formula writing
TRANSCRIPT
Chapter 3
Atoms and Molecules
“Formula Writing”
H2O
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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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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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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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Steps in Formula Writing
1. Determine the symbols for the elements in a compound.
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K
Na
Ca
Mg
Al
Ag 6 Archana Singh
2. Determine the valence of each of the atoms.
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K
Na
Ca
Mg
Al
Ag
+ 1
+ 1
+ 2
+ 2
+ 3
+ 1 8 Archana Singh
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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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. 11 Archana Singh
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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RULES IN WRITING CHEMICAL FORMULA
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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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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 15 Archana Singh
+1 -1
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+1 -2
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+1 -1
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+2 -1
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Rule 2: When the subscript is number 1, subscript is not written.
Example:
Sodium Chloride- Na+1 Cl+1 --- NaCl
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Sodium hydroxide write
symbol
Remember
charge
Cross the
charge NaOH21 Archana Singh
Sodium Chloride
chloride write
symbol
Remember
charge
Cross the
charge NaCl22 Archana Singh
Potassium Hydroxide
hydroxide write
symbol
Remember
charge
Cross the
charge KOH23 Archana Singh
Potassium chloride write
symbol
Remember
charge
Cross the
charge KCl
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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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Calcium oxide write
symbol
Remember
charge
Cross the
charge CaO26 Archana Singh
Iron(II) Oxide oxide write
symbol
Remember
charge
Cross the
charge FeO27 Archana Singh
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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Ammonium Oxide write
symbol
Remember
charge
Cross the
charge ONH 24)(
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Iron(lll) Sulfate Sulphate write
symbol
Remember
charge
Cross the
charge 342 )(SOFe
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Iron(lll) Carbonate Carbonate write
symbol
Remember
charge
Cross the
charge 332 )(COFe
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Ammonium Sulfate
Sulphate write
symbol
Rememb
er charge
Cross the
charge 424)( SONH
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Ammonium Carbonate write
symbol
Rememb
er charge
Cross the
charge 324)( CONH
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Complete the following grid (Chemical formula)
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