Battery Chemistry
The Bohr model of the atom.
The nucleus consists of positively charged protons and uncharged neutrons.
Negatively charged electrons surround the nucleus.
A neutral atom has equal numbers of protons and electrons.
Some electrons are strongly attracted to the nucleus; others are weakly attracted to the
nucleus. A weakly attracted electron can be associated with another atom which then has
a net negative charge. The atom left by the electron has a net positive charge.
Below is an illustration of the Bohr model of a sodium atom. The electron in the M-shell
is weakly attracted to the nucleus.
[http://www.cbu.edu/~mcondren/bohr.gif]
Images and text below are from: http://www.ewart.org.uk/science/Structures/str13.htm
Sodium
Chlorine
If you look at the diagrams of the sodium and chlorine atoms you can see that sodium normally has eleven
electrons in shells around the nucleus. You can also see that chlorine has seventeen electrons around its
nucleus. Both of these atoms would be happier if they had eight electrons in their outside shells.
The easiest way for sodium to do this is for it to lose an electron - and chlorine can do this by gaining an
extra electron.
The sodium atom and the chlorine atom want to make their outside shells contain only eight electrons. The
sodium atom wants to lose an electron and the chlorine atom wants to gain an electron.
When the two atoms come together the electron from the sodium atom jumps into the gap in the outer shell
of the chlorine atom.
If you look at the diagram the sodium ion now contains only ten electrons and the new chloride ion (an
anion) has eighteen electrons.
However, both ions now have eight electrons in their outer shell. [End of reference] The sodium ion has a
net positive charge of +1 because it has one fewer electrons than protons; the chlorine ion has a net
negative charge of -1 because it has one more electrons than protons.
In a sodium chloride (salt) solution, there are sodium chloride (NaCl) molecules, sodium ions (Na+) and
chloride ions (Cl-). We can write a chemical equilibrium equation:
Na+ + Cl-   NaCl
Balanced charges
Or we can do electron accounting equations:
Na + 1 e-  Na+
Cl + 1 e-  Cl-
Electron accounting
Some metals have a single electron in their outer shell and are good conductors. Examples are copper and
silver. Zinc has two electrons. The weakly attracted electrons of metals can move through the metal under
the influence of an electric field that results if a voltage is applied across the metal. Carbon has four
electrons in its outer shell and is in the form of a stable hexagonal crystal. Its outer shell electrons do not
move easily under the influence of an electric field, so carbon is not a good conductor. This property makes
carbon a good substance to use as a resistor, and many resistors are made of carbon.
The diagrams are from http://www.chemicalelements.com/
Copper
Silver
Zinc
Carbon
The following is taken from Modern Chemistry, QD33, D8, 1962 in the USM Portland Library.
Oxidation-reduction reactions involve a transfer of electrons from the substance oxidized
to the substance reduced. NaNa+ + 1 e- is an oxidation of Na (sodium).
Such reactions that occur spontaneously can be used as sources of electric energy. By
separating electrodes consisting of different substances in an electrolyte solution the
transfer of electrons may take place through a conducting wire connected between the
electrodes. Such a configuration is known as an electrochemical cell. The flow of
electrons is an electric current.
Following is the chemistry of an alkaline battery, such as Duracell or Eveready
Energizer. The battery is discharging. Zinc metal gives up two electrons that are available
to flow through the external electrical circuit. The Z++ ions then combine with two OHions that are available from the reaction at the other electrode. As the battery depletes
there is a decrease in the amount of zinc and MnO2 (manganese dioxide) and an increase
in the amount of Zn(OH)2 and Mn2O3. This type of battery is not rechargeable; that is, we
cannot force the opposite chemical reactions to occur by pumping electrons into the
battery.
e-
+
e+
>
Powdered Zinc Metal
Carbon surrounded by MnO2
Zn + 2OH- Zn(OH)2 +2e-
2MnO2+H2O+2e- Mn2O3+2OHIn solution KOH K++OH-
Following is the chemistry of a Nickel Metal Hydride Battery, from Handbook of
Batteries
29.2 The active material of the positive electrode of the NiMH battery, in the charged
state, is nickel oxyhydroxide (NiOOH). The negative active material, in the charged state,
is hydrogen in the form of a metal hydride. This metal alloy is capable of undergoing a
reversible hydrogen adsorbing-desorbing reaction as the battery is charged and
discharged. An aqueous solution of potassium hydroxide is the major component of the
electrolyte.
e- ->
+
Nickel oxyhydroxide
Metal hydride
MHx+OH- MHx-1+H2O+e-
NiOOH+H2O+e-Ni(OH)2+OHOverall: MH+NiOOHM-Ni(OH)2
The process is reversed by recharging the battery, during which electrons are obtained
from an external electrical source and “pumped” back into the battery.
The metal of the negative electrode is Misch metal, a naturally occurring mixture of rare
earth metals: cesium, neodymium, gadolinium, praseodymium, and yttrium.