Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What is an element? An element is a substance that cannot be split into two of more simpler substances by chemical processes or electricity. For example, copper is an element. Sugar is not an element as it can be broken down into carbon and water.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Chemical Symbols of Elements Chemists use symbols to represent elements. For example, O represents oxygen while Fe represents iron. ElementSymbolElementSymbol CalciumCaMercuryHg CarbonCNeonNe HydrogenHSiliconSi IronFeSodiumNa

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Classification of Elements – Metals and Non-metals There are two major groups of elements – metals and non-metals. Iron is a metal. Oxygen is a non-metal. There are some elements called metalloids which behave like both metals and non-metals. Metals and non-metals are grouped separately on the Periodic Table.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Differences between metals and non-metals MetalsNon-metals Shiny (lustrous)Dull Solid at r.t.p (except Hg)Usually gases or liquids at r.t.p Malleable, sonorous, ductileBrittle (if solid) Usually high melting and boiling points Usually low melting and boiling points Good conductors of heatUsually poor conductors of heat Good conductors of electricity Usually poor conductors of electricity

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Atoms An element is made of tiny particles called atoms. The atoms of an element is different from that of another element.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Molecules Very few elements exists as atoms besides elements such as helium and neon. Most elements exist as molecules. For example, hydrogen is H 2. Ozone is O 3.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Hydrogen (H 2 ) is a diatomic molecule. Ozone (O 3 ) is a triatomic molecule. Molecules consisting of a few atoms are called polyatomic molecules. Molecules

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Compounds A compound is a pure substance that contains two or more elements chemically combined. CompoundElements present Common salt (sodium chloride) Sodium, chlorine Marble (calcium carbonate)Calcium, carbon, oxygen Copper(II) sulphateCopper, sulphur, oxygen Hydrogen chlorideChlorine, hydrogen

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we name compounds? Rule 1 A compound made up of two elements has a name that ends in -ide. Sodium chloride — made up of the elements sodium and chlorine Zinc oxide — made up of the elements zinc and oxygen Carbon dioxide — made up of the elements carbon and oxygen

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we name compounds? Rule 2 A compound that contains hydroxide ions, OH – (a negatively charged ion made up of oxygen and hydrogen) is named a hydroxide. Potassium hydroxide — contains potassium ions and hydroxide ions

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we name compounds? Rule 3 A compound that contains a negatively charged polyatomic ion containing oxygen usually has a name ending in –ate. Copper(II) sulphate — contains oxygen atoms in the sulphate ion Sodium nitrate — contains oxygen atoms in the nitrate ion

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Fixed Composition of Compounds For example, water (H 2 O) is a compound made only by joining together two atoms of hydrogen to one atom of oxygen. That is, the ratio of hydrogen atoms to oxygen atoms in water is always 2 : 1. A compound is made up of different elements chemically combined in a fixed ratio.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Chemical Formula of a Compound A compound can be represented by a chemical formula. The chemical formula states the types of atoms (i.e. elements) in the compound, the ratio of the different atoms in the compound. Types of atoms: hydrogen, oxygen Ratio of H to O = 2:1

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we write chemical formula? Rule 1 For many compounds that contain both metallic and non-metallic elements, the symbol of the metallic element is written first. calcium oxide (CaO) sodium chloride (NaCl) magnesium carbonate (MgCO 3 )

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we write chemical formula? Rule 2 The number of atoms is written as a subscript, to the right of the atom’s symbol. water (H 2 O, not H2O or 2HO) magnesium carbonate (MgCO 3, not MgCO 3 or MgC 3 O)

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we write chemical formula? Rule 3 It is not necessary to write the subscript ‘1’. water (H 2 O, not H 2 O 1 ) calcium oxide (CaO, not Ca 1 O 1 )

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we write chemical formula? Rule 4 The oxygen atom is usually written at the end of the formula. water (H 2 O, not OH 2 ) carbon dioxide (CO 2, not O 2 C) nitric acid (HNO 3, not O 3 NH)

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How do we calculate the number of atoms in a formula? Pb(NO 3 ) 2 Number of nitrogen (N) atoms = 1 x 2 = 2 Number of lead (Pb) atoms = 1 Number of oxygen (O) atoms = 3 x 2 = 6

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Heat can be used to break down compounds into elements or simpler compounds. Such a chemical reaction is called thermal decomposition. Compounds can be decomposed mercury(II) oxide oxygen

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What are mixtures? Mixtures are formed when two or more substances are added together without chemical bonds being formed. Examples of mixtures include muddy water and air. Air is made up of gases such as nitrogen and oxygen mixed together.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Different Types of Mixtures The ratios of the components of a mixture are not fixed. They can be present in any ratio. A mixture can exist in various ways: a) two elements, b) a mixture of two compounds, c) a mixture of one element and one compound.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. A mixture of 2 elements A mixture of two elements, e.g. neon (Ne) and hydrogen (H 2 )

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. A mixture of 2 compounds A mixture of two compounds, e.g. water vapour (H 2 O) and carbon dioxide (CO 2 )

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. A mixture of 1 element and 1 compound A mixture of one element and one compound, e.g. hydrogen (H 2 ) and ammonia (NH 3 )

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Comparison of Mixtures with Compounds A mixture is different from a compound in the following ways: S eparation P roperties E nergy changes C omposition

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Comparison of Mixtures with Compounds - Separation The components of a mixture can be separated by physical methods, e.g. filtration, distillation or chromatography. CompoundMixture The elements in a compound can only be separated by chemical reactions or by using electricity.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Comparison of Mixtures with Compounds - Properties The chemical properties of a mixture are the same as those of its components. CompoundMixture The physical and chemical properties of a compound are different from those of the elements in the compound.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Comparison of Mixtures with Compounds – Energy Changes No chemical reaction takes place when a mixture is formed — usually there is little or no energy change. CompoundMixture A chemical reaction takes place when a compound is formed — usually there is an energy change, e.g. the reactants get hot.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Comparison of Mixtures with Compounds - Composition The components of a mixture can be mixed in any proportion. CompoundMixture The elements in a compound are always combined in a fixed proportion (by mass).

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Alloys Alloys are widely used and in great demand because they tend to be stronger than pure metals. An alloy is a mixture of metals with other elements (usually metals but sometimes non-metals such as carbon). An alloy is an example of a mixture.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Examples of Alloys lead, tinsolder aluminium, copper, magnesiumduralumin copper, tinbronze copper, zincbrass iron, chromium, nickel, carbonstainless steel iron, carbonsteel ConstituentsAlloy