Describe the model now used for the structure of an atom. Refer to the constituent particles including their relative masses and charges.

In an atom, the mass is concentrated in the nucleus which is at the centre of the atom. The nucleus is composed of neutrons and protons, each with a relative mass of 1. By comparison, the third component of an atom is the electrons which orbit the nucleus and have a relatively neglible mass.The charge of the atom is defined by the total charge of its components. Electrons have a charge of -1, protons of +1 and neutrons have no charge.