Structure of the atom

Last revised by Mohammad Taghi Niknejad on 23 Jan 2023

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. An atom is the smallest constituent of a chemical element that still maintains the chemical properties of that element.

The following is a simplified overview of the structure of the atom.


The composition of an atom is - in classical physics - principally 'empty space'. Its mass is concentrated within the central nucleus which is composed of a specific number of nucleons. Nucleons are either protons or neutrons, and the total number of nucleons is assigned the symbol A (also known as the mass number). The total number of protons within the nucleus is called the atomic number and assigned the symbol Z (the A and Z symbols are always italicized in atomic physics). Protons and neutrons are each assigned a relative mass of one.

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

Protons and neutrons are not elementary particles; they each consist of quarks (three) and gluons. Two up quarks and one down quark form the proton; two down quarks and one up quark form the neutron. Protons are positively charged; neutrons have no charge (specifically, the algebraic sum of the electric charges of their constituents is zero) 4

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 an atom (having 6 protons) is carbon. However, more than one nuclide may exist for a given element, i.e. carbon may exist with six neutrons (12C) or eight (14C), these atoms of the same element with different numbers of neutrons are called isotopes: in each case, there are still six protons.

Orbiting electrons

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

In classical physics, the electrons orbit the nucleus in a similar way to planets orbiting the sun (Rutherford–Bohr model). They orbit in one of the 'shells' that surround the nucleus: these are named K, L, M, N etc. from the center outwards. The number of electrons each shell can contain is 2n2 (where n = number of the shell).

For the first three 'shells':

  • K: 2 electrons
  • L: 8 electrons
  • M: 18 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. The innermost shell is filled first because it has the lowest energy and must be filled completely before the next outer one can be filled.

NB: It is now known, from quantum theory, that the electrons are not in 'shells' but occupy specific energy levels, i.e. are quantized, but for the purposes of Radiopaedia the simplistic shell model suffices.

History and etymology

The first “scientist” to consider, albeit theoretically, the intimate constitution of the matter was – in the fifth century BCE – the Greek philosopher Democritus. He hypothesized the existence of the atom (from the Greek word for 'indivisible', 'indestructible') as that portion of matter that cannot be further divided 5.

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