Chemistry help!?

Question

Can somebody help me solve this multi-step chemistry problem? We learned about them at the beginning of the year and I completely forgot the rules and stuff. Thanks!

3. Use simple structure and bonding models to account for each of the following:
(a) The bond length between two carbon atoms is shorter in C2H4 than in C2H6.
(b) The H-N-H bond angle is 107.5 degrees in NH3.
(c) The bond lengths in SO3 are all identical and are shorter than a sulfur-oxygen single bond.
(d) The I3 - ion is linear.

Answer 1

a. C2H4 has a double bond in it. C2H6 does not. Double bonds are stringer, therefore shorter than single bonds.

b. NH3 is a smushed tetrahedron It has three H's bonded to it, it also has a lone pair of electrons. The electrons push the N-H bonds away slightly,so the angles deviate from the angles seen in a perfect tetrahedron (like CH4).

c. The S is the central atom, to it there are three O atoms bonded. Each of these SO bonds is a double bond, thus violating the octect rule for S. But, recall that S is an exception to the octet rule.

d. I3- ion has 22 valence electrons. Using Lewis structure rules, you can make a structure where the 3 I atoms are bonded together and they share a double bond via resonance. It would be a linear molecule.

Answer 2

a. C2H4 is an alkene while C2H6 is an alkane. Alkene double bonds are shorter than alkane single bonds.

b. In ammonia, N has a lone pair, making the bond angle about 107 degrees (3 sides of a tetrahedron)

c. Sulfur has a lone pair in SO3

d. I3- is covalent - electrons are shared. Each I has 8 electrons with no lone pairs, so molecule is linear.