Structure of the atom

The structure of the atom is key to the majority of the techniques used in radiology and a general understanding of atomic structure is worthwhile.

Nucleus

The atom is composed mainly of empty space. Its mass is concentrated within the central nucleus which is composed of a certain number of nucleons. Nucleons are either protons or neutrons and the total number of nucleons is given the symbol A (also known as the mass number). The total number of protons within the nucleus is called the atomic number and given the symbol Z. Protons and neutrons have a mass of 1, protons are positively charged and neutrons have no charge.

• A (mass number): total number of nucleons = protons + neutrons
• Z (atomic number): number of protons

A great many different configurations of the nucleus may occur. In each case, the atomic number defines the element, i.e. it is the number of protons that determines that carbon (having 6 protons) is carbon. However, more than one nuclide may exist for a given element, i.e. carbon may exist with 6 neutrons (carbon-12) or 8 (carbon-14): in each case, there are still 6 protons.

Orbiting electrons

Orbiting the positively charged nucleus is a cloud of electrons, which have negligable mass and a negative charge. In order for an element to have a neutral overall charge, the number of electrons will be equal to the number of protons in the nucleus.

The electrons orbit the nucleus in a similar way to planets orbiting a sun. They orbit in one of the 'shells' that surround the nucleus: these are named K, L, M, N etc. from the centre outwards. Only a certain number of electrons may occupy a particular shell:

• K: 2 electrons
• L: 8 electrons
• M: 8 electrons

In each atom, the outermost shell is called the valence shell and may only be partially full. It has a role to play in the properties that the atom has.