Electron 'd' orbital models, c 1977.
© Science Museum / Science & Society Picture Library
Each of these models consists of a steel rod with support wires and eighteen wooden lobes in five colours. Atomic orbitals are the region surrounding the nucleus of an atom or molecule where an electron is most likely to be found. Quantum theory dictates that the position of an electron cannot be known for certain. Some positions are more likely than others, however, and these more probable positions make up the orbital. An atom or molecule has numerous orbitals, each with a fixed size and shape. They fall into four types, defined by their shapes, and are known as 's', 'p', 'd' and 'f' orbitals. 'D' orbitals have complex shapes as these models show.