Chapter-3. Atoms and Molecules Science class 9 in english Medium CBSE Notes

3. Atoms and Molecules

The rules that you have to follow while writing a chemical formula are as follows:

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

(ii) When 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), copper oxide (CuO) etc.

(iii) Where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

(iv) In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

Binary Compounds: The simplest compounds, which are made up of two different elements are called binary compounds.

Writing Formula of Simple Compounds: 

While writing the chemical formulae for compounds, we write the constituent
elements and their valencies as shown below. Then we must crossover the valencies of the combining atoms.

There are two non-metals which participate in molecule

Example 1: 

Formula of Hydrogen Compound: 

Therefore formula: HCl

Example 2: 

Formula of Hydrogen Sulphide: 

Therefore formula:H₂O

Example 3: 

Formula of Carbon Tetrachloride: 

Therefore formula: CCl₄

Formula of Ionic Compound: When the First element is Metal which forms cation (+) and Second element is non-metal which form anion (-).  

Formula of Magnesium Chloride:  

For finding the the formula of Magnesium Chloride, we write symbol of cation with element like this (Mg⁺), After this we write the symbol of anion of (Cl^-). Then we crossover the charges we obtain formula.

Example 4:

Therefore formula: MgCl₂

Example 5:

Formula of Alumunium Oxide: 

Therefore formula:Al₂O₃

The Compound of Polyatomic Ions: 

Example 6:

Formula of Sodium Nitrate: 

Therefore formula: NaNO₃

Example 6:

Therefore formula: (NH₄)₂SO₄