Atoms are very small. An atom has a radius of about 1 × 10-10 m. At the centre is a positively charged nucleus made of two kinds of particle, protons and neutrons. Negatively charged electrons surround the nucleus.
The nucleus is far smaller than the atom. Its radius is less than 1/10 000 of the radius of the atom, so it is less than 1 × 10-10 ÷ 10 000 = 1 × 10-14 m across. This means the atom is more than 10000 times wider than its nucleus. Even so, most of the mass of an atom is concentrated in the nucleus.
The electrons are arranged at different distances from the nucleus. These distances are called energy levels. If an atom absorbs electromagnetic radiation, an electron can move further from the nucleus to a higher energy level. If an electron falls closer to the nucleus, to a lower energy level, the atom emits electromagnetic radiation.
