Chemical formulae
Science
Unit: Atoms, elements and compounds
Outcome
I can use chemical symbols to write down chemical
formulae for elements and compounds.
Keywords
chemical symbol
chemical formula
Keywords
A chemical symbol is one or two letters used to represent a
chemical element. The first letter is always a capital letter, e.g.
bromine is Br and carbon is C.
A chemical formula is a representation of either the number of
atoms or ratio of atoms of each element in a substance, e.g.
H2O is water which has 2 hydrogen atoms and 1 oxygen atom.
Lesson outline
Chemical formulae
Representing compounds using chemical symbols
Variations of compounds
Representing compounds using chemical symbols
Explanation
The chemical formula of a compound contains the
symbols and numbers of each atom of the element.
For example, carbon monoxide, CO, contains one oxygen
atom bonded (chemically joined) to a carbon atom.
C
O
Different colours are used to represent
different atoms so you can tell them
apart.
There is a 1:1 ratio of C to O atoms.
Representing compounds using chemical symbols
Explanation
Elements ...
Compounds ...
contain one type of atom.
contain two or more different
types of atom.
N N
O O
NNO O
N2
O2
NO
2 nitrogen atoms 2 oxygen atoms
1 nitrogen atom
1 oxygen atom
H Cl
HCl
1 hydrogen atom
1 chlorine atom
Representing compounds using chemical symbols
Check
Which of the following represent compounds?
Na
O
C Cl
H Cl
Li
a
b
c
Representing compounds using chemical symbols
Explanation
The subscript numbers in a formula tell you how
many atoms of the element are in a molecule.
H2S
two atoms of
hydrogen
CH4
one atom of
sulfur
2:1 ratio of hydrogen to sulfur
one atom of
carbon
four atoms of
hydrogen
1:4 ratio of carbon to hydrogen
O
Representing compounds using chemical symbols
Check
Which diagram shows ammonia (NH3)?
N
H
N H
H N
N
a
H
N H
H
b
c
Representing compounds using chemical symbols
Check
What is the formula for the compound shown below?
a
b
c
CH3
C2H6
C2H6
H
H
H C
C
H
H
H
Representing compounds using chemical symbols
What is the correct formula for carbon dioxide?
a
b
CO2
CO2
c
C2O
d
C2O
O
C
O
carbon dioxide
Check
Representing compounds using chemical symbols
Explanation
Some compounds form giant structures.
For example:
sodium chloride
sodium atom
chlorine atom
NaCl = 1:1 ratio
The formula gives the ratio of the elements in the compound.
Task A Representing compounds using chemical symbols
Practice
Complete the table of information.
compound
formula
symbols of
atoms in
compound
number of each
type of atom in
compound
ratio of atoms in
the compound
KBr
K, Br
1 K, 1 Br
1:1
NaCl
N2 O
C3H8
CuSO4
H,S,O
2:1:4
Task A Representing compounds using chemical symbols
Feedback
compound
formula
symbols of
elements in
compound
number of each
type of atom in
compound
ratio of elements
in the compound
KBr
K, Br
1 K, 1 Br
1:1
NaCl
Na, Cl
1 Na, 1 Cl
1:1
N, O
2 N, 1 O
C, H
3 C, 8 H
2:1
Cu, S, O
1 Cu, 1 S, 4 O
N2O
C3H8
CuSO4
H2SO4
H, S, O
2 H, 1 S, 4 O
3:8
1:1:4
2:1:4
Lesson outline
Chemical formulae
Representing compounds using chemical symbols
Variations of compounds
Variations of compounds
Explanation
Some compounds contain the same atoms as other
compounds but in a different ratio.
These different compounds will have different properties.
For example: H2O and H2O2
corrosive
H2 O
H2O2
harmful
water
hydrogen peroxide
Variations of compounds
Explanation
Ethanol (C2H6O) and ethanoic acid (C2H4O2) have the
same elements; carbon, hydrogen, and oxygen, but in
a different ratio. As they are different compounds,
their physical and chemical properties are different.
H
ethanol
H
H
C
C
H
H
O
● boiling point 78°C
● density 0.8 g/cm3
● flammable
H
H
H C
ethanoic acid
H
O
C
O
● boiling point 118°C
● density 1.1 g/cm3
● flammable
H
Variations of compounds
Which compound(s) contain the same atoms in
different ratios?
a
CO2
b
CH2O
c
SO2
d
C2H6O
Check
Variations of compounds
Check
True or false?
Different compounds containing the same elements will always have
the same chemical and physical properties.
T
True
F
False
Justify your answer
a
Compounds with different ratios of the same elements will
have properties similar to the individual elements.
b
Compounds with different ratios of the same elements will
behave differently.
Variations of compounds
Task B
Practice
1) Which three compounds contain the same elements
bonded in different ratios?
2) Name the elements in the compounds you selected.
3) Write the formulae of the compounds.
a
O
O
d
O
N
N N
b
O
C
c
H
O
O
O
e
N
H
H
f
O
ON N
O
O
O
H
H
H C
C
H
H
H
Variations of compounds
Task B
Feedback
1) Which three compounds contain the same elements bonded in
a, c and d
different ratios?
2) Name the elements in the compounds you selected.
nitrogen (N) and oxygen (O)
1) Write the formulae of the compounds. NO, NO2, N2O4
O
a
O
O
d
O
N
N N
b
O
C
c
H
O
O
O
e
N
H
H
f
ON N
O
O
O
H
H
H C
C
H
H
H
Task B
Variations of compounds
Practice
4) Correct the student’s ideas about compounds.
If compounds contain the
same elements in different
ratios, they will have the
same properties.
Jun
Some compounds contain
the same elements that
are stuck together in
different ways.
Carbonic acid (H2CO3),
glucose (C6H12O6) and
ethanol (C2H6O) all contain
the elements nitrogen,
cobalt, osmium and
hydrogen.
Sam
Aisha
Task B
Variations of compounds
Feedback
4) Correct the student’s ideas about compounds.
If compounds contain the
same elements in
different ratios, they will
have different properties.
Jun
Some compounds contain
the same elements that
are bonded together in
different ways.
Carbonic acid (H2CO3),
glucose (C6H12O6) and
ethanol (C2H6O) all contain
the elements carbon,
oxygen and hydrogen.
Sam
Aisha
Summary Chemical formulae
The symbols of each element a substance contains appear
in its chemical formula.
The subscript numbers in a formula tell you how many
atoms of the element are in a molecule.
Each different compound contains a different combination
of elements.
Some compounds contain the same elements but in a
different ratio.
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