AP Chemistry Question?

Question

NF3 and PF5 are stable molecules. Write the electron-dot formulas for these molecules. On the basis of structural and bonding considerations, account for the facts that NF3 and PF5 are stable molecules but NF5 does not exist.

This is the 1976 AP Chemistry Free Response Question, #9. Couldn't quite figure out the answer. Anyone know the answer?

Answer 1

P is in Period 3 and can therefore expand its valence shell to include not only 3s and 3p but 3d. More than 4 orbitals are now available to hold electrons - can make five bonds. N is in Period 2 - there is no 2d sublevel so you cannot expand and only have the 2s and 2p = 4 orbitals (8 electrons - 3 bonded pairs anbd on lone pair)

Answer 2

AP Chemistry and AP Biology are perfect classes for the Health science careers. Physics not so much. I was, like you, trying to decide between those two classes. I had to ask myself: Which class is more related to my career? What class am I better at? And I chose to do AP Biology. Since you're a sophomore, is there any possibility you can take the other class (besides the one you chose) senior year? That will make the choice a bit easier. I suggest maybe doing AP Bio junior year and AP Chem (which uses both Bio and Chemistry) senior year. AP Biology is not quite as hard as AP Chemistry. Colleges typically put more pressure on the junior year of high school, so taking the easier alternative is a good option so that your GPA/class rank won't be messed up.

Answer 3

The key lies in the phrase "expansion of the valency octet". P can have more than 8 in its outer level, but N can't.