CBSE Class 9 Answered

Dear student,

Valencies do not get "cut" even while writing the formula of MgO.MgO is an electrovalent compound .It consists of giant structures containing metal ions and oxide ions thus existing as (MgO)_n.Hence while writing its formula it is written in the simplest form as MgO.On the other hand C₂H_{2 (Ethyne)} is an covalent compound.It exists as H-C≡C-H .In this simplest form the valency requirements of both carbon and hydrogen are fulfilled.Hence it's simplest formula being C₂H_{2 only.}