L EARNING O UTCOMES Write formulae to represent ions and molecules Write balanced equations including state symbols to represent chemical reactions referred to in the syllabus Chapter 4 Elements and Compounds

An element When mercuric oxide is heated, it decomposes into oxygen and mercury. However, both oxygen and mercury cannot be broken down into anything simpler. Oxygen and mercury are examples of elements. An element is a substance which cannot be broken down into any simpler substances by chemical means. Elements and Compounds Chapter 4

Elements Elements are the fundamental building blocks of matter in our universe. There are about 92 natural elements and more than 10 man-made elements. Each element has a name and a chemical symbol. A list of elements with their symbols is given in the Periodic Table. The Periodic Table of the Elements Elements and Compounds Chapter 4

Atoms of elements An element is made up of only one kind of atom. For example, gold is made up of only gold atoms. Oxygen is made up of molecules each consisting of two oxygen atoms, while ozone is made up of molecules each containing three oxygen atoms. Elements and Compounds Chapter 4

Metals and non-metals Elements can be classified into metals and non-metals. Metals are usually hard and shiny. They are malleable and ductile and are good conductors of heat and electricity. Non-metals are usually soft and brittle, and are poor conductors of heat and electricity. There are more metals than non-metals. Copper: a metal Sulphur: a non- metal Elements and Compounds Chapter 4

Quick check 1 1. “Magnesium is an element”. Explain what this statement means. 2. “A piece of copper can be broken down into very tiny pieces, hence copper is not an element.” Explain what is wrong with this statement. 3. Give the symbol for each of the following elements. State whether it is a metal or non-metal. (a) Mercury, (b) Lead, (c) Silver, (d) Chlorine, (e) Strontium, (f) Tungsten. Solution Elements and Compounds Chapter 4

Solution to Quick Check 1 1. Magnesium is an element because it cannot be broken down into simpler substances. Magnesium is made up of magnesium atoms and nothing else. 2. A piece of copper can be broken down into very tiny pieces but each tiny piece of copper is still made up of only copper atoms, hence copper is an element. 3. (a) Mercury: Hg (metal), (b) Lead: Pb (metal), (c) Silver: Ag (metal), (d) Chlorine: Cl (non-metal), (e) Strontium: Sr (metal), (f) tungsten: W (metal). Return Elements and Compounds Chapter 4

A Compound A compound is a substance made up of two or more elements chemically combined together. Mercuric oxide is not an element because it is made up of mercury and oxygen. It is called a compound. Element + Compound Elements and Compounds Chapter 4

Atoms of compounds A compound is made up of molecules. A molecule of a compound is made up of two or more different types of atoms chemically joined together. water molecules carbon dioxide molecules methane molecules Elements and Compounds Chapter 4

Formulae of compounds As a compound has a fixed composition, it can be represented by a formula. The formula of a compound shows:  the symbols of the elements present  the ratio of the atoms present For example, water has the formula H 2 O. H 2 O shows 2 hydrogen atoms and 1 oxygen atom Elements and Compounds Chapter 4

Formulae of some compounds CompoundFormulaRatio of atoms Carbon dioxideCO 2 1 carbon atom with 2 oxygen atoms MethaneCH 4 1 carbon atom with 4 hydrogen atoms Sulphuric acidH 2 SO 4 2 hydrogen atoms with 1 sulphur atom and 4 oxygen atoms Magnesium hydroxideMg(OH) 2 1 magnesium atom with 2 oxygen atoms and 2 hydrogen atoms Elements and Compounds Chapter 4

To help us write chemical formulae more easily, we can use the valency of an element. The valency of an element can be treated as the “combining power” of an element. The valency of an element is related to the electronic structure of the atom and the Group number of the element in the Periodic Table. Valency of an element Elements and Compounds Chapter 4

Valencies of some common elements (metals) MetalValencySymbol (cation) (Hydrogen)1H+H+ Sodium1Na + Potassium1K+K+ Copper(I)1Cu + Calcium2Ca 2+ Magnesium2Mg 2+ Zinc2Zn 2+ Copper(II)2Cu 2+ Iron(II)2Fe 2+ Aluminium3Al 3+ Iron(III)3Fe 3+ Elements and Compounds Chapter 4

Valencies of some common elements (non-metals) Non-metalValencySymbol (anion) Chlorine1Cl − Bromine1Br − Iodine1I−I− Oxygen2O2−O2− Sulphur2S2−S2− Nitrogen3N3−N3− Elements and Compounds Chapter 4

Valencies of some group atoms NameValencySymbol Ammonium1NH 4 + Hydroxide1OH − Nitrate1NO 3 − Hydrogen carbonate1HCO 3 − Sulphate2SO 4 2 − Carbonate2CO 3 2 − Elements and Compounds Chapter 4

Metals (or cations) can react with non-metals (or anions). When a formula is formed, the positive charges of the cation have to be balanced by the negative charges of the anion. The total charge of a neutral compound must be zero. Examples: 1. Sodium chloride: Na + + Cl −  NaCl 2. Copper(II) oxide: Cu 2+ + O 2 −  MgO Writing formulae Elements and Compounds Chapter 4

Example 3: magnesium chloride Mg 2+ + (Cl − ) x 2  MgCl 2 Example 4: aluminium oxide (Al 3+ ) x2 + (O 2 − ) x3  Al 2 O 3 Example 5: ammonium sulphate (NH 4 + ) x2 + (SO 4 2 − )  (NH 4 ) 2 SO 4 Writing formula Elements and Compounds Chapter 4

