The Structure of an Atom

What an atom is made of

Every atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons that move around it in shells (energy levels).

• Protons — positive charge (+1), found in the nucleus.
• Neutrons — no charge (0), found in the nucleus.
• Electrons — negative charge (−1), arranged in shells around the nucleus.

How big is an atom?

• The radius of an atom is about 1 × 10⁻¹⁰ m.
• The nucleus is tiny — its radius is less than 1/10 000 of the atom's radius (about 1 × 10⁻¹⁴ m).
• Almost all of the atom's mass is concentrated in the nucleus.

Why atoms have no overall charge

An atom has the same number of protons as electrons. The positive and negative charges are equal and cancel out, so an atom is neutral overall.

Electrons and energy levels

Electrons sit at different distances from the nucleus (different energy levels). They can move:

• Up to a higher energy level (further out) by absorbing electromagnetic radiation.
• Down to a lower energy level (closer in) by emitting electromagnetic radiation.