1.1.2—Protons, neutrons and electrons

Atoms contain three main subatomic particles: protons and neutrons in the nucleus, and electrons in shells around the nucleus. The particles differ in relative charge, relative mass and location.

Protons have relative charge +1 and relative mass 1; neutrons have charge 0 and relative mass 1; electrons have charge −1 and a much smaller relative mass. Protons and neutrons are nuclear particles, whereas electrons occupy the space around the nucleus.

Because protons and neutrons are much more massive than electrons, nearly all atomic mass is concentrated in the nucleus. In a neutral atom, equal numbers of protons and electrons make the total charge zero.

Relative values are used to compare the particles; they are not exact masses. A change in electron number makes an ion, but changing the proton number changes the element. Detailed electron configurations belong to the neighbouring objectives.