<Tr> <Td> Np </Td> <Td> orthorhombic </Td> <Td> highly distorted bcc structure . Lattice parameters: a = 666.3 pm, b = 472.3 pm, c = 488.7 pm </Td> <Td> </Td> </Tr> <Tr> <Td> Pu </Td> <Td> monoclinic </Td> <Td> slightly distorted hexagonal structure . 16 atoms per unit cell . Lattice parameters: a = 618.3 pm, b = 482.2 pm, c = 1096.3 pm, β = 101.79 ° </Td> <Td> </Td> </Tr> <P> Many metals adopt close packed structures i.e. hexagonal close packed and face centred cubic structures (cubic close packed). A simple model for both of these is to assume that the metal atoms are spherical and are packed together in the most efficient way (close packing or closest packing). In closest packing every atom has 12 equidistant nearest neighbours, and therefore a coordination number of 12 . If the close packed structures are considered as being built of layers of spheres then the difference between hexagonal close packing and face centred cubic is how each layer is positioned relative to others . Whilst there are many ways that can be envisaged for a regular buildup of layers: </P> <Ul> <Li> hexagonal close packing has alternate layers positioned directly above / below each other, A, B, A, B,...(also termed P6 / mmc, Pearson symbol hP2, strukturbericht A3). </Li> <Li> face centered cubic has every third layer directly above / below each other, A, B, C, A, B, C,...(also termed cubic close packing, Fm3m, Pearson symbol cF4, strukturbericht A1). </Li> <Li> double hexagonal close packing has layers directly above / below each other, A, B, A, C, A, B, A, C,...of period length 4 like an alternative mixture of fcc and hcp packing (also termed P6 / mmc, Pearson Symbol hP4, strukturbericht A3'). </Li> <Li> α - Sm packing has a period of 9 layers A, B, A, B, C, B, C, A, C,...(R3m, Pearson Symbol hR3, strukturbericht C19). </Li> </Ul>

Name an element that exists in a crystal lattice at stp