A note on units, symbols and chemical names

Throughout this book, we have, as far as possible, used SI units or derived imits, and the symbols for physical quantities and the printing conventions recommended by the International Union of Pure and Applied Chemistry (lUPAC) [3]. The single significant deviation from this approach is the use of the (A) as the unit of length. 

The reciprocal Angstrom unit is also conventionally used for momentum (A ) throughout the neutron scattering literature. There was neither a common optical spectroscopic alternative (as there was to justify the adoption of the wavenmnber, cm, for the unit of energy) nor chemical reason (as there was to justify the adoption of the atomic mass unit, u or amu, for the unit of mass). For quantities not defined by lUPAC we have mostly used symbols consistent with the neutron scattering literature [14,15] but have, on rare occasions, been forced to invent our own symbol for the sake of clarity. We provide a table of symbols and units (p. xix). 

In handling physical quantities and their units we have used the method of quantity calculus [3] (also called dimensional analysis). The value of a physical quantity is expressed as the product of a numerical value and a unit 

In tabulating numerical values of physical quantities and labelling the axes of graphs, we use the quotient of a physical quantity and a unit in such a form that the values are pure numbers, for example y/cm = 500 (1.5) 

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