CHAPTER 4
Elements, Compounds and
Mixtures
© 2013 Marshall Cavendish International (Singapore) Private Limited
Chapter 4 Elements, Compounds
and Mixtures
4.1 Elements
4.2 Compounds
4.3 Mixtures
2
4.1
Elements
Learning Outcomes
At the end of this section, you should be able to:
• define an element;
• state the chemical symbols for a number of elements.
3
4.1
Elements
What is an Element?
It is a pure substance that cannot be broken down
into two or more simpler substances by chemical
processes.
Examples: Carbon, copper, oxygen
Sugar is not an element.
When heated, it breaks down
into carbon and water
vapour.
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4.1
Elements
Is Water an Element?
Water is not an element.
It can be broken down into hydrogen and oxygen.
Hydrogen and oxygen are elements.
URL
5
4.1
Elements
Chemical Symbols of Elements
• Chemists use chemical symbols to represent
elements.
• Each symbol is unique, consisting of one or
two letters.
Element
Symbol
Element
Symbol
calcium
Ca
mercury
Hg
carbon
C
neon
Ne
hydrogen
H
silicon
Si
iron
Fe
sodium
Na
The names and symbols of all known elements are found
in Periodic Table.
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4.1
Elements
Using the Periodic Table, find the symbols
of the following elements.
Element
Symbol
Aluminium
Al
Magnesium
Mg
Lead
Pb
Copper
Cu
Nitrogen
N
Neon
Ne
Bromine
Br
Sulfur
S
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4.1
Elements
Classification of Elements
Based on their properties, elements are classified into:
• metals;
e.g. sodium, magnesium, calcium
• non-metals;
e.g. hydrogen, carbon, chlorine
• metalloids.
e.g. boron, silicon, germanium,
arsenic
(Metalloids have properties of metals and non-metals.)
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4.1
Elements
Differences Between Metals and Non-Metals
Properties
Metals
Metalloids
Non-metals
Appearance
Shiny (lustrous)
Shiny
(lustrous)
Dull (nonlustrous)
Physical state
at r.t.p.
Mostly solids
(except mercury)
Solids
Gases, volatile
liquids or solids
Melting and
boiling points
High (except for
High
sodium, potassium
and mercury)
Low (except for
carbon and
silicon)
Heat and
Electrical
Conductivity
Good
Poor (except
carbon in the
form of graphite
and diamond)
Moderate
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4.1
Elements
Differences Between Metals and Non-Metals
Properties
Metals
Ductility and  Ductile (can be
Malleability
drawn into wires)
 Malleable (can be
hammered into
different shapes
without breaking
 Sonorous (makes a
ringing sound when
struck)
Metalloids Non-metals
Brittle (easily Brittle if solid
broken when
hammered)
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4.1
Elements
Atoms and Molecules
• An element is made up of tiny particles called atoms.
• Atoms are the smallest particles of an element that have
the chemical properties of that element.
• The atoms of an element are different from that of another
element.
neon atom
gold atom
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4.1
Elements
Atoms and Molecules
A molecule is a group of two or more atoms that
are chemically combined (joined together).
Elements like hydrogen, oxygen, ozone and sulfur
exist as molecules.
Diatomic molecules are those
that are formed by the
combination of two atoms.
E.g. hydrogen, oxygen, nitrogen.
12
4.1
Elements
Polyatomic Molecules
Polyatomic molecules consists of three or more
atoms that are chemically combined together.
phosphorus (P4)
ozone (O3)
sulfur (S8)
13
Chapter 4 Elements, Compounds
and Mixtures
4.1 Elements
4.2 Compounds
4.3 Mixtures
14
4.2
Compounds
Learning Outcomes
At the end of this section, you should be able to:
• define a compound;
• describe the difference between elements and
compounds;
• deduce the formula of a compound from the
ratio of elements present.
15
4.2
Compounds
What is a Compound?
A compound is a pure substance that contains two
or more elements that are chemically combined in a
fixed ratio.
What are compounds made of?
sodium
ion
water
molecule
chloride
ion
• molecules
• ions
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4.2
Compounds
Properties of Compounds
A compound has different properties from
the elements that form it.
Example:
When magnesium is burned, it combines with
oxygen in the air to form a compound called
magnesium oxide.
magnesium
shiny grey solid
+ oxygen
colourless gas
 magnesium oxide
white solid
URL
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4.2
Compounds
Names of Compounds
Each compound has a chemical name indicating
the elements it contains.
Compound
Elements present
sodium chloride
sodium, chlorine
carbon dioxide
carbon, oxygen
zinc oxide
zinc, oxygen
calcium carbonate (marble)
calcium, carbon, oxygen
copper(II) sulfate
copper, sulfur, oxygen
hydrogen chloride
hydrogen, chlorine
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4.2
Compounds
Chemical Formula
A compound can be represented by a chemical
formula.
The chemical formula
shows:
• the types of elements
present;
• the ratio of the different
atoms present.
Chemical formula for lead(II) nitrate
Pb(NO3)2
Pb
1 lead
atom
(N)2 (O3)2
1×2=
2 nitrogen
atoms
3×2=
6 oxygen
atoms
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4.2
Compounds
Composition of Compounds
Every compound has fixed compositions of
the different elements it contains.
