Building Atoms With PhET: A Practical Guide
The PhET Build An Atom simulation is a browser-based tool from the University of Colorado Boulder. It runs without installation, which means most teachers and students just open a tab and start dragging particles around. The simulation lives at phet.colorado.edu. You can also download it as an HTML5 package if your school blocks phet domains or you need offline access. The core mechanic is straightforward. You have three particle types: protons, neutrons, and electrons. Drag them onto the central canvas. The simulation updates the atom name, element symbol, charge, and mass number in real time. It also shows whether the result is a stable isotope or something that would decay. Most people miss the middle panel at first. That's the particle display area where you can see counts labeled explicitly. Below that, the "Atom Symbol" box updates live. Underneath the symbol, it shows the full isotope notation with the mass number superscripted and the atomic number subscripted. If you build carbon-14, for example, it labels it correctly and flags the neutron count as too high for stability. The simulation doesn't actually simulate radioactive decay in real time, though. It just tells you the isotope is unstable based on known nuclear data.
I spent a semester running this with AP Chemistry students. The first time someone built an atom with 6 protons and 8 neutrons, they assumed the charge would be negative because there were more neutrons. That's a genuinely common misconception. The simulation makes the charge calculation visible when you add or remove electrons separately, which helps, but the connection between neutron excess and nuclear instability isn't intuitive from the interface alone. I had to explicitly point out that the red "unstable" label refers to nuclear binding energy, not electric charge.
What You Can Build — And What You Cannot
The simulation supports all elements from hydrogen through oganesson (element 118). The particle limits are soft constraints rather than hard barriers. You can drag in more protons than any real nucleus could hold, and the simulation will just label it accordingly. This is useful for teaching the concept of atomic number but misleading if students think these superheavy configurations are physically achievable with the current drag mechanics. It does not model molecular bonding. There is no covalent or ionic interaction engine. When you switch to the "Multiple Atoms" view, you can place separate atoms on the canvas side by side, but they do not form bonds. I tried building sodium chloride by placing Na and Cl next to each other and expecting the simulation to show electron transfer. It does not. The ions remain visually separate and the simulation gives no indication of lattice formation or electrostatic attraction.
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Using It for Ion and Isotope Problems
The ion section is where the tool is most useful. Add or remove electrons and watch the charge indicator change. Remove one electron from neon and it becomes Ne+. Add two to oxygen and it shows O2-. The mass number stays constant during ionization, which reinforces that electrons contribute negligibly to atomic mass. I have students work through a set of ten ion-building problems in about 12 minutes using the simulation, compared to roughly 30 minutes when they were doing it on paper. For isotopes, the simulation flags stable versus unstable automatically. This is based on the known nuclear chart data built into the tool. Boron-8, for instance, registers as unstable immediately. The simulation does not tell you the decay mode, though. If a student asks whether boron-8 undergoes beta-plus or beta-minus decay, you have to provide that externally. The tool itself stops at the stability judgment.
Download and Access Options
The primary access method is the browser version at the Colorado PhET site. It requires WebGL and a reasonably modern browser. Internet Explorer is not supported. The downloadable HTML5 package is available from the same page and can be run locally by opening the main index file in any browser. I've seen it run on Chromebooks from 2017 without issues, but older iPads running iOS 12 or earlier sometimes have touch latency that makes precise particle dragging frustrating. Students on those devices tend to overshoot and drop particles in the wrong spot repeatedly. While supervising a lab session, I noticed several students consistently building atoms that the simulation labeled "unstable" and then assuming those atoms could not exist at all. One student spent eight minutes trying to force nitrogen-14 to show as stable by adjusting electron count. The electron count has zero effect on nuclear stability, but the interface does not make that separation obvious. The charge panel and the isotope panel sit close together visually. My workaround was simple. I had students build nitrogen-14, note the stable label, then manually change only the neutron count to 5 or 9 and observe the label flip to unstable. The cause-effect becomes visible when you isolate the variable. I also had them compare the charge panel before and after neutron changes to demonstrate explicitly that charge remained zero throughout. This took about four minutes per student and eliminated the confusion for the rest of the session.
Counter-Intuitive Detail Most Beginners Miss
The mass number shown in the simulation is an integer sum of protons and neutrons. It is not the same as the atomic mass you see on the periodic table. Carbon-12 has a mass number of exactly 12, but its atomic mass is 12.00000 by definition. Carbon-13 has a mass number of 13 but an atomic mass of approximately 13.003355 amu. The simulation displays the mass number, not the precise atomic mass. Students who conflate the two will get confused when they cross-reference with a periodic table and the numbers do not match exactly. The discrepancy comes from nuclear binding energy and the mass defect, which the simulation does not calculate or display. Another detail that trips people up: the electron count does not affect the element identity. You can have 6 protons and 0 electrons and the simulation still labels it as carbon, just with a +6 charge. Some students interpret the empty electron cloud as a system error. It is not. Bare nuclei exist and the simulation handles them correctly within its model.
Limitations Worth Knowing
The simulation treats electrons as discrete draggable particles rather than as a cloud or orbital system. There is no shell model, no quantum numbers, no orbital diagrams. If you are teaching electron configuration beyond the basic count, this tool will not help. It also has no way to represent isotopic abundance. Building carbon-12 and carbon-13 side by side does not show that natural carbon is roughly 98.9% carbon-12. The simulation crashes occasionally on older hardware when switching between the single atom and multiple atoms views while particles are actively being dragged. I have seen it freeze at least twice per class session on a lab computer from 2015. Reloading the page resets progress. There is no autosave feature. If you need to simulate nuclear reactions, decay chains, or orbital structures, this tool is insufficient. For those topics, a dedicated nuclear physics simulator or a quantum chemistry package would be more appropriate. PhET Build An Atom is designed for introductory atomic structure concepts, not advanced physical chemistry.
When to Use It and When to Skip It
Use this simulation when your objective is teaching the relationship between proton count and element identity, how charge arises from electron imbalance, or the difference between isotopes and ions. It works well for grades 9 through 12 and first-year college chemistry. Skip it if you need students to visualize electron orbitals, understand hybridization, or model actual chemical reactions. In those cases, a different tool will save everyone time. The simulation runs indefinitely with no account required. No data is collected unless the browser sends analytics to Colorado's servers, which can be disabled in browser settings. It is free. The download is free. There are no premium features locked behind a paywall.
Quick Reference
URL: phet.colorado.edu/sims/html/build-an-atom
Download: Available from the same page via the "HTML5" download button
Offline capable: Yes, with the downloaded package
Supported elements: 1 through 118
Particle tracking: Protons, neutrons, electrons
Stability data: Based on published nuclear chart values
Estimated class time for a basic activity: 15 to 20 minutes
Browser requirements: Modern browser with HTML5 and WebGL support That is the tool as it exists. It is adequate for its intended scope and nothing more.
