The sum of all the oxidation numbers in a compound have to be equal to zero.
The sum of all the oxidation states in a complex ion is the same as the charge on the ion.
Semiconductors
A covalent element such as silicon or germanium which has a higher conductivity than that of a typical non-metal but a much lower conductivity than that of a metal is described as a semiconductor.
Semiconductors are also referred to as metalloids and they occur at the division between metals and non-metals in the Periodic Table.
Key facts : 1. The electrical conductivity of
semiconductors increases with
increasing temperature.
2: Doping pure crystals of silicon or
germanium with certain other elements
produces 2 types of semiconductors: n- type and p-type
semiconductors.
The addition of small quantities of certain substances to pure silicon greatly enhance its conductivity and makes possible the construction of electronic devices. This controlled addition of impurities is called ‘doping’.
- doping pure silicon with phosphorus
or arsenic (group 5 elements)
these elements have 5 valence electrons, a few of the silicon atoms are replaced by P, As atoms --- 1 electron is left over after the 4 bonds have been formed. The extra electron is free to conduct an electric current and the phosphorus-doped silicon becomes a conductor called n-type.
B. doping with boron or aluminium (group 3 elements). By doping with an element having 3 valence electrons some of the silicon atoms are replaced by boron atoms, but because each boron has only 3 electrons, one of the four bonds to each boron atom has only 1 electron in it. We can think of this as a vacancy or hole in the bonding orbital. An electron from a neighbouring atom can move in to occupy this vacancy. As a result of this movement this type of conductor is called p-type.
Superconductors
1. Superconductors are a special class of
materials that have zero electrical
resistance at temperatures near
absolute zero.
2. Achieving temperatures near absolute
zero is difficult and costly so
application of superconduction at these
temperatures is impractical.
3. Recently superconductors have been
discovered which have zero resistance
up to temperatures above the boiling
point of liquid nitrogen- temperatures
which are less costly to attain.
4. Superconductors may have a future
applications in power transmission and
electrically powered forms of
transport.