The Build an Atom simulation from PhET lets you assemble an atom with your own hands. Drag protons, neutrons and electrons into place and watch the element, charge and mass change instantly. It is free, runs in any browser on a phone, tablet or laptop, and it is one of the quickest ways to understand atomic structure for Tamil Nadu State Board (Samacheer Kalvi) Science, Physics and Chemistry.
What is the Build an Atom simulation?
You build an atom out of protons, neutrons and electrons, then see how the element, charge and mass change. After exploring, you can play a game to test your ideas. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to university
Atom, Symbol and Game
Phone, tablet, Chromebook, PC
English
What students can learn from it, according to PhET’s own learning goals: use the numbers of protons, neutrons and electrons to draw an atom model and identify the element; predict how adding or removing a particle changes the element, charge and mass; work backwards from an element’s name, mass and charge to the particle counts; define proton, neutron, electron, atom and ion; and write an isotope symbol.
Launch the Build an Atom simulation online
Tap or click inside the player to start building. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Build an Atom simulation, step by step
- Start on the Atom screen. Drag protons and neutrons into the nucleus, and electrons onto the orbit rings.
- Watch the live readout. The element name, mass number and net charge update every time you add or remove a particle.
- Change one particle at a time. Add a proton and the element changes. Add a neutron and only the mass changes. Add an electron and only the charge changes.
- Open the Symbol screen. Practise writing the isotope symbol from the particle counts.
- Finish with the Game screen. Answer the challenges to check what you have learned.
Tip for students: before you drag anything, say your prediction aloud (“one more neutron will change the mass, not the element”). Then test it. Predicting first is what turns a game into learning.
Atomic structure key terms explained
These are the words you will meet while using the Build an Atom simulation, with simple meanings.
Atom
The smallest unit of an element that keeps the element’s identity.
Nucleus
The dense centre of the atom that holds the protons and neutrons.
Proton (+)
Positive particle in the nucleus. The number of protons decides the element.
Neutron (0)
Neutral particle in the nucleus. It adds mass and helps keep the nucleus stable.
Electron (−)
Tiny negative particle that moves around the nucleus.
Atomic number (Z)
The number of protons. It is also the element’s place in the periodic table.
Mass number (A)
Protons plus neutrons. Electrons are so light that they are ignored.
Ion
An atom with unequal protons and electrons. Fewer electrons gives a positive cation; more electrons gives a negative anion.
Isotope
Atoms of the same element (same protons) with different numbers of neutrons, such as carbon-12 and carbon-14.
Isotope symbol
A shorthand with mass number at the top left, atomic number at the bottom left, and charge at the top right.
Worked example: A chlorine atom with 17 protons, 20 neutrons and 18 electrons has a mass number of 37 and a charge of 17 − 18 = −1. It is the chloride ion of chlorine-37. Build it in the simulation and check the readout matches.
Common mistakes: Mixing up mass number with atomic mass, thinking electrons change the element (they do not), and confusing an ion (different electrons) with an isotope (different neutrons).
Try the atom calculator (practice what you build)
Enter particle counts for elements 1 to 20 and see the element, mass number, charge and isotope symbol, just like the Symbol screen in the Build an Atom simulation. Or tap a preset.
Proton, neutron and electron compared
| Particle | Charge | Relative mass | Where it is |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | about 1/1836 | Outside the nucleus |
Classroom activities for teachers
The Build an Atom simulation fits a single 40-minute period.
- Element detectives (10 min): give pairs a target element and charge; they build it and record the particle counts.
- Isotope pairs (10 min): build carbon-12, then change only the neutrons to make carbon-14. Ask what stayed the same and why.
- Ion hunt (10 min): start from a neutral atom, remove or add electrons, and write the new charge and symbol.
- Game exit ticket (10 min): students play the Game screen and note one idea they got wrong.
Looking for the matching chapters? Browse the Science and Chemistry books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the atomic structure lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: Test your atomic structure
Score: 0 / 5
Build an Atom simulation: frequently asked questions
Is the Build an Atom simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does Build an Atom work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
What is the difference between an ion and an isotope?
An ion has a different number of electrons from protons, so it carries a charge. An isotope has a different number of neutrons from other atoms of the same element, so it has a different mass.
Which class is the Build an Atom simulation best for?
It suits middle school, high school and early university chemistry. For Samacheer Kalvi students it supports the atomic structure topics in Science and Chemistry.
The Build an Atom simulation turns abstract atomic structure into something you can drag, test and see. Explore it, take the quiz above, and keep building until predicting the element, charge and mass feels easy.