Quick check 3 Write the formula for each of the following compounds: NameFormula Sodium bromide Potassium sulphate Calcium hydrogen carbonate Magnesium nitrate Copper(I) oxide Copper(II) carbonate Iron(II) chloride Iron(III) chloride Aluminium sulphate Solution Elements and Compounds Chapter 4

Solution to Quick check 3 NameFormula Sodium bromideNaBr Potassium sulphateK 2 SO 4 Calcium hydrogen carbonateCa(HCO 3 ) 2 Magnesium nitrateMg(NO 3 ) 2 Copper(I) oxideCu 2 O Copper(II) carbonateCuCO 3 Iron(II) chlorideFeCl 2 Iron(III) chlorideFeCl 3 Aluminium sulphateAl 2 (SO 4 ) 3 Return Elements and Compounds Chapter 4

Writing chemical equations A chemical equation tells us what chemical changes take place during a reaction. It tells us what the reactants (things that react) and what the products (things that are formed) are. A chemical equation must be balanced. This means that the total number and types of atoms on the right side of the equation must be equal to those on the left side of the equation. This is because atoms cannot be created or destroyed. Mercury + oxygen  mercuric oxide [ Reactants ][ product ] Word equation Elements and Compounds Chapter 4

Writing chemical equations Step 1: Write down the chemical formula for each reactant and product: Mercury + oxygen  mercuric oxide Hg + O 2  HgO Step 2: Count the number of atoms on each side of the equation: Left side: 1 Hg atom + 2 O atoms Right side: 1 Hg atom + 1 O atom Elements and Compounds Chapter 4

Balancing chemical equations The equation is not balanced because the right side has 1 less oxygen atom. Step 3: To balance the equation, add 2 in front of HgO, and again count the number of atoms on both sides of the equation: Left side: 1 Hg atom + 2 O atoms Right side: 2 Hg atoms + 2 O atoms Hg + O 2  2 HgO The equation is still not balanced because the left side has 1 less mercury atom. Elements and Compounds Chapter 4

Writing chemical equations Step 4: To balance the equation, add 2 in front of Hg, and again count the number of atoms on both sides of the equation: Left side: 2 Hg atom + 2 O atoms Right side: 2 Hg atoms + 2 O atoms 2 Hg + O 2  2 HgO The equation is now correctly balanced. 2 Hg + O 2  2 HgO Elements and Compounds Chapter 4

State symbols in chemical equations The state symbols tell us the physical states of the reactants and products in a chemical reaction.  (s)  solid state  (l)  liquid state  (g)  gaseous state  (aq)  aqueous state (solution in water) HCl(aq) + CaCO 3 (s)  CaCl 2 (aq) + H 2 O(l) + CO 2 (g) E.g. Write a balanced chemical equation, with state symbols, for the reaction between dilute hydrochloric acid and limestone chips (calcium carbonate). Elements and Compounds Chapter 4 2

Quick check 4 1.Balance the following chemical equations by writing numbers in the blank spaces provided: a)__ Ca + __ O 2  __ CaO b)__ N 2 + __ H 2  __ NH 3 c)__ C 2 H 4 + __ O 2  __ CO 2 + __ H 2 O d)__ Mg + __ Fe 2 O 3  __ MgO + __ Fe e)__ NH 3 + __ O 2  __ N 2 + __ H 2 O f)__ Pb(NO 3 ) 2  __ PbO + __ NO 2 + __ O 2 g)__ KClO 3  __ KCl + __ O 2 h)__ CaCl 2 + __ Na 2 CO 3  __ CaCO 3 + __ NaCl Solution Elements and Compounds Chapter 4

2.Write balanced chemical equations with state symbols for the following word equations: a)Magnesium + Oxygen  Magnesium oxide b)Hydrogen + Oxygen  Water c)Mercuric(II) oxide  Mercury + Oxygen d)Sodium + Oxygen  Sodium oxide e)Ammonia + Sulphuric acid  Ammonium sulphate f)Ammonium chloride + Sodium hydroxide  Sodium chloride + water + ammonia g)Zinc + Hydrochloric acid  Zinc chloride + hydrogen Solution Elements and Compounds Quick check 4 Chapter 4

Solution to Quick check 4 Q1. a)2_Ca + __ O 2  2_ CaO b)__ N 2 + 3_H 2  2_ NH 3 c)__ C 2 H 4 + 3_O 2  2_CO 2 + 2_H 2 O d)3 Mg + __ Fe 2 O 3  3_ MgO + 2_ Fe e)4_NH 3 + 3_O 2  2_N H 2 O f)2_ Pb(NO 3 ) 2  2_PbO + 4_ NO 2 + __ O 2 g)2_ KClO 3  2_KCl + 3_O 2 h)__ CaCl 2 + __ Na 2 CO 3  __ CaCO 3 + 2_NaCl Return Elements and Compounds Chapter 4

a)2Mg(s) + O 2 (g)  2MgO(s) b)2H 2 (g) + O 2 (g)  2H 2 O(l) c)2HgO(s)  2Hg(l) + O 2 (g) d)4Na(s) + O 2 (g)  2Na 2 O(s) e)2NH 3 (g) + H 2 SO 4 (aq)  (NH 4 ) 2 SO 4 (aq) f)NH 4 Cl(s) + NaOH(aq)  NaCl(aq) + H 2 O(l) + NH 3 (g) g)Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g) Return Elements and Compounds Solution to Quick check 4 Chapter 4 Q2.