Lewis structure

Lewis structure of AlCl₃

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

Molecule or ion

AlCl₃Aluminium chloride

Trigonal planar

AB₃: 3 bond pairs, no lone pairs

i

This is the single AlCl3 molecule. In the vapour at lower temperature it pairs up as Al2Cl6, where each Al is sp3.

24 valence electrons

i

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

Formal charges: none

Al 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 Al 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 AlCl₃ is non-polar.

Working

How to draw it

Count the valence electrons: Al 3 + 3 × Cl 7 = 24 valence electrons, 12 pairs.
Put Al 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 Al-Cl single bonds, with no lone pair on Al, so Al 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 AlCl3?

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

What are the formal charges in the Lewis structure of AlCl3?

Every atom has a formal charge of zero.

How many lone pairs are in the Lewis structure of AlCl3?

9 in all, none on the central Al.