1. Chapter 4
Elements and Compounds
LEARNING OUTCOMES
Write formulae to represent ions and molecules
Write balanced equations including state symbols
to represent chemical reactions referred to in the
syllabus

2. 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.

3. Chapter 4
Elements and Compounds
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
http://www.chemicool.com/

4. Chapter 4
Elements and Compounds
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.


5. Chapter 4
Elements and Compounds
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

6. Chapter 4
Elements and Compounds
Quick check 1
1.
2.
3.
“Magnesium is an element”. Explain what this statement
means.
“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.
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

7. Chapter 4
Elements and Compounds
Solution to Quick Check 1
1.
2.
3.
Magnesium is an element because it cannot be broken down into
simpler substances. Magnesium is made up of magnesium atoms
and nothing else.
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.
(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).
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8. Chapter 4
Elements and Compounds
A Compound
 Mercuric oxide is not an element because it is made
up of mercury and oxygen.
 It is called a compound.
A compound is a substance made up of two or
more elements chemically combined together.
Element
+
Element
Compound

9. Chapter 4
Elements and Compounds
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

10. Chapter 4
Elements and Compounds
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

11. Chapter 4
Elements and Compounds
Formulae of some compounds
Compound
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
Magnesium hydroxide
H2SO4
2 hydrogen atoms with
1 sulphur atom and 4
oxygen atoms
Mg(OH)2
1 magnesium atom
with 2 oxygen atoms
and 2 hydrogen atoms

12. Chapter 4
Elements and Compounds
Valency of an element



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.

13. Chapter 4
Elements and Compounds
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+

14. Chapter 4
Elements and Compounds
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−

15. Chapter 4
Elements and Compounds
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−

16. Chapter 4
Elements and Compounds
Writing formulae




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: Cu2+ + O2−  CuO

17. Chapter 4
Elements and Compounds
Writing formula

Example 3: magnesium chloride
Mg2+ + (Cl−)x 2  MgCl2

Example 4: aluminium oxide
(Al3+)x2 + (O2−)x3  Al2O3

Example 5: ammonium sulphate
(NH4+)x2 + (SO42−)  (NH4)2SO4

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

19. Chapter 4
Elements and Compounds
Solution to Quick check 3
Name
Formula
Sodium 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
FeCl2
Iron(III) chloride
FeCl3
Aluminium sulphate
Al2(SO4)3
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20. Chapter 4
Elements and Compounds
Writing chemical equations
Word equation
Mercury + oxygen  mercuric oxide
[ Reactants ]




[ product ]
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.

21. Chapter 4
Elements and Compounds
Writing chemical equations
Mercury + oxygen  mercuric oxide
Step 1: Write down the chemical formula for each
reactant and product:
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

22. Chapter 4
Elements and Compounds
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:
Hg + O2  2 HgO
Left side: 1 Hg atom + 2 O atoms
Right side: 2 Hg atoms + 2 O atoms
The equation is still not balanced because the left side has
1 less mercury atom.

23. Chapter 4
Elements and Compounds
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:
2 Hg + O2  2 HgO
Left side: 2 Hg atom + 2 O atoms
Right side: 2 Hg atoms + 2 O atoms
The equation is now correctly balanced.
2 Hg + O2 
2 HgO

24. Chapter 4
Elements and Compounds
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)
E.g. Write a balanced chemical equation, with state symbols, for the
reaction between dilute hydrochloric acid and limestone chips (calcium
carbonate).
2 HCl(aq) + CaCO3(s)  CaCl2(aq) + H2O(l) + CO2(g)

25. Chapter 4
Elements and Compounds
Quick check 4
1.
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

26. Chapter 4
Elements and Compounds
Quick check 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

27. Chapter 4
Elements and Compounds
Solution to Quick check 4
Q1.
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
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28. Chapter 4
Elements and Compounds
Solution to Quick check 4
Q2.
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)
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