Atoms and Elements

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There is a huge number of different chemicals in our world. Some of these chemicals that will be familiar to you are glass, wood, water, salt, sugar, petrol and plastic. These chemicals are all made out of extremely small particles called atoms, which are joined together in molecules (groups of atoms) or lattices (networks). All of the thousands of different chemicals that exist are made up from a limited number of different kinds of atoms. An element is a simple chemical made up of only one type of atom. Pure gold is an element because it contains gold atoms only. An element cannot be broken down into simpler substances. Water can be broken down into hydrogen and oxygen gases, so it is not an element. Liquid mercury cannot be broken down into simpler substances, so it is an element. Most of the elements are solids. Eleven are gases (hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, etc.) and just two are liquids (mercury and bromine). Most of the elements are metals. Only 20 or so are non-metals (carbon, nitrogen, oxygen, fluorine, neon, phosphorus, sulfur, chlorine, iodine, etc.) Each element has been given a distinct name. Often the name was given to the element by the scientist who discovered the element. Each element also has a chemical symbol – a letter code for the element. For example, C is the code for carbon, N for nitrogen, and S for sulfur. Some elements have single-letter codes, but most have two-letter codes, e.g. Mg for magnesium. Most symbols are derived from the element’s English name, but a few come from the element’s name in another language. An element is a chemical formed out of one type of atom only. A compound is a chemical in which atoms of different elements are joined by chemical bonds. Table salt is a compound in which atoms of the elements sodium and chlorine are bonded together to give sodium chloride crystals. Compounds are new substances produced by chemical reactions. A compound has different physical and chemical properties from those of the elements out of which it is formed. A compound has fixed proportions of different elements (e.g. table salt, NaCl, always has equal numbers of sodium and chlorine atoms). In the compound water (H2O) there are always twice as many hydrogen atoms as there are oxygen ones. The chemical name of a compound often tells you what elements it is made of Chemistry is the study the properties and behaviour of all substances. We can classify substances into pure substances and mixtures. There are 2 kinds of pure substances: 1. Elements are pure substances made of one type of atom only. Elements cannot be changed into simpler chemical substances. 2. Compounds are pure substances that contain different types of atoms bonded together. Compounds can be broken down into elements in chemical reactions. Mixtures are impure substances that occur when pure substances (e.g. salt and sand) are mixed together without a chemical reaction occurring. The substances making up a mixture can usually be separated using physical processes such as filtering, evaporating or distilling. For example, if water is added to a sand and salt mixture, the sand can be filtered out and dried, and the salt regained by evaporating the water.

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