NOTE: All Chem 1E03-01 students will have a formal tutorial during the week of October 25. Solutions will be posted.

1. For each of the following twelve species, draw an adequate Lewis structure, including formal charges, give the AX_nE_m class, predict the shape, and state if the octet rule is being violated.

a) Cl₂O

There are 20 valence electrons. The Lewis structure is:
 

The molecule is AX₂E₂, and is bent.

b) H₂CO

There are 12 valence electrons. The Lewis structure is:
 

The molecule is AX₃, and is triangular planar.

c) SF₄

There are 34 valence electrons. The Lewis structure is:
 

Note that the nbp on S is an equatorial nbp.

The molecule is AX₄E, and has a seesaw shape (or distorted tetrahedron).
Note that a 2-dimensional Lewis structure does not show the shape. (The shape is inferred from the VSEPR treatment.)

The octet rule is violated here.

d) CCl₂Br₂

There are 32 valence electrons.
 

The molecule is AX₄, and is tetrahedral.

e) ICl₃

There are 28 valence electrons. The Lewis structure is:
 

The molecule is AX₃E₂, and is T-shaped (the 2 nbps are equatorial).

The octet rule is violated here.

f) IF₅

There are 42 valence electrons. The Lewis structure is:
 

The molecule is AX₅E, and is square-based pyramidal.

The octet rule is violated here.

g) PF₅

There are 40 valence electrons. The Lewis structure is:
 

The molecule is AX₅, and is triangular bipyramidal.

The octet rule is violated here.

h) NO₂^-

There are 18 valence electrons. The Lewis structure is:
 

The ion is AX₂E, and is bent.

We will soon see that there are two resonance structures for NO₂^-; this is one of them; both N-O bonds are equivalent.

i) SO₃

There are 24 valence electrons. The Lewis structure is:
 

The molecule is AX₃, and is triangular planar.

The octet rule is violated here.

j) SO₃^2-

There are 26 valence electrons. The Lewis structure is:
 

Note that the diagram has a typo. It is missing the non-bonding pair around S.

The molecule is AX₃E, and is triangular pyramidal.

The octet rule is violated here.

There are three resonance structures for SO₃^2-;this is one of them;all three S-O bonds are equivalent.

k) OCS

There are 16 valence electrons. The Lewis structure is:
 

The molecule is AX₂, and is linear.

l) PO₄^3-

There are 32 valence electrons. The Lewis structure is:
 

NOTE THAT THERE IS A TYPO IN THE DIAGRAM. THE DOUBLY-BONDED
OXYGEN HAS NO FORMAL CHARGE, BUT THE THREE SINGLY-BONDED OXYGENS
EACH HAVE A FORMAL CHARGE OF -1.

The molecule is AX₄, and is tetrahedral.

The octet rule is violated here.

There are four resonance structures for PO₄^3-;this is one of them;all four P-O bonds are equivalent.

2. Match the molecules SiH₄, PI₃, BBr₃, GeCl₂, and SF₆ with the bond angles 90^o, 109.5^o, 120^o, slightly less than 109.5^o, and slightly less than 120^o
.

SiH₄, an 8 valence electron system, is AX₄; it has 109.5^o bond angles.

PI₃, a 26 valence electron system, is AX₃E; its bond angles are slightly less than 109.5^o.

BBr₃, a 24 valence electron system, is AX₃; it has 120^o bond angles.

GeCl₂, an 18 valence electron system, is AX₂E; its bond angle is slightly less than 120^o.

SF₆, a 48 valence electron system, is AX₆;it has 90^o bond angles.

(Hint: for all of these species, count the valence electrons, deduce the AX_nE_m class, and determine the geometry. With sufficient practice, you can often do these steps without drawing a Lewis structure.)