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LEARNING OUTCOMES
Write formulae to represent ions and moleculesWrite 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 CompoundsChapter 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 Elementshttp://www.chemicool.com/
Elements and CompoundsChapter 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 CompoundsChapter 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 CompoundsChapter 4
Quick check 11. “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.
SolutionSolution
Elements and CompoundsChapter 4
Solution to Quick Check 11. 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 CompoundsChapter 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 + Element Compound
Elements and CompoundsChapter 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 CompoundsChapter 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 H2O. H2O
shows 2 hydrogen atoms and 1 oxygen atom
Elements and CompoundsChapter 4
Formulae of some compoundsCompound Formula Ratio of atoms
Carbon dioxide CO2 1 carbon atom with 2 oxygen atoms
Methane CH4 1 carbon atom with 4 hydrogen atoms
Sulphuric acid H2SO4 2 hydrogen atoms with 1 sulphur atom and 4 oxygen atoms
Magnesium hydroxide Mg(OH)2 1 magnesium atom with 2 oxygen atoms and 2 hydrogen atoms
Elements and CompoundsChapter 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 CompoundsChapter 4
Valencies of some common elements (metals)Metal Valency Symbol (cation)
(Hydrogen) 1 H+
Sodium 1 Na+
Potassium 1 K+
Copper(I) 1 Cu+
Calcium 2 Ca2+
Magnesium 2 Mg2+
Zinc 2 Zn2+
Copper(II) 2 Cu2+
Iron(II) 2 Fe2+
Aluminium 3 Al3+
Iron(III) 3 Fe3+
Elements and CompoundsChapter 4
Valencies of some common elements (non-metals)
Non-metal Valency Symbol (anion)
Chlorine 1 Cl−
Bromine 1 Br−
Iodine 1 I−
Oxygen 2 O2−
Sulphur 2 S2−
Nitrogen 3 N3−
Elements and CompoundsChapter 4
Valencies of some group atoms
Name Valency Symbol
Ammonium 1 NH4+
Hydroxide 1 OH−
Nitrate 1 NO3−
Hydrogen carbonate 1 HCO3−
Sulphate 2 SO42−
Carbonate 2 CO32−
Elements and CompoundsChapter 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− NaCl2. Copper(II) oxide: Cu2+ + O2− MgO
Writing formulae
Elements and CompoundsChapter 4
Example 3: magnesium chlorideMg2+ + (Cl−)x 2 MgCl2
Example 4: aluminium oxide(Al3+)x2 + (O2−)x3 Al2O3
Example 5: ammonium sulphate(NH4
+)x2 + (SO42−) (NH4)2SO4
Writing formula
Elements and CompoundsChapter 4
Quick check 3Write the formula for each of the following compounds:
Name FormulaSodium 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 CompoundsChapter 4
Solution to Quick check 3
Name FormulaSodium bromide NaBr
Potassium sulphate K2SO4
Calcium hydrogen carbonate Ca(HCO3)2
Magnesium nitrate Mg(NO3)2
Copper(I) oxide Cu2O
Copper(II) carbonate CuCO3
Iron(II) chloride FeCl2Iron(III) chloride FeCl3Aluminium sulphate Al2(SO4)3
Return
Elements and CompoundsChapter 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 CompoundsChapter 4
Writing chemical equations
Step 1: Write down the chemical formula for each reactant and product:
Mercury + oxygen mercuric oxide
Hg + O2 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 CompoundsChapter 4
Balancing chemical equationsThe 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 atomsRight side: 2 Hg atoms + 2 O atoms
Hg + O2 2 HgO
The equation is still not balanced because the left side has 1 less mercury atom.
Elements and CompoundsChapter 4
Writing chemical equationsStep 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 atomsRight side: 2 Hg atoms + 2 O atoms
2 Hg + O2 2 HgO
The equation is now correctly balanced.
2 Hg + O2 2 HgO
Elements and CompoundsChapter 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) + CaCO3(s) CaCl2(aq) + H2O(l) + CO2(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 CompoundsChapter 4
2
Quick check 41. Balance the following chemical equations by writing numbers in the blank
spaces provided:
a) __ Ca + __ O2 __ CaO
b) __ N2 + __ H2 __ NH3
c) __ C2H4 + __ O2 __ CO2 + __ H2O
d) __ Mg + __ Fe2O3 __ MgO + __ Fe
e) __ NH3 + __ O2 __ N2 + __ H2O
f) __ Pb(NO3)2 __ PbO + __ NO2 + __ O2
g) __ KClO3 __ KCl + __ O2
h) __ CaCl2 + __ Na2CO3 __ CaCO3 + __ NaCl Solution
Elements and CompoundsChapter 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 4Q1.
a) 2_Ca + __ O2 2_ CaO
b) __ N2 + 3_H2 2_ NH3
c) __ C2H4 + 3_O2 2_CO2 + 2_H2O
d) 3 Mg + __ Fe2O3 3_ MgO + 2_ Fe
e) 4_NH3 + 3_O2 2_N2 + 6 H2O
f) 2_ Pb(NO3)2 2_PbO + 4_ NO2 + __ O2
g) 2_ KClO3 2_KCl + 3_O2
h) __ CaCl2 + __ Na2CO3 __ CaCO3 + 2_NaCl Return
Elements and CompoundsChapter 4
a) 2Mg(s) + O2(g) 2MgO(s)
b) 2H2(g) + O2(g) 2H2O(l)
c) 2HgO(s) 2Hg(l) + O2(g)
d) 4Na(s) + O2(g) 2Na2O(s)
e) 2NH3(g) + H2SO4(aq) (NH4)2SO4(aq)
f) NH4Cl(s) + NaOH(aq) NaCl(aq) + H2O(l) + NH3(g)
g) Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)Return
Elements and CompoundsSolution to Quick check 4
Chapter 4
Q2.