Atoms and Molecuels

The chemical Formula of a compound is a symbolic representation of its composition.

Using chemical formula, a compound can be represented easily compare to writing its name in word.

Example:

Chemical formula of Calcium chloride: CaCl₂

Chemical formula of Water : H₂O

Chemical formula of Ammonia : NH₃

Writing the Formula of Compounds

Rules to write the chemical formula of compound

(a) The valencies or charges on the ion must balance.

(b) When a compound consists of a metal and a non metal, the name or symbol of the metal is written first.

(c) 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.

Formula of Simple Compounds

Binary Compound: Compounds which are made of two different elements are called Binary Compounds.

Steps to write formula of binary compounds or simple compounds

(a) Write the symbol of constituent elements of compound

(b) Write respective valency below the elements

(c) Cross over valencies of the combining atom

(1) Write the formula of Hydrogen Chloride

Symbol of hydrogen : H

Symbol of Chloride : Cl

Valency of Hydrogen (H) : 1

Valency of Chloride (Cl): 1

Steps to write the formula of Hydrogen Chloride

(a) Write symbols of hydrogen (H) and chloride (Cl) slightly apart in one line

Symbol : H       Cl

(b) Now write their respective valencies below the symbol of elements

Symbol : H       Cl

Valency: 1       1

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula

H₁Cl₁

(e) For convenience "1" is not written as subscript of an element.

Thus, formula of the Hydrogen chloride: HCl

(2) Write the formula of Potassium chloride

Symbol of Potassium : K

Symbol of Chloride : Cl

Valency of Potassium (K) : 1

Valency of Chloride (Cl): 1

Steps to write the formula of Potassium Chloride

(a) Write symbols of Potassium (K) and chloride (Cl) slightly apart in one line

Symbol: K       Cl

(b) Now write their respective valencies below the symbol of elements

Symbol: K       Cl

Valency: 1       1

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

K₁Cl₁

(e) For convenience "1" is not written as subscript of an element while writing the formula of a compound.

Thus, formula of the Potassium chloride is KCl

(3) Write the formula of Calcium chloride

Symbol of Calsium : Ca

Symbol of Chloride : Cl

Valency of Calcium (Ca) : 2

Valency of Chloride (Cl): 1

Steps to write the formula of Calcium Chloride

(a) Write symbols of Calcium (Ca) and chloride (Cl) slightly apart in one line

Symbol: Ca       Cl

(b) Now write their respective valencies below the symbol of elements

Symbol: Ca       Cl

Valency: 2       1

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Ca₁Cl₂

(e) For convenience "1" is not written as subscript of an element while writing the formula of a compound.

Thus, formula of the Calcium chloride is CaCl₂

(4) Write the formula of Magnesium chloride

Symbol of Magnesium : Mg

Symbol of Chloride : Cl

Valency of Magnesium (Mg) : 2

Valency of Chloride (Cl): 1

Steps to write the formula of Magnesium Chloride

(a) Write symbols of Magnesium (Mg) and chloride (Cl) slightly apart in one line

Symbol: Mg       Cl

(b) Now write their respective valencies below the symbol of elements

Symbol: Mg       Cl

Valency: 2       1

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Mg₁Cl₂

(e) For convenience "1" is not written as subscript of an element while writing the formula of a compound.

Thus, formula of the Magnesium chloride is MgCl₂

(5) Write the formula of Hydrogen sulphide

Symbol of Hydrogen : H

Symbol of Sulphide : S

Valency of Hydrogen (H) : 1

Valency of Sulphide (S): 2

Steps to write the formula of Hydrogen sulphide

(a) Write symbols of Hydrogen (H) and sulphide (S) slightly apart in one line

Symbol: H       S

(b) Now write their respective valencies below the symbol of elements

Symbol: H       S

Valency: 1       2

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

H₂S₁

(e) For convenience "1" is not written as subscript of an element while writing the formula of a compound.

Thus, formula of the Hydrogen sulphide is H₂S

(6) Write formula of Carbon tetrachloride

Symbol of Carbon : C

Symbol of Chloride : C

Valency of Carbon (C) : 4

Valency of Chloride (Cl): 1

Writing the formula

Step (a) write symbol of elements and their valency

Symbol: C       Cl

Valency: 4       1

Step (b) Cross over valency and symbols

Step (c) write the result after cross over (cross multiplication)

C₁Cl₄

Step (d) write the formula of given compound by omitting unnecessary subscripts.

