Formal ChargesMonday 11/08/15

Monday, November 9, 15

Agenda

Finish Topic 14 practice questions

Formal Charges and expanded octets

Homework

Monday, November 9, 15

Formal ChargeWhy do charges appear and disappear in organic mechanisms such as

Monday, November 9, 15

Formal ChargeAnd how do we predict the most likely Lewis structure from a list of alternatives like these proposed structures for the sulfate ion

Monday, November 9, 15

Formal Charge

If this results in the atom having more electrons than the isolated atom, the formal charge is negative (-1 for one electron, -2 for two extra electrons etc).

If this results in the atom having less electrons than in the isolated atom, the formal charge of the atom is positive (+1 for one less electron, and so on)

Monday, November 9, 15

Formal Charge

For example:•The carbon in the carbonate in has 4 x 1 = 4 electrons assigned to it (one from each of its four bonds), so it has a FC of zero (neutral).•The top oxygen has two lone pairs and two bonding pairs giving a total of 4 + 2 = 6 electrons. So FC on this oxygen is also zero.

Monday, November 9, 15

Formal Charge

For example:•The two bottom oxygen atoms each have three lone pairs and one bonding pair, giving 6 +1 = 7 electrons and has a FC of -1. The sum of the FC on all the atoms equals the total charge on the whole ion.

Monday, November 9, 15

Formal ChargeVisually, this can get complicated to keep track of so we do it mathematically given the following formula:

F = V - (L + 1/2B)Where:F = formal chargeV = number of valence electrons in the isolated/central atom L = number of lone pair electronsB = number of bonding pair electrons on that particular atom

Monday, November 9, 15

Formal ChargeIf the FC’s on the individual atoms are low, this means less charge transfer has taken place when (considering the formation of the compound or ion from atoms)

This leads to high stability and lowest energy arrangements.

This will be the most accurate description of that compound

Monday, November 9, 15

Formal ChargeLet’s go back to the structures for the sulfate ion:

If the FC’s on the atoms in each structure are calculated, it becomes clear that structure c) has involved the least charge transfer from the isolated atoms, and is the most accurate description of the sulfate ion. Why??

Monday, November 9, 15

Formal ChargeSulfur is in the third period (n = 3) and has an unoccupied d-sublevel. It has the ability to expand its octet and can form compounds with more than eight electrons in its outer shell

Monday, November 9, 15

Homework

Reading will be due next week - won’t be able to get through content by this week

Go to website and download/answer questions on handout about Formation and Destruction of Ozone and Dissociation of Oxygen and Ozone

These handouts, the one from today and reading of HL Topic 4 will be DUE by next Wednesday.

Monday, November 9, 15

Topic 14.1

Any topics you want me to cover specifically this week on Topic 14.1?

Sigma and Pi bondsLewis Structures and Formal ChargesDelocalized Pi ElectronsOzone and/or oxygen dissociation/formation?

If not, I will start on MOT tomorrow and specifically will discuss bonding/antibonding orbitals

Monday, November 9, 15