Class 9th – CHEMICAL FORMULAE

• It determines the composition of a molecule of the Compound, with the symbols of the elements present in it.
• When a metal and a non metal combine it result in the formation of Ionic Compound. The compounds that are formed by Ions.
• The ions present in KNO₃ (Potassium nitrate) are K⁺ & NO₃ˉ
• Rules for writing chemical formula of Ionic Compound:
  • Metal element is followed by non metal element.
  • Metal element is named as such as it is, but the non-metal is modified. Like- chloride, oxide, nitrite etc.
  • NaCl is written and named as Sodium chloride.
• To write the formula of a compound we should know the valency of an atom.
• Metals have the tendency to lose electrons whereas non metals have the tendency to gain electrons.
• A COMPOUND WHICH IS MADE UP OF TWO ELEMENTS IS CALLED BINARY COMPOUNDS.   
• When an element contains more than one atom, prefixes are added to them after the 1^st element i.e. a word is used before the naming of the compound.

EXCEPTIONS

H₂S is written as Hydrogen sulphide and not as hydrogen disulphide, it shows that if hydrogen being the 1^st element no prefix is added.