The PhET Build an Atom lab is one of the easiest ways to make atomic structure click for high school chemistry students. Instead of memorizing that “protons determine the element,” students drag protons, neutrons, and electrons into an atom and watch the element name, charge, and mass number change in real time.
Build an Atom is a free simulation from the PhET Interactive Simulations project at the University of Colorado Boulder, and it runs in any modern browser, including Chromebooks. This guide gives you a one-period lab procedure, a data table, practice on ions and isotopes, and discussion questions you can copy straight into your lesson.
In this guide7 sections
What the simulation includes
The sim has three screens:
- Atom: students build an atom from buckets of protons, neutrons, and electrons. Optional readouts show the element, net charge, and mass number, and students can switch between an orbit model and an electron cloud model.
- Symbol: the same building space, plus a periodic-table-style symbol showing atomic number, mass number, and charge.
- Game: leveled challenges where students identify or build atoms and ions and earn stars.
The sim focuses on the lightest elements (hydrogen through neon), which is ideal for an introductory unit. It also indicates whether a nucleus is stable or unstable, a nice hook for later work on isotopes and radioactivity.
Learning goals
- Explain that the number of protons determines the identity of an element.
- Calculate mass number from protons and neutrons.
- Predict the charge of an atom or ion from protons and electrons.
- Describe isotopes as atoms of the same element with different numbers of neutrons.
- Read and write isotope and ion notation.
Lab procedure (about 50 minutes)
Part 1: Explore (10 minutes)
Open the Atom screen and give students five minutes of free play with no worksheet. Then ask them to write down three “rules” they discovered. Most will notice that adding a proton changes the element, adding a neutron changes the mass, and adding an electron changes the charge.
Part 2: Build specific atoms (15 minutes)
Have students turn on all readouts and build the following neutral atoms, recording their data in a table like the one below.
| Target | Protons | Neutrons | Electrons | Mass number | Net charge |
|---|---|---|---|---|---|
| Hydrogen-1 | 1 | 0 | 1 | 1 | 0 |
| Helium-4 | 2 | 2 | 2 | 4 | 0 |
| Lithium-7 | 3 | 4 | 3 | 7 | 0 |
| Carbon-12 | 6 | 6 | 6 | 12 | 0 |
| Oxygen-16 | 8 | 8 | 8 | 16 | 0 |
| Fluorine ion (F⁻), mass 19 | 9 | 10 | 10 | 19 | -1 |
| Lithium ion (Li⁺), mass 7 | 3 | 4 | 2 | 7 | +1 |
Give students a blank version with only the first column filled in. The completed table above is your answer key.
Part 3: Isotopes (10 minutes)
Ask students to build carbon-12, then add neutrons one at a time. Prompt them to record what changes (mass number, stability indicator) and what stays the same (element name, charge). Repeat with hydrogen-1, hydrogen-2, and hydrogen-3. Then have them write a one-sentence definition of an isotope in their own words.
Part 4: Symbol screen (5 minutes)
Switch to the Symbol screen and have students build an atom, then check that the symbol shows the atomic number at the lower left and the mass number at the upper left, with charge at the upper right. Ask them to write the symbol for oxygen-18 and for a nitrogen atom with 7 protons, 7 neutrons, and 10 electrons (N³⁻ with mass number 14).
Part 5: Game (10 minutes)
Finish with the Game screen. Students can work in pairs and race to complete a level. This doubles as a review game, similar to the quick-fire formats in our review games for high school roundup.
Discussion questions
- Which particle changes the identity of the element? How do you know?
- Why does adding an electron barely change the mass number?
- Two atoms both have 6 protons. One has 6 neutrons and one has 8. Are they the same element? What do we call them?
- How many electrons would a magnesium ion with a +2 charge have? (Magnesium has 12 protons, so 10 electrons.)
- What happened to the stability indicator when you added too many neutrons? Why might that matter?
Teacher tips
- Hide readouts first: start with the element and charge readouts off and ask students to predict before revealing.
- Use the cloud model on purpose: after students master counting, switch to the cloud view and talk about why scientists moved away from fixed orbits.
- Close with an exit ticket: one atom, one ion, one isotope to identify. Our exit ticket ideas include fast formats.
- Connect to the next sim: once students can build atoms, move on to bonding with the Build a Molecule lab, then to reactions with the Balancing Chemical Equations lab.
For more free virtual labs organized by topic, browse our classroom simulations page.
Frequently asked questions
Is PhET Build an Atom free?
Yes. PhET simulations are free to use online, and the HTML5 version runs on Chromebooks, laptops, and tablets without installing anything.
What grade level is Build an Atom for?
It works well for middle school physical science and introductory high school chemistry. Older students can move quickly to the ions, isotopes, and Game screen.
How long does the Build an Atom lab take?
The full procedure above fits in about 50 minutes. For a shorter class, skip the Symbol screen or assign the Game as homework.
Does the simulation show the whole periodic table?
No. It focuses on the first ten elements, hydrogen through neon, which keeps the counting manageable while students learn the core ideas.
Wrap-up
The Build an Atom lab turns abstract particle rules into something students can see and test. Use it to launch your atomic structure unit, and students will walk away knowing exactly what protons, neutrons, and electrons each do.
