Atoms are not the smallest particles of matter. Atoms are made up of even smaller, subatomic particles called protons, neutrons and electrons. By studying the light given out by elements, scientists have found out about the structure of atoms, and even discovered new elements. The number of protons in the atom of an element determines its place in the periodic table. The number of electrons in an atom is the same as the number of protons. These electrons are arranged in shells or 'energy levels' around the nucleus. The arrangement of electrons determines the chemical properties of an element.

Protons, neutrons and electrons.

Structure of the atom. At the centre of every atom is a nucleus containing protons and neutrons. All atoms of the same element have the same number of protons. This number is used to arrange the elements in the periodic table, beginning with hydrogen, which has just one proton. Electrons are contained in shells around the nucleus. In a neutral atom the total number of electrons is always the same as the number of protons in the nucleus.

Energy levels.

These shells are also called energy levels. The number of shells, and the number of electrons in the outer shell, varies from one element to another. For example, a lithium atom has two shells, with two electrons in the inner shell and one in the outer shell. A carbon atom also has two shells, but with two electrons in the inner shell and four in the outer shell.

Relative masses and charges.

Protons and neutrons have the same mass, which is much larger than the mass of an electron. Protons and electrons have an electrical charge. This electrical charge is the same size for both, but protons are positive and electrons are negative. Neutrons have no electrical charge; they are neutral. These properties are summarised in the table:

Summary of the relative mass and charge of particles.

Particle.Relative mass.Relative charge. Proton. 1. +1. Neutron. 1. 0. Electron. Negligible. -1.

Flame colours.

When the atoms of some metals are heated, they give off coloured light. The colour given off by each metal is different, and can be used to identify them. A small piece of metal compound on the end of a piece of Nichrome wire is introduced into a hot Bunsen flame. The Bunsen flame shows a colour that is characteristic of the metal in the compound. The table shows the colours given out by some group 1 elements:

Flame colours of group 1 elements.

Element.Symbol.Flame colour. Lithium. Li. Red. Sodium. Na. Yellow. Potassium .K. Lilac.

Line spectra.

All atoms give off light when heated, although sometimes this light is not visible to the human eye. A prism can be used to split this light to form a spectrum, and each element has its own distinctive line spectrum. This technique is known as spectroscopy. Some examples of what line spectra look like are shown here: A prism…