Lewis dot structures are useful to predict the geometry of a molecule. Sometimes, one of the atoms in the molecule does not follow the octet rule for arranging electron pairs around an atom. This example uses the steps outlined in How to Draw A Lewis Structure to draw a Lewis structure of a molecule where one atom is an exception to the octet rule.
Draw the Lewis structure of the molecule with molecular formula ICl3.
Step 1: Find the total number of valence electrons.
Iodine has 7 valence electrons
Chlorine has 7 valence electrons
Total valence electrons = 1 iodine (7) + 3 chlorine (3 x 7)
Total valence electrons = 7 + 21
Total valence electrons = 28
Step 2: Find the number of electrons needed to make the atoms "happy"
Iodine needs 8 valence electrons
Chlorine needs 8 valence electrons
Total valence electrons to be "happy" = 1 iodine (8) + 3 chlorine (3 x 8)
Total valence electrons to be "happy" = 8 + 24
Total valence electrons to be "happy" = 32
Step 3: Determine the number of bonds in the molecule.
number of bonds = (Step 2 - Step 1)/2
number of bonds = (32 - 28)/2
number of bonds = 4/2
number of bonds = 2
This is how to identify an exception to the octet rule. There are not enough bonds for the number of atoms in molecule. ICl3 should have three bonds to bond the four atoms together.
Step 4: Choose a central atom.
Halogens are often the outer atoms of a molecule. In this case, all the atoms are halogens. Iodine is the least electronegative of the the two elements. Use iodine as the center atom.
Step 5: Draw a skeletal structure.
Since we do not have enough bonds to connect all four atoms together, connect the central atom to the other three with three single bonds.
Step 6: Place electrons around outside atoms.
Complete the octets around the chlorine atoms. Each chlorine should get six electrons to complete their octets.
Step 7: Place remaining electrons around the central atom.
Place the remaining four electrons around the iodine atom to complete the structure. The completed structure appears at the beginning of the example.