Chemistry_Grade_10-12 (1).pdf

3 the symbols of the elements can be used to

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3. The symbols of the elements can be used to represent compounds e.g. FeS, NaCl and KBr. These are called chemical formulae . In these three examples, the ratio of the elements in each compound is 1:1. So, for FeS, there is one atom of iron for every atom of sulfur in the compound. 4. A compound may contain compound ions . Some of the more common compound ions and their names are shown below. Name of compound ion formula Carbonate CO 3 2 sulphate SO 4 2 Hydroxide OH Ammonium NH 4 + Nitrate NO 3 Hydrogen carbonate HCO 3 Phosphate PO 4 3 Chlorate ClO 3 Cyanide CN Chromate CrO 4 2 Permanganate MnO 4 5. When there are only two elements in the compound, the compound is often given a suffix (ending) of -ide. You would have seen this in some of the examples we have used so far. When a non-metal is combined with oxygen to form a negative ion (anion) which then combines with a positive ion (cation) from hydrogen or a metal, then the suffix of the name will be ...ate or ...ite. NO 3 for example, is a negative ion, which may combine with a cation such as hydrogen (HNO 3 ) or a metal like potassium (KNO 3 ). The NO 3 anion has the name nitr ate . SO 3 in a formula is sulph ite , e.g. sodium sulphite (Na 2 SO 3 ). SO 4 is sulph ate and PO 4 is phosph ate . 6. Prefixes can be used to describe the ratio of the elements that are in the compound. You should know the following prefixes: ’mono’ (one), ’di’ (two) and ’tri’ (three). CO (carbon monoxide) - There is one atom of oxygen for every one atom of carbon NO 2 (nitrogen dioxide) - There are two atoms of oxygen for every one atom of nitrogen SO 3 (sulfur trioxide) - There are three atoms of oxygen for every one atom of sulfur Important: When numbers are written as ’subscripts’ in compounds (i.e. they are written below the element symbol), this tells us how many atoms of that element there are in relation to other elements in the compound. For example in nitrogen dioxide (NO 2 ) there are two oxygen atoms for every one atom of nitrogen. In sulfur trioxide (SO 3 ), there are three oxygen atoms for every one atom of sulfur in the compound. Later, when we start looking at chemical equations, you will notice that sometimes there are numbers before the compound name. For example, 2 H 2 O means that there are two molecules of water, and that in each molecule there are two hydrogen atoms for every one oxygen atom. 11
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1.3 CHAPTER 1. CLASSIFICATION OF MATTER - GRADE 10 Exercise: Naming compounds 1. The formula for calcium carbonate is CaCO 3 . (a) Is calcium carbonate a mixture or a compound? Give a reason for your answer. (b) What is the ratio of Ca:C:O atoms in the formula? 2. Give the name of each of the following substances. (a) KBr (b) HCl (c) KMnO 4 (d) NO 2 (e) NH 4 OH (f) Na 2 SO 4 3. Give the chemical formula for each of the following compounds.
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