Inside an Atom

An atom has a tiny, heavy core called the nucleus, surrounded by electrons moving in shells at different distances. The nucleus holds protons and neutrons, while electrons fill the shells from the inside outward.

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In the picture

Nucleus — the cluster of spheres at the center, holding almost all of the atom’s mass in a very small space.
Proton — a crimson sphere inside the nucleus, carrying a positive charge.
Neutron — a pearl-grey sphere inside the nucleus, carrying no charge at all.
Inner shell — the small brass hoop closest to the nucleus, the first level electrons occupy.
Electron — a blue bead riding the outer hoop, carrying a negative charge.

Examples

A neon sign glows because electrons in the gas jump to a higher shell and give off light when they drop back down.
The shock you feel touching a car door comes from outer-shell electrons rubbed from one surface onto another.

Key takeaways

Nearly all the mass sits in the nucleus, but nearly all the volume is the electron shells.
The outermost shell decides how an atom bonds with other atoms.