Chemistry revision notes Essay

The oxidization assign or oxidisation number, is a value appoint to a chemical species which represents its actual or nominal galvanic flash. The oxidation number in a free or uncombined chemical element is zero. This means that, met tout ensembleic magnesium has an oxidation number of zero, and chlorine in Cl2 form has an oxidation state of zero. For simple ionic compounds, the oxidation state of the single fraction ions is simply the charge on the ions. In virtually compounds the oxidation number for hydrogen is +1 and for oxygen is -2.The exceptions include the hydrides (the oxidation state is -1) and the peroxides (the oxidation state for oxygen is -1). In the compounds, halides usually hold up the oxidation number -1. The sum of all the oxidation numbers in a compound shit 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. Semi mannerors A covalent element such as te or germanium which has a higher conductiv ity than that of a distinctive 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 guide 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 veritable 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 face of negatronic devices.This controlled addition of impurities is called doping. 1. doping pure silicon with phosphorus or arsenous anhydride (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 oer after the 4 bonds have been formed. The extra electron is free to conduct a n electric current and the phosphorus-doped silicon becomes a conductor called n-type. B. doping with vitamin B complex or aluminium (group 3 elements).By doping with an element having 3 valence electrons more or less 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 dear(p) 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 liquefied nitrogen- temperatures which are less costly to attain. 4. Superconductors may have a afterlife applications in power transmission and electrically powered forms of transport.