A distortion in a one-dimensional metal that makes it energetically favourable for there to be alternating single and double bonds, and hence alternating distances between metal atoms (as opposed to electrons propagating along the chain with the same distance between atoms). This result was proved by Sir Rudolf Peierls (1907–95) in 1955. The Peierls instability is found in certain organic compounds, such as polyacetylene, and is responsible for the Hückel rule for (4n + 2) pi electrons in an aromatic compound breaking down when n is greater than 6.