Lewis structure

Lewis structure of BCl₃

24 valence electrons. Three B-Cl single bonds, with no lone pair on B, so B has only 6 electrons around it.

Molecule or ion

BCl₃Boron trichloride

Trigonal planar

AB₃: 3 bond pairs, no lone pairs

24 valence electrons

i

B has only six electrons around it, an incomplete octet.

Formal charges: none

B has 3 valence electrons. It uses three in three single bonds to Cl, so none are left over.
Steric number = three sigma bonds + no lone pairs = 3, so B is sp² hybridised.
Three electron pairs point to the corners of a flat triangle (trigonal planar electron geometry). With no lone pairs the atoms take the same shape: trigonal planar.
All the bond pairs repel each other equally, so the angle is the ideal 120°.
The bond dipoles cancel in the symmetric trigonal planar shape, so BCl₃ is non-polar.

Working

How to draw it

Count the valence electrons: B 3 + 3 × Cl 7 = 24 valence electrons, 12 pairs.
Put B in the middle and join it to the three outer atoms with single bonds.
Give each outer atom lone pairs until it has an octet (H needs only two electrons), then put any electrons left over on the central atom.
Result: three B-Cl single bonds, with no lone pair on B, so B has only 6 electrons around it.
Formal charge = valence electrons − lone pair electrons − number of bonds. Here: none.

Class 11, Chapter 4 Chemical Bonding and Molecular Structure.

Questions

How many valence electrons are in BCl3?

B 3 + 3 × Cl 7 = 24 valence electrons, 12 pairs.

What are the formal charges in the Lewis structure of BCl3?

Every atom has a formal charge of zero.

How many lone pairs are in the Lewis structure of BCl3?

9 in all, none on the central B.