Formula writing

Chapter 3: Atoms and Molecules

1. Chapter 3
Atoms and Molecules
“Formula Writing”


H
2O
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2. Formulas
 Formulas are combinations of symbols
that represent a compound. A formula
indicates which elements are involved
and the number of atoms of each element
contained in the compound.

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3. In writing formulas, we use subscripts, and
parentheses in addition to the symbols of
the elements.
Subscripts indicate the number of atoms of
an element, as in H
2 where two is the
subscript meaning two hydrogen atoms. If
there is no subscript with a symbol, it is
assumed there is only one atom of that
element.

H
2

Subscript

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4. Parentheses or brackets are used to
separate a radical/ polyatomic ion from
the rest of the formula when it would
be confusing.

Example: Fe
2 (CO
3)
3


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5. Steps in Formula Writing
1.Determine the symbols for
the elements in a
compound.


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6. K
Na
Ca
Mg
Al
Ag
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7. 2. Determine the valence
of each of the atoms.

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8. K
Na
Ca
Mg
Al
Ag
+ 1
+ 1
+ 2
+ 2
+ 3
+ 1
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9. 3. Write the positive
element's symbol first,
followed by that of the
negative element.
Example:
Al
+3
S
-2
--- Al
2S
3


Fe
+3
Cl
-1
--- FeCl
3

Cu
+1
O
-2
--- Cu
2O


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11. The rules for writing a chemical formula :
• the valencies or charges on the ion must balance.
• if a compound consists of a metal and a non-
metal, the name or symbol of the metal is written
first. For example: calcium oxide (CaO), sodium
chloride(NaCl), iron sulphide (FeS),etc., where
oxygen, chlorine, sulphur are non-metals and are
written on the right, whereas calcium, sodium,iron
are metals, and are written on the left.
• in compounds formed with polyatomic ions, the
ion is enclosed in a bracket before writing the
number to indicate the ratio. In case the number of
polyatomic ion is one, the bracket is not required.
For example, NaOH.
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12. 4. Balance the positive and
negative charges.

Try to balance the following
Fe
+3
Cl
-1

Al
+3
S
-2

Cu
+1
O
-2

Ca
+2
N
-3

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13. RULES IN WRITING
CHEMICAL FORMULA
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14. Rule 1: CRISS-CROSS rule
1. Write the correct symbols of the elements.
2. Above each symbol, write the correct
valence. Ex. Al
+3
O
-2

3. To obtain the proper subscript, criss-cross
the valence and drop the algebraic sign.
Ex. Al
+3
O
-2
became Al
2
O
3


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15. The positive ion
charge without its
sign becomes the
subscript of the
negative ion
The negative ion
charge without its
sign becomes the
subscript of the
positive ion
1 (+3) + 3(-1) = 0
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16. +1 -1
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17. +1 -2
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18. +1 -1
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19. +2 -1
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20. Rule 2: When the subscript
is number 1, subscript is not
written.
Example:
Sodium Chloride- Na
+1
Cl
+1
--- NaCl

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21. Sodium hydroxide
write
symbol
Remember
charge
Cross the
charge NaOH
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22. Sodium Chloride
chloride
write
symbol
Remember
charge
Cross the
charge NaCl
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23. Potassium Hydroxide
hydroxide
write
symbol
Remember
charge
Cross the
charge KOH
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24. Potassium chloride
write
symbol
Remember
charge
Cross the
charge KCl
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25. Rule 3: When the valence of both
elements are numerically equal
but grater than 1, the subscript
are not also written.
Example:
Calcium Oxide~ Ca
+2
O
-2
----- CaO

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26. Calcium oxide
write
symbol
Remember
charge
Cross the
charge CaO
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27. Iron(II) Oxide
oxide
write
symbol
Remember
charge
Cross the
charge FeO
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28. Rule 4: All radicals take more
than once( the subscript is 2 or
more) must be enclosed in
Brackets().
Example:
Ammonium Sulfate~~ NH
4
+1
SO
4
-2-----
(NH
4)
2 SO
4

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29. Ammonium Oxide
write
symbol
Remember
charge
Cross the
charge ONH24)(
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30. Iron(lll) Sulfate
Sulphate
write
symbol
Remember
charge
Cross the
charge 342)(SOFe
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31. Iron(lll) Carbonate
Carbonate
write
symbol
Remember
charge
Cross the
charge 332)(COFe
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32. Ammonium Sulfate
Sulphate
write
symbol
Rememb
er charge
Cross the
charge 424)( SONH
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33. Ammonium Carbonate
write
symbol
Rememb
er charge
Cross the
charge 324)( CONH
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35. Complete the following grid (Chemical formula)
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