Inside an Atom

Almost all of an atom’s mass is packed into a tiny clump at its centre, which takes up nearly none of its volume. The much lighter electrons stay far outside it, in shells set at fixed distances, and the shell closest to the centre fills first.

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In the picture

The dense central clump: the nucleus, made of protons and neutrons.
The blue sphere on the inner hoop: an electron in the shell nearest the nucleus.
The silver band of the outer hoop: a shell, a fixed distance and energy level an electron can occupy.
The blue sphere on the outer hoop: an electron further out, carrying more energy.

Examples

A helium party balloon: each helium atom has two electrons filling its single shell, so the gas hardly reacts with anything.
Copper wire: copper’s outermost electrons are held loosely, so they drift through the metal and carry the current.

Key takeaways

The nucleus holds nearly all the mass in a very small part of the volume.
Electrons occupy shells at set distances; the first holds up to 2, the next up to 8.