Direct Energy, 2018a

202 9.2 Measures of the Ability of Charges to Flow
gigawatt hours [129].
grown larger.
More recently, industry-wide investment has only
9.2 Measures of the Ability of Charges to Flow
The idea of ow of charges is fundamental to both electrical engineering
and chemistry. However, electrical engineers and chemists make dierent
assumptions, and they use dierent notations to describe closely related
phenomena. Engineers prefer to work with solids because solids are durable.
Electrical engineers assume all discussions involve solids unless otherwise
specied. Chemists, however, are quite interested in, and assume all discussions
involve, liquids, with special focus on aqueous solutions. Batteries
and fuel cells typically involve charge ow through both liquids and solids,
so to understand these devices, we have to be familiar with notations and
assumptions from both elds of study.
In solid conductors, valence electrons ow. Inner shell electrons are
assumed to be so tightly bound to atoms that their movements can be
ignored. Nuclei are so much heavier than electrons that their movements
can also be ignored. In solid semiconductors, both valence electrons and
holes ow. Electrical engineers measure the ability of charges to ow in
materials by the electrical conductivity.
Positive and negative ions can ow more easily in liquids than solids, so
chemists are concerned with the ow of both electrons and ions. Semiconductor
physicists tend to use the terms electrical conductivity, resistivity,
Fermi level, and energy gap. Chemists are so interested in the ability of
charges to ow that they have many interrelated measures to describe it.
We'll discuss the following measures:
• Mulliken electronegativity
• Ionization energy
• Electron anity
• Electronegativity
• Chemical potential
• Chemical hardness
• Redox potential
• pH