Electronegativity The Tendency of Atoms to Attract Electrons

When an electron attaches to an atom to form a stable anion, the electron becomes trapped in the potential well of the atom, the energy of the products is lower than the energy of the reactants, and A 0. 

Ionization energy, which measures the difficulty with which an atom gives up an electron to form a cation, is defined in Section 3.3. The energy change of the opposite reaction, in which an atom accepts an extra electron to form an anion, is the electron affinity of the atom. 

For historical reasons, electron affinity has been defined as the amount of energy released when an electron is attached to a neutral atom, but it is always expressed as a positive number. This is a time-honored, if frustrating, exception to the otherwise universal convention adopted by chemists and physicists that energy liberated in a process is assigned a negative number, an exception that must simply be remembered. 

It is difficult to measure the electron attachment energy directly, and to obtain the electron affinity from EA = —AE (electron attachment). It is easier to obtain EA from another measurement that determines the stability of the gaseous anion in the same way that ionization energies are measured. The reaction 

The values of /Ej and EA determine how readily an atom might form a positive or a negative ion. To combine these propensities into a single quantity with predictive value, in 1934, the American physicist Robert Mulliken defined electronegativity 

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