Build an atom simulator

What this simulator shows

Atoms are made of three kinds of particle. Protons (positive) and neutrons (no charge) sit together in the tiny nucleus at the center. Electrons (negative) are arranged in shells around it. This simulator has plus and minus buttons for each: protons from 1 to 20, neutrons from 0 to 24 and electrons from 0 to 20. The picture shows the nucleus and the electrons filling shells.

• The number of protons decides the element. One proton is always hydrogen; six is always carbon.
• The mass number is protons plus neutrons. Electrons are so light they are not counted.
• The charge is protons minus electrons. Equal numbers give a neutral atom; fewer electrons give a positive ion; more electrons give a negative ion.
• Electrons fill shells in order: 2 in the first, then 8, then 8, then the rest in the fourth. This simple pattern works for the first 20 elements.

Common misconceptions

Many students think changing the number of neutrons or electrons changes the element. It does not. Add neutrons to carbon and you get a heavier carbon atom, an isotope. Take away an electron from sodium and you get a sodium ion. Only the number of protons decides which element it is, and the simulator makes this easy to test.

Another common error is about the sign of an ion. Removing electrons makes an ion positive, not negative, because electrons carry the negative charge. Students also picture electrons as tiny planets on fixed tracks. The shell picture is a useful model, not a photograph, and real atoms are mostly empty space with the nucleus far smaller than this drawing suggests.

Where this fits in school

• England, Key Stage 3: pupils use a simple atomic model and learn the differences between atoms, elements and compounds. Many schools introduce protons, neutrons and electrons in Year 9.
• England, GCSE (Years 10 and 11): students learn atomic number, mass number, isotopes, ions and the electron arrangement of the first 20 elements.
• US, grades 7 and 8: many middle school physical science courses introduce the parts of the atom alongside NGSS MS-PS1-1 on modeling atoms and molecules.
• US, high school chemistry: NGSS HS-PS1-1 asks students to use the periodic table and the pattern of outer electrons to predict properties of elements.

A 10-minute atom activity

• Start with 1 proton, 0 neutrons and 1 electron: hydrogen. Ask what the charge is and why.
• Add protons one at a time, adding an electron each time too. Watch the element change and the shells fill.
• Stop at sodium (11 protons, 12 neutrons, 11 electrons). Remove one electron. Ask: "Is it still sodium? What is the charge now?"
• Build chlorine (17 protons, 18 neutrons) and give it 18 electrons. Ask why the charge is negative.
• Finish with two or three Make a real atom challenges.

The simulator lets students build ions that do not occur in nature, such as oxygen with 20 electrons. That is a good prompt for discussion: real atoms usually gain or lose only a few electrons, typically enough to end up with a full outer shell.

Learning more

Science is part of Eurekami's skill map, with lessons that draw science diagrams live and narrate them in English or Spanish. For older students, Exam Prep includes GCSE Combined Science practice papers.