The document discusses the rules and steps for writing chemical formulas: 1) Formulas use symbols and subscripts to show which elements are present and how many atoms of each. 2) Rules include balancing charges by "criss-crossing" valence numbers, omitting subscripts of 1, and using parentheses for polyatomic ions. 3) The steps are to determine symbols and valences, write the positive element first, and balance charges to obtain the correct subscripts.

1. Chapter 3
Atoms and Molecules
“Formula Writing”
H2O
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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 H2 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.
H2 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: Fe2 (CO3)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 --- Al2S3
Fe+3 Cl-1 --- FeCl3
Cu+1 O-2 --- Cu2O
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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. 11Archana Singh

12. 4. Balance the positive and
negative charges.
Try to balance the following
Fe+3 Cl-1
Al+3S-2
Cu+1 O-2
Ca+2N-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+3O-2 became Al2O3
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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 15Archana Singh

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
chloridewrite
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+2O-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
oxidewrite
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~~ NH4
+1 SO4
-2-----(NH4)2 SO4
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29. Ammonium Oxide
write
symbol
Remember
charge
Cross the
charge ONH 24)(
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30. Iron(lll) Sulfate
Sulphatewrite
symbol
Remember
charge
Cross the
charge 342 )(SOFe
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31. Iron(lll) Carbonate
Carbonatewrite
symbol
Remember
charge
Cross the
charge 332 )(COFe
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32. Ammonium Sulfate
Sulphatewrite
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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