CHEMISTRY 1E03-01 TUTORIAL #6

Dr. J. D. Laposa October 26 - 30, 1998

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.)