Chemical formula can be deduced by the ratio
of atoms present in a compound.
Compound
Chemical formula
Ratio of atoms
hydrogen chloride
HCl
H : Cl = 1 : 1
carbon dioxide
CO2
C:O=1:2
carbon monoxide
CO
C:O=1:1
sulfuric acid
H2SO4
H:S:O=2:1:4
sulfurous acid
H2SO3
H:S:O=2:1:3
ethanol
C2H5OH
C:H:O=2:6:1
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4.2
Compounds
Decomposition of Compounds
A chemical reaction is necessary to separate the
elements in a compound.
In thermal decomposition, heat is used to break down
compounds into elements or simpler compounds.
oxygen gas
mercury(II)
oxide
mercury
Compounds can also be broken down by using electricity.
This process is known as electrolysis.
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Chapter 4 Elements, Compounds
and Mixtures
4.1 Elements
4.2 Compounds
4.3 Mixtures
22
4.3
Mixtures
Learning Outcomes
At the end of this section, you should be able to:
• define a mixture;
• differentiate between compounds and mixtures.
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4.3
Mixtures
What are Mixtures?
Mixtures are made up of two or more
substances that are not chemically combined.
Examples of mixtures:
• petrol
• air
• muddy water
• alloys such as brass, bronze and steel
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4.3
Mixtures
Types of Mixtures
A mixture can exist as a combination of
•
two elements;
•
two compounds;
•
one element and one compound.
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4.3
Mixtures
Mixture of Two Elements
Example:
Neon and Hydrogen
Neon is monatomic.
Hydrogen is diatomic.
hydrogen
(H2)
neon
(Ne)
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4.3
Mixtures
Mixture of Two Compounds
Example:
Water and Carbon dioxide
A water molecule is made up
of two hydrogen atoms and
one oxygen atom.
A carbon dioxide molecule is
made up of two oxygen atoms
and one carbon atom.
water
(H2O)
carbon
dioxide
(CO2)
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4.3
Mixtures
Mixture of an Element and a Compound
Example:
Hydrogen and Ammonia
Hydrogen is an element.
Ammonia is a compound.
Each ammonia molecule
consists of three hydrogen
atoms and one nitrogen
atom.
hydrogen
(H2)
ammonia
(NH3)
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4.3
Mixtures
Differences between Mixture and Compound
Mixture
Compound
Separation
The components of a mixture
can be separated by physical
processes such as magnetic
separation, filtration or
distillation.
A compound can only be
broken down into its elements
or into simpler compounds by
chemical processes (e.g.
thermal decomposition or
electrolysis).
Properties
The chemical properties of a
mixture are the same as
those of its components.
The physical and chemical
properties of a compound are
different from those of its
constituent elements.
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4.3
Mixtures
Differences between Mixture and Compound
Mixture
Compound
Energy changes
No chemical reaction takes
place when a mixture is
formed — usually there is
little or no energy change.
A chemical reaction takes
place when a compound is
formed — usually there is an
energy change, e.g. the
reactants get hot or cold.
Composition
The components of a mixture
can be mixed in any
proportion.
The elements in a compound
are always combined in a
fixed proportion.
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4.3
Mixtures
Example:
Comparing a mixture of iron filings and sulfur
with a compound iron(II) sulfide
Separation
Mixture
Can be separated by
physical methods
Compound
Can only be separated by
chemical methods or by
using electricity
E.g. In a mixture of iron
E.g. Iron in iron(II) sulfide
filings and sulfur, iron
cannot be simply separated
filings can be separated
with a magnet.
with the use of a magnet.
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4.3
Mixtures
Example:
Comparing a mixture of iron filings and sulfur
with a compound iron(II) sulfide
Physical and Chemical Properties
Mixture
Compound
Has the same chemical
properties as its
components
Has different physical and
chemical properties from
those of its components
E.g. In a mixture of iron
filings and sulfur, each
component retains their
colour, texture and
magnetic property (iron).
E.g. Iron(II) sulfide has a
different appearance from
iron filings or sulfur; has no
magnetic property.
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4.3
Mixtures
Example:
Comparing a mixture of iron filings and sulfur
with a compound iron(II) sulfide
Energy Change
Mixture
Compound
No chemical reaction
takes place. Little or no
energy change.
Chemical reaction takes
place. Heat and light
usually given off.
E.g. Mixture of iron filings
and sulfur – no chemical
change takes place
without heating.
E.g. When a mixture of iron
filings and sulfur is heated,
it glows red hot to form
iron(II) sulfide.
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4.3
Mixtures
Example:
Comparing a mixture of iron filings and sulfur
with a compound iron(II) sulfide
Composition
Mixture
Compound
• No fixed composition.
• Fixed composition.
E.g. A mixture of iron
filings and sulfur can
have more or less of
each component.
E.g. The compound iron(II)
sulfide is formed in the ratio
of iron to sulfur (by mass) of
7:4.
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Chapter 4 Elements, Compounds
and Mixtures
Concept Map
35
Chapter 4 Elements, Compounds
and Mixtures
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