Units of electricity
The unit of measure of electrical charge is the Coulomb. When 1 Coulomb of electrical charge passes through a given point per second, we say that 1 ampere of current is flowing. Electrical current is measured in amperes or amps, for short.
96,485 Coulombs = 1 Faraday of electrical charge. It is the charge carried by 1 mole of electrons.
The voltage (measured in volts) is the difference in electrical potential between the positive and negative regions that the electricity flows to and from. It is the 'push', or energy that the electrons are given.
The voltage and the current are related by Ohms Law:
voltage = current x resistance (in ohms)
V = I x R
Giant covalent structures
These can only conduct if there are delocalised electron shells over the whole giant structure. Effectively, among the common substances dealt with this only happens with graphite. Graphite has layers of carbon hexagons in which all of the carbon atoms have one 'p' orbital parallel to the plane of the layer. All of these 'p' orbitals can overlap to make one giant delocalised electron layer.
All metals have delocalised electrons over the whole structure and therefore conduct electricity. The degree of conductivity depends on the ease with which electrons are lost to the delocalised orbitals and the number of electrons provided by the metal atoms.
Giant ionic compounds
In the solid state, although there are charged particles, they are not able to move and therefore cannot carry electrical charge.
If the ionic compound is melted, however, the ions become free to move and can carry the electrical charge. Molten ionic salts are conductors.
When an ionic compound is dissolved in water the ions become free to move and once again they are able to carry charge and conduct electricity. The conductivity of an ionic solution is proportional to the concentration of dissolved ions.
Simple molecular substances
As the molecules are not physically connected there should be no electrical conductivity under any conditions. They are insulators. However, there is a certain amount of disagreement among the 'experts' as regards definitions of conductivity. As usual the situation is not black and white, but rather different shades of grey.
Many substances behave as semiconductors although they are classified as non-conductors. Conductance depends on the applied voltage (electrical potential difference) and even a gas conducts providing the voltage is high enough. Fluorescent light tubes being an example.
The IBO expects students to classify C60 fullerene as a conductor, even though it is simple molecular. There seems to be confusion between C60 (simple molecular) fullerene and the nano tubes made from similarly bonded carbon structures. The best advise for IB students is to follow syllabus requirements, while being able to argue according to the structure.
The requirements for electrical conductivity are charged particles which are free to move. These charged particles may be ions or electrons. Different types of structure have different characteristics resulting in either electrical conductivity, or non-conductivity. The following table summarises the situations that may give rise to conductivity in different structures.
|Giant molecular||only with delocalisation||only with delocalisation||-|