Structure of the Atom

A tiny centre surrounded by electrons

Matter is made of atoms. An atom is not a solid ball: it has a small central region called the nucleus, surrounded by electrons.

The nucleus is positively charged, while electrons are negatively charged. These charges belong to different parts of the atom: the positive charge is concentrated at the centre, and the negative electrons occupy the surrounding space.

Inside the nucleus are protons, which have positive charge, and neutrons, which have no electrical charge. The protons therefore account for the nucleus’s positive charge. Electrons are outside the nucleus, not mixed in with its protons and neutrons.

A nucleus much smaller than its atom

A radius is a distance measured from the centre to the outside of an object. The nuclear radius measures the size of the nucleus; the atomic radius measures the much larger extent of the whole atom, including its surrounding electrons.

The nuclear radius is roughly 10,000 times smaller than the atomic radius. This means the nucleus occupies only a tiny part of the atom’s overall volume: most of an atom is empty space rather than tightly packed particles.

Diagrams usually enlarge the nucleus so that you can see and label it. They show where the parts are, but do not normally show their sizes to scale.

Almost all the mass in one small region

Protons and neutrons have similar masses. An electron has a much smaller mass—roughly one two-thousandth of the mass of a proton or neutron.

Because the relatively heavy protons and neutrons are packed into the nucleus, almost all of an atom’s mass is concentrated there. The electrons contribute very little to its total mass, even though they occupy the region that determines the atom’s overall size.

Keep these two ideas separate: the atom’s size includes the large region surrounding the nucleus, but its mass is concentrated in the tiny nucleus. This makes the nucleus very dense: it contains a large amount of mass in a very small volume.