chart.

Ions

Charge = protons − electrons. Adding or removing electrons never changes the element.
Cations have lost electrons and are positive: Na⁺, Mg²⁺, Al³⁺. Metals usually form cations.
Anions have gained electrons and are negative. Their names end in -ide: chloride Cl⁻, oxide O²⁻, nitride N³⁻. Nonmetals usually form anions.
Many atoms gain or lose electrons until their outer shell is full, like a noble gas: 8 electrons, or 2 when only the first shell is left.

Is an atom really like this?

The rings come from the Bohr model (1913). They mark energy levels, not paths.
Electrons don't circle the nucleus like planets. Each one is spread out in a cloud around the nucleus. Switch to Cloud to see this.
The nucleus is drawn far too big. A real nucleus is tens of thousands of times smaller across than the whole atom, yet it holds almost all of the mass.

Practice Quiz

Use the atom builder to check your answers

Score: 0 Streak: 0 Answered: 0

Question 1 of 18

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