Electronegativity, symbol χ, is a chemical property that escribes the ability of an atom (or, more rarely, a functional group) to attract electrons (or electron density) towards itself in a covalent bond. First proposed by Linus Pauling in 1932 as a development of valence bond theory, it has been shown to correlate with a number of other chemical properties. Electronegativity cannot be directly measured and must be calculated from other atomic or molecular properties. Several methods of calculation have been proposed and, although there may be small differences in the numerical values of the electronegativity, all methods show the same periodic trends between elements.