Answer
General guidance
Hybridization is the process of mixing the atomic orbitals of comparable energies in an atom to generate a set of new atomic orbitals which are equal in energy and shape is called hybridization.
To determine the steric number for calculating the hybridization of an atom, count the number of bond pairs + lone pairs which gives the type of orbitals involved in hybridization. The hybridization and the corresponding geometry resulting from the hybridization is listed below:
Only single bonds and lone pairs are included in determining hybridization. Double bonds and triple bonds are not used in determining the steric number as well as the geometry of the molecule.
Step-by-step
Step 1 of 2
Draw the given structure and label the geometry of left carbon atom in acetonitrile by calculating steric number as shown below:
The steric number for left carbon atom is 4. Therefore, the hybridization is 
and the corresponding geometry is tetrahedral.
The electronic configuration of carbon is 
, this implies that carbon atom has four valence electrons.
Now, in the structure left carbon atom is bonded to three hydrogens and one carbon atom, therefore, the number of bond pair is equal to four. As all the valence electrons are used in forming the bonds so, there is no lone pair left. Thus,
Therefore, the orbitals that participate in hybridization is one 
orbital and three 
orbitals. Hence, the hybridization of left carbon atom is 
and the geometry corresponding to the hybridization is tetrahedral.
Therefore, in general for carbon atom having no multiple bond indicates hybridization, .
Step 2 of 2
Draw the given structure and label the geometry of right carbon atom in acetonitrile by calculating steric number as shown below:
The steric number for right carbon atom is 2. Therefore, the hybridization is 
and the corresponding geometry is linear.
The right carbon atom is bonded to one carbon atom and one nitrogen atom, therefore, the number of bond pair is equal to two (multiple bonds are not included in determining steric number). As two valence electrons are used in forming the single bonds and the remaining two are used to form the multiple bonds. Thus,
Therefore, the orbitals that participate in hybridization is one orbital and one orbital. Hence, the hybridization of right carbon atom is 
and the corresponding hybridization is linear.
Therefore, in general for carbon atom having a triple bond indicates hybridization, 
.
Answer
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