as low as reasonably achievable (ALARA).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. The first “scientist” to consider, albeit theoretically, the intimate constitution of the matter was – in the fifth century BCE – the Greek philosopher Democritus. are quantized, but for the purposes of Radiopaedia the simplistic shell model suffices. NB: It is now known, from quantum theory, that the electrons are not in 'shells' but occupy specific energy levels, i.e. 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. It has a role to play in the properties that the atom has. In each atom, the outermost shell is called the valence shell and may only be partially full. The number of electrons each shell can contain is 2n 2 (where n = number of the shell). They orbit in one of the 'shells' that surround the nucleus: these are named K, L, M, N etc. In classical physics, the electrons orbit the nucleus in a similar way to planets orbiting the sun (Rutherford–Bohr model).

For an element to have a neutral overall charge, the number of electrons will be equal to the number of protons in the nucleus. Orbiting the positively charged nucleus is a cloud of electrons, which have negligible mass and a negative charge. 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. However, more than one nuclide may exist for a given element, i.e. it is the number of protons that determines that an atom (having 6 protons) is carbon. In each case, the atomic number defines the element, i.e. Protons are positively charged neutrons have no charge (specifically, the algebraic sum of the electric charges of their constituents is zero) 4.Ī great many different configurations of the nucleus may occur. Two up quarks and one down quark form the proton two down quarks and one up quark form the neutron. Protons and neutrons are not elementary particles they each consist of quarks (three) and gluons.

A (mass number): total number of nucleons = protons + neutrons.Protons and neutrons are each assigned a relative mass of one. 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). Nucleons are either protons or neutrons, and the total number of nucleons is assigned the symbol A (also known as the mass number). Its mass is concentrated within the central nucleus which is composed of a specific number of nucleons. The composition of an atom is - in classical physics - principally 'empty space'. The following is a simplified overview of the structure of the atom. An atom is the smallest constituent of a chemical element that still maintains the chemical properties of that element. 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.

Structure of an atom