Direct Energy, 2018a

206 9.2 Measures of the Ability of Charges to Flow
In SI units, both chemical potential andelectronegativity are measured
in J
atom
, but sometimes they are also expressedin eV
atom or kJ
mol
. As if
the three names, chemical potential, negative of the electronegativity, and
Fermi energy level, weren't enough, this quantity is also known as the
partial molar free energy [60, p. 145].
Electronegativity is usedto describe a collection of atoms, molecules,
or ions all of the same ionization state [131]. Less energy is requiredto
rip the rst electron o an atom than the secondor thirdelectron. The
denition of electronegativity is specic to potential V , in volts, due to the
nucleus andelectrons aroundan atom. For example, we can talk about
the electronegativity, energy requiredto rip o the electron, of a neutral
magnesium atom. We can also talk about the electronegativity, energy
requiredto rip o an electron, from a Mg + ion. The electric eld, and
hence potential V , arounda neutral Mg atom andthe electric eld, and
hence potential V , arounda magnesium ion Mg + are necessarily dierent
because of the number of electrons present. The energies requiredto rip
o the next electron from these atoms will also necessarily be dierent.
So, electronegativity of a material always refers to a specic ionization
state. Electronegativity incorporates both the energy requiredor gained
by ripping o an electron andthe energy requiredor gainedby acquiring an
electron. Qualitatively, it is the average of the ionization energy required
to rip o an electron andthe electron anity releasedwhen an electron
is captured. In the case of the Mg atom from the example above, the
energy gainedby releasing an electron is the signicant term, but that is
not always the case.
In most energy conversion devices, and most chemical reactions, we are
interestedin only the valence electrons. So, even if an atom has dozens
of electrons aroundit andthe energy to rip o each electron is dierent,
we are just interestedin the rst few valence electrons. We will see that
batteries andfuel cells involve energy storedin chemical bonds. Only
the valence electrons are involvedin the reactions of batteries andfuel
cells, so in studying batteries andfuel cells, we are most interestedin the
electronegativity of neutral or singly ionizedatoms.
Equation 9.3 denes electronegativity as the energy required to rip o
the next electron from the atom. Again consider Fig. 9.1. The energy level
known as the valence bandto semiconductor physicists andthe highest
occupiedstate to chemists is lledwith electrons. The next highest band,
calledeither the conduction bandby semiconductor physicists or lowest
unoccupiedstate by chemists, is not lledwith electrons. The electronegativity
according to this denition is the energy required to rip o the next
electron. On average, it is again graphically representedby the Fermi level.