CSEC Chemistry

1. LEARNING OUTCOMES
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

2. 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

3. 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/
Elements and Compounds
Chapter 4

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

5. 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

6. 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

7. 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

8. 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 Compounds
Chapter 4

9. 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

10. 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 Compounds
Chapter 4

11. 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 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 Compounds
Chapter 4

12.  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

13. 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 Compounds
Chapter 4

14. 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 Compounds
Chapter 4

15. Valencies of some group atoms
Name Valency Symbol
Ammonium 1 NH4
+
Hydroxide 1 OH−
Nitrate 1 NO3
−
Hydrogen carbonate 1 HCO3
−
Sulphate 2 SO4
2−
Carbonate 2 CO3
2−
Elements and Compounds
Chapter 4

16.  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−  MgO
Writing formulae
Elements and Compounds
Chapter 4

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

18. 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
Elements and Compounds
Chapter 4

19. 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
Return
Elements and Compounds
Chapter 4

20. 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

21. 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 Compounds
Chapter 4

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

23. 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 + O2  2 HgO
The equation is now correctly balanced.
2 Hg + O2  2 HgO
Elements and Compounds
Chapter 4

24. 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 Compounds
Chapter 4
2

25. 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
Elements and Compounds
Chapter 4

26. 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

27. 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 Return
Elements and Compounds
Chapter 4

28. 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 Compounds
Solution to Quick check 4
Chapter 4
Q2.

29. References
 Chemistry for CSEC Examinations by
Mike Taylor and Tania Chung
 Longman Chemistry for CSEC by Jim
Clark and Ray Oliver
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