Question

how to determine planarity of molecules​

Answer 1

For this we find out hybridisation of compound.

Rules to find hybridization

Count valence electron of each atom present in compound. ( here one caution to be maintained is count valence electron of hydrogen as 7)

Add them

Then divide it by 8 (don't write the result in decimal form, write it in quotient & remainder form)

Quotient will represent bond pair &( remainder/2) represent loan pair

Now add quotient &( remainder /2) Say this as x.

If x=3 then compound is planar(hybridisation : sp2)

If x=2 then compound is linear(hybridisation :sp)

If x=4 then compound is non planar(hybridisation :sp3)

Example

Determine which is planar

NH3

Valence electron (v.e.) of N is 5

& for H is 7 (here number of H atom is 3. So v. e. =7*3=21)

Adding this becomes 5+21=26

Now 26/8 ( quotient =3, remainder=2)

(remainder/2) =2/2=1

Now x=quotient +(remainder /2)=4

So hybridisation :sp3 & compound NH3 is non planar

2. BeF2

v. e. of Be is 2 & v. e. of F=7(here F atom is 2 so total v. e. =7*2=14)

Adding we get 16

Now 16/8 ( quotient =2 & remainder =0)

(Remainder/2)=0

x=quotient+(remainder/2)=2+0=2

So hybridization :sp & linear

3. BF3

v.e.of B :3 & for F :7(here F atom is 3, total v.e. is 7*3=21)

Adding we get 3+21=24

Now 24/8(quotient =3, remainder =0)

(Remainder/2)=0

x=quotient +(remainder /2)=3+0=3

So hybridization :sp2 & the given compound is planar.

Hope this will help you.

If any doubt, please don't hesitate to ask.

All the best.