Thus, formula of the Carbon tetrachloride is CCl₄

(7) Write the formula of Aluminium Oxide

Symbol of Aluminium : Al

Symbol of Oxide : O

Valency of Aluminium (Al) : 3

Valency of Oxide (O): 2

Steps to write the formula of Aluminium oxide

(a) Write symbols of Aluminium (Al) and Oxide(O) slightly apart in one line

Symbol: Al       O

(b) Now write their respective valencies below the symbol of elements

Symbol: Al       O

Valency: 3       2

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Al₂O₃

(e) Here since both the valencies are greater than one, thus both of the valencies are written as subscript with respective symbols.

Thus, formula of the Aluminium oxide is Al₂O₃

(8) Write the formula of Calcium Oxide

Symbol of Calcium : Ca

Symbol of Oxide : O

Valency of Calcium (Ca) : 2

Valency of Oxide (O): 2

Steps to write the formula of Calcium oxide

(a) Write symbols of Calcium (C) and Oxide(O) slightly apart in one line

Symbol: C       O

(b) Now write their respective valencies below the symbol of elements

Symbol: C       O

Valency: 2       2

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Ca₂O₂

(e) Here since both of the the valencies equal thus, both of the valencies are omitted from subscript.

Thus, formula of the Calcium oxide is CaO

(9) Write the formula of Sodium Nitrate

Symbol of Sodium : Na

Symbol of Nitrate : NO₃

Valency of Sodium (Na) : + 1

Valency of Nitrate (NO₃): –1

Steps to write the formula of Sodium nitrate

(a) Write symbols of Sodium (Na) and Nitrate (NO₃) slightly apart in one line

Symbol: Na       NO₃

(b) Now write their respective valencies below the symbol of elements

Symbol: Na       NO₃

Valency: +1       –1

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Na₁(NO₃)₁

(e) For convenience "1" is not written as subscript of an element while writing the formula of a compound.

Thus, formula of the Sodium nitrate is NaNO₃

(10) Write the formula of Calcium hydroxide

Symbol of Calcium : Ca

Symbol of Hydroxide : OH

Valency of Calcium (Ca) : 2

Valency of Hydroxide (OH): –1

Steps to write the formula of Calcium hydroxide

(a) Write symbols of Calcium (Ca) and hydroxide(OH) slightly apart in one line

Symbol: Ca       OH

(b) Now write their respective valencies below the symbol of elements

Symbol: Ca       OH

Valency: 1       –1

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Ca₁OH₂

(e) Here since hydroxide ion has symbol with two letters, thus symbol of hydroxide (OH) is surrounded with brackets and valency of calcium which is equal to 2 is given as sub script of whole of (OH).

Thus, formula of the Calcium oxide is Ca(OH)₂

(11) Write the formula of Sodium Carbonate

Symbol of Sodium : Na

Symbol of Carbonate : CO₃

Valency of Sodium (Na) : 1

Valency of Carbonate (CO₃): –2

Steps to write the formula of Sodium carbonate

(a) Write symbols of Sodium (Na) and Carboante(CO₃) slightly apart in one line

Symbol: Na       CO₃

(b) Now write their respective valencies below the symbol of elements

Symbol: Na       CO₃

Valency: 1       –2

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

Na₂(CO₃)₁

(e) Here since carbonate ion has symbol with two letters, but it has subscript as 1 (one), thus it is written without any bracket.

Thus, formula of the Sodium carbonate is Na₂CO₃

(12) Write the formula of Ammonium sulphate

Symbol of Ammonium : NH₄

Symbol of Sulphate : SO₄

Valency of Ammonium (NH₄) : +1

Valency of Sulphate (SO₄): –2

Steps to write the formula of Ammonium sulphate

(a) Write symbols of Ammonium (NH₄) and Sulphate (SO₄) slightly apart in one line

Symbol: NH₄       SO₄

(b) Now write their respective valencies below the symbol of elements

Symbol: NH₄       SO₄

Valency: 1         –2

(c) Now, cross over valencies similar to cross multiplication

(d) Write the resulting Formula by writing valencies as subscript

(NH₄)₂(SO₄)₁

(e) Here Ammonium (NH₄) has smbol with two letters and a subscript, thus it is written as bracket and whole 2 as subscript

Thus, formula of the Ammonium sulphate is (NH₄)₂SO₄