2. 3.5 Chemical Formulae
 Used
to represent a chemical compound
 It shows:- the elements (denoted by symbols)
- the relative numbers (indicated by subscript
after the symbol)
 Example:-
H2 O

3. Chemical formulae of some covalent
compounds
Name of compound
Chemical
formula
Number of each element in
the compound
Oxygen
O2
2 oxygen atoms
Water
H2O
2 hydrogen atoms
1 oxygen atom

4. Chemical formulae of some ions
(cations)
Charge
Cation
Symbol
+1
Sodium ion
Na+
+2
Magnesium
ion
Mg 2+
+3
Iron(III) ion
Fe3+

5. Chemical formulae of some ions
(anions)
Charge
Anion
Symbol
-1
Fluoride ion
F-
-2
Oxide ion
O2-
-3
Nitride ion
N3-

6.  To
write the chemical formula of an ionic
compounds:- write the formula of the ions involved in
forming the compound
- balance the positive and negative charge
(use subscript)
- Write the chemical formula of the ionic
compound without the charges.

7. Example:-

8. Formulae of some ionic compounds
Cation
Anion
Chemical formula
Na+
Cl-
NaCl
Ca2+
Cl-
CaCl2
Al3+
N3-
AlN

9.  Empirical
formulae of a compound:- shows the simplest ratio of the atoms of the
elements that combine to form a compound
- steps to determine the empirical formula of a
compound:1) write the mass / percentage of each
element in the compound
2) calculate the number of moles for each
element
3) Divide each number by the smallest
number to obtain simplest ratio
4) Write the empirical formula of the
compound

10. Example:-

11.  Molecular
formulae of the compound:- shows the actual numbers of the atoms of the
elements that combine to form the compound
Compound
Molecular
formula
Simplest
ratio of the
elements
Empirical
formula
Water
H2O
H:O = 2:1
H2O
Ethene
C2H4
C:H=1:2
CH2
Glucose
C6H12O6
C:H:O=1:2:1
CH2O

12. 3.6 Chemical Equations
 Chemical
reaction can be represented by
a chemical equation
 Reactants – chemicals that are reacting.
Written on LHS.
 Products – chemicals formed in the
reaction. Written on the RHS

13.  Writing
a chemical equation:1) write the correct formulae of all reactants on
the LHS of the equation
2) write the correct formulae of all products on
the RHS of the equation
3) the equation is then balanced.
4) make sure the number of atoms before and
after reaction are the same
5) Write the physical state of each reactants
and products

14. Example:-