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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