1.1.4—Distribution of mass and charge within an atom

An atom is mostly empty space, but its mass and charge are not spread evenly. Nearly all the mass and all the positive charge are concentrated in a very small nucleus.

Protons and neutrons each have relative mass about 1, whereas an electron has a much smaller relative mass. This is why the nucleus contains nearly all the atom’s mass even though electrons occupy the surrounding region.

The nucleus is positive because it contains protons; the surrounding electrons contribute negative charge. In a neutral atom these charges balance overall, although the positive charge remains concentrated at the centre.

Do not describe an atom as a solid sphere or place electrons inside the nucleus. Mass concentration and charge concentration are related but distinct ideas; particle identities and electron arrangements are handled by neighbouring objectives.