Predict the approximate molecular geometry around each carbon atom of acetonitrile

Solution 

With respect to the left side carbon (C₁)

The left side of carbon is surrounded by three hydrogen atoms and one right carbon atom i.e. means it has 4 regions of electron density and no lone pair present on it.

So, it implies tetrahedral geometry. You can also check its molecular geometry through the AXN method.

AXN notation for left side carbon:

• A denotes the central atom, therefore, A = Carbon
• X denotes the bonded atoms to the central atom, three hydrogens, and one right-side carbon is bonded to it, therefore, X = 4
• N denotes the lone pair on the central atom, according to the CH3CN lewis structure, No lone pair is present on the left side of carbon. Hence, N = 0

So, the AXN generic formula for carbon (C₁) becomes AX₄.

As per the VSEPR chart, if a central atom is attached with four bonded atoms and has no lone pair then the molecular geometry with respect to that central atom is tetrahedral in nature, and electron geometry is also tetrahedral.

With respect to the right side carbon (C₂)

The right side of carbon is surrounded by only two atoms (one is nitrogen and another one is left side carbon) i.e. means it has 2 regions of electron density and no lone pair present on it.

So, it implies linear geometry. You can also check its molecular geometry through the AXN method.

AXN notation for right-side carbon:

• A denotes the central atom, therefore, A = Carbon
• X denotes the bonded atoms to the central atom, one nitrogen, and one left side carbon is bonded to it, therefore, X = 2
• N denotes the lone pair on the central atom, according to the CH3CN lewis structure, No lone pair is present on the right side of carbon. Hence, N = 0

So, the AXN generic formula for carbon (C₂) becomes AX₂.

As per the VSEPR chart, if a central atom is attached with two bonded atoms and has no lone pair then the molecular geometry with respect to that central atom is linear in nature, and electron geometry is also linear.