Ordinary matter is built from atoms. Each atom contains a very small nucleus surrounded by electrons.

The nucleus contains protons and neutrons. Protons carry positive electric charge, electrons carry negative electric charge, and neutrons have no net electric charge. An atom with equal numbers of protons and electrons is electrically neutral.

Nearly all of an atom's mass is concentrated in the nucleus, while the region occupied by its electrons determines most of the atom's size.

The number of protons in the nucleus is the atomic number $Z$. It identifies the chemical element: every carbon atom has $Z=6$, every oxygen atom has $Z=8$, and changing the number of protons changes the element.

Changing the number of electrons does not change the element but can give the atom a net electric charge; such a charged atom is an ion. Atoms of the same element can also contain different numbers of neutrons.

Electrons should not be pictured as tiny planets following fixed classical paths around the nucleus. Their detailed behavior requires a more refined physical model developed later.

This basic structure provides a common starting point for electron shells and bonding, isotopes, crystalline materials, atomic nuclei and more microscopic descriptions of matter.