Shell Rush

Shell Rush: electronic structure and the lowest available energy level

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

Electrons fill the lowest available energy level first

The electrons in an atom are not scattered at random. They sit in energy levels, also called shells, around the nucleus. For AQA GCSE Chemistry (spec point 4.1.1.7) the key rule is that the electrons in an atom occupy the lowest available energy levels: the innermost shells that still have space. You can answer exam questions using either word, energy levels or shells.

In Shell Rush the atom is drawn as an energy well: four steps going down to the nucleus. The lower the step, the lower the energy level and the closer it is to the nucleus. You point at a shell and hold to pour electrons in. Pour into a higher shell while a lower one still has space and the electron rolls straight down into the lower shell, so you see the rule happen in front of you.

How many electrons fit in each shell?

For the first 20 elements, which is all you need at GCSE, learn the pattern 2, 8, 8:

• The first shell holds up to 2 electrons and always fills first.
• The second shell holds up to 8 electrons.
• The third shell takes 8 electrons before any go into the fourth shell.
• Any electrons left over go into the fourth shell. Of the first 20 elements, only potassium (2,8,8,1) and calcium (2,8,8,2) have electrons there.

The number of electrons in an atom equals its atomic number (proton number), not its mass number. In the game you choose the right number from an AQA-style periodic table tile before each build: argon-40 has 18 electrons, not 40.

Numbers and diagrams: sodium is 2,8,1

The electronic structure of an atom can be written as numbers or drawn as a diagram. The spec’s example is sodium, 2,8,1: two electrons in the lowest energy level, eight in the second energy level and one in the third.

To draw the diagram, put a small circle for the nucleus in the middle (you can write the symbol in it), draw one circle for each shell that has electrons, and show each electron as a cross or dot on its circle. Never draw empty shells. Every time you finish an atom in Shell Rush, the 3D energy well flattens and the camera looks straight down, so the atom turns into exactly the pencil diagram you draw on paper, with its numbers beside it. It is a neat way to see how a flat 2D diagram represents a 3D atom.

Each finished atom also shows its link to the periodic table: the number of shells is the period, and the number of outer electrons is the group (a full outer shell means Group 0).

How to play Shell Rush

Point at a shell in the energy well (or press 1 to 4, use the arrow keys or the shell buttons) and hold the mouse button, your finger or Space to pour. A tap adds one electron and holding pours a stream. Each shell seals with a chime when it is full. There are six levels: shell 1 (hydrogen and helium), shell 2 (lithium, carbon and neon), the atomic number (sodium, aluminium and argon), the Inspector, where you mark five students’ diagrams and tap the mistake, shell 4 (potassium and calcium), and a 75 second Shell Rush with random elements from the first 20. Mistakes cost points and open a short explanation with a link to the lesson. Build combos, beat your best score and fill all 20 elements on the periodic table.

Exam tip: asked why the first two electrons are in the first shell? Say that electrons occupy the lowest available energy level, and that the first shell is the lowest energy level and holds 2 electrons, so it fills first. Always check that your numbers add up to the atomic number.