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Define valency by taking examples of silicon and oxygen.
 
Valency or valence refers to an atom's ability to  a pair of electrons to form a chemical compound.
  
Oxygen:

Atomic number of oxygen = 8

Electronic configuration of oxygen =

In oxygen atom, the valence electrons are . To fill the orbit,  electrons are required. Hence, the valency of oxygen is .
 
Silicon:

Atomic number of silicon = 14

Electronic configuration of silicon = 

In the silicon atom, the valence electrons are . To fill this orbit, electrons are required. Hence, the valency of silicon is  .