← Atoms and Molecules

Lesson 2 of 7 · Textbook 8.1

Inside the atom

“Cannot be cut” turned out to be wrong. There are three kinds of particle inside, and where each one sits matters as much as what it is.

Three sub-atomic particles

An atom contains three types of sub-atomic particle: the proton, the neutron and the electron.

Where each one sits Protons and neutrons form the nucleus of the atom.
Electrons move around the nucleus, at high speed.
Particle Relative charge Relative mass Position
proton (p)+11within the nucleus
neutron (n)01within the nucleus
electron (e)−11/1840around the nucleus
Two things that table is quietly telling you

Why an atom is neutral. In an atom the number of protons equals the number of electrons, so the +1s and the −1s cancel exactly. The atom is electrically neutral. Neutrons have no charge, so they never affect it either way.

Where the mass is. An electron has only 1/1840 of the mass of a proton — so small that it is treated as negligible. The mass of an atom is therefore contributed almost entirely by the protons and neutrons in its nucleus.

Build an atom

Set the protons and neutrons and watch what changes — and, more importantly, what does not.

Atom builder

C carbon
Proton number—
Nucleon number—
Electrons—
Overall charge—

Red + = proton, grey 0 = neutron, blue = electron. The electron count follows the proton count automatically — that is what keeps the atom neutral.

The planetary model

An atom cannot be seen, so scientists represent it with models. The one your textbook uses is the planetary model: electrons move around the nucleus rather as planets move around the sun.

Two things it shows well:

The models it replaced

How the model changed

Science involves constant revision so that it stays current and reliable. As new evidence is found, what scientists accept today may be changed or replaced in future.
A model is not a photograph No one knows what an atom looks like exactly. The planetary model is only a simplified representation that allows atoms to be studied scientifically. Saying “an atom looks like this” loses marks; saying “this model represents an atom as…” does not.

Check yourself