Chemistry revision notes Essay Example for Free

Chemistry revision notes Essay The oxidation state or oxidation number, is a value assigned to a chemical species which represents its actual or nominal electrical charge. The oxidation number in a free or uncombined element is zero. This means that, metallic 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 atom ions is simply the charge on the ions. In most 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 have the oxidation number -1. 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’. 1. 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.