The atom builder sim
Protons (mass 1, charge +1) and neutrons (mass 1, charge 0) sit in the nucleus; electrons (mass 1/1840, charge −1) sit in shells round it.
The atom builder · covers the GCSE, IGCSE, O-Level topic
What the sim shows
Protons (mass 1, charge +1) and neutrons (mass 1, charge 0) sit in the nucleus; electrons (mass 1/1840, charge −1) sit in shells round it. The proton number Z names the element, and the mass number A = protons + neutrons. In nuclide notation A is written on top and Z below, to the left of the symbol. Electrons fill the shells from the inside, 2, then 8, then 8, which holds for the first 20 elements; the shells in use give the Period and the valence electrons the Group (1 or 2, or valence + 10 for 3 to 8; helium is Group 18). An ion forms when electrons move, never protons: a metal loses electrons to give a cation (protons more than electrons), a non-metal gains them to give an anion (electrons more than protons), each reaching a full outer shell. Isotopes have the same protons and electrons, so the same chemistry, and different neutrons. Relative atomic mass is the weighted average: (mass × % for each isotope, added) ÷ 100. The lesson's round abundances are used unless Standard is chosen; standard abundances and atomic weights are IUPAC values typed by the desk. Group and Period always come from the neutral atom.
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
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