Announcements. 10pm Room assignments for Exam III and TA Exam Review Sessions are posted on website. Look under Exam Info link

Transcription

1 Announcements 1. Exam #3: Thursday, Dec. 6 th, 7:00-8:15pm (Conflict: 5:15-6:30pm) No calculators allowed 2. Activity 3: Making Models of Molecules lab write-up due tomorrow in discussion 3. Lon-capa HW Remaining 1. Homework 7 Type 1 due Monday, 12/3 by 7pm 2. Homework 8 Type 1 due Tuesday, 12/4 by 7pm 3. Review Questions for Exam III due Wed, 12/5 by 10pm Room assignments for Exam III and TA Exam Review Sessions are posted on website Look under Exam Info link

2 TA Exam Review Sessions

3 General Rules for Lewis Structures 1. Follow the octet rule. Follow the duet rule for hydrogen. 2. The total number of electrons in your Lewis structure must equal the total number of valence electrons. 3. The least electronegative atom is typically the central atom (except for hydrogen).

4 Clicker #1 Which of the following supports why Lewis structures are not a completely accurate way to draw molecules? A) The Lewis structure for O 2 shows all electrons as paired but when liquid O 2 is poured onto a magnet, it behaves as a molecule with an unpaired electron. B) There are sometimes multiple valid Lewis structures for a single chemical species. C) When adding up the number of valence electrons for a molecule (such as nitrogen monoxide), it is possible to get an odd number which would make it impossible to satisfy the octet rule for all atoms. D) All of the above statements support why Lewis structures are not a completely accurate way to draw molecules. E) Lewis structures are the most accurate way to draw molecules and are completely correct.

5 Exceptions to the rules There are problems with model of Lewis structures As we saw with the paramagnetism of O 2 last time We don t know that electrons are exactly where we placed them (placement based on probability) Because the model is not perfect, there are exceptions to the rules

6 Example: BH 3 Valence electrons for B: 3 Valence electrons for H: 1 Total valence electrons: 3 + (1*3) = 6 H B H H Even though boron does not have 8 valence electrons, this Lewis structure is valid (boron can have only 6 valence electrons in a molecule)

7 Example: BeF 2 Valence electrons for Be: 2 Valence electrons for F: 7 Total valence electrons: 2 + (2*7) = 16 F Be F Even though beryllium does not have 8 valence electrons, this Lewis structure is valid (Be can have only 4 valence electrons in a molecule)

8 Exceeding the octet rule If central atom is in third row or below, can exceed 4 bonds (8 electrons) if needed Theorized that d orbitals can contribute to expanded bonding Cl Example: PCl 5 P: 5 valence electrons Cl: 7 valence electrons Cl P Cl Total valence e-: 40 Cl Cl

9 Br Example: XeBr 4 Xe: 8 valence electrons Br: 7 valence electrons Total valence electrons: 36 Br Xe Br Noble gases can sometimes exceed the octet rule as well Br

10 Determining Polarity Determining if a molecule is is very important because it helps us to understand why it has certain properties (for example, why H 2 O is a liquid but H 2 and O 2 are gases at room temperature) Polar uneven distribution of electron pairs Molecule overall is asymmetrical (not symmetrical) Non even distribution of electron pairs Molecule overall is symmetrical

11 Determining Polarity First: draw Lewis structure for molecule Second: determine geometry (how atoms and lone pairs are arranged around central atom) Third: determine shape of molecule (how does molecule look with just atoms, not including lone pairs) Fourth: Ask yourself - is the molecule symmetrical?

12 Terminology Effective pairs: composed of all valence electron pairs around central atom (bonding pairs and lone pairs) Multiple bonds are treated like single bonds, so they only count as one effective pair Lone pairs: do not participate in bonding Geometry: how effective pairs are arranged around central atom to minimize electron-electron repulsion 5 types of geometry Shape: how bonded atoms are arranged (does not include lone pairs!)

13 To determine geometry and shape, refer to charts in lab book, pp You need to memorize these charts for the exam!!! Thing that drives determining geometry and shape is Valence Shell Electron Pair Repulsion theory (VSEPR) The key is to minimize repulsion between electron pairs by moving them as far apart as possible This results in lower energy = more stable!

14 Geometry terms Linear Number of effective pairs: 2 Bond angles: 180 Examples: CO, CO 2

15 Geometry terms Trigonal planar Number of effective pairs: 3 Bond angles: 120 Examples: BF 3, NO 3-, SO 2

16 Geometry terms Tetrahedral Number of effective pairs: 4 Bond angles: Examples: CH 4, NH 3, H 2 O, HCl

17 Geometry terms Trigonal bipyramid Number of effective pairs: 5 Bond angles: 90, 120, 180 Examples: PCl 5, SF 4, ClF 3, I 3 -

18 Geometry terms Octahedral Number of effective pairs: 6 Bond angles: 90, 180 Examples: SF 6, ICl 5, XeF 4

19 Geometry and Shapes While C-O bond is, both pull equally in opposite directions; non C-O bond is more than C-S, uneven pull on center atom; Molecules with more than one possible structure have resonance Usually molecules with multiple bonds that can be drawn between more than one atom Lone pair does not pull, and C-O bond is ;

20 Geometry and Shapes While B-F bond is, all three pull equally in opposite directions; non B-F bond is more than B-H, uneven pull on center atom; Lone pair does not pull, and Se-O bond is ;

21 Clicker #2 Does TeS 2 have an overall net dipole moment (is it overall)? A) Yes, the electrons are found more towards the sulfurs. B) Yes, the electrons are found more towards the tellurium. C) No, the electron distribution is symmetrical around the central atom.

22 Geometry and Shapes non

23 Clicker #3 Does CCl 4 have an overall net dipole moment (is it overall)? A) Yes, the electrons are found more towards carbon. B) Yes, the electrons are found more towards the chlorines. C) No, the electron distribution is symmetrical around the central atom.

24 Geometry and Shapes non non

25 Clicker #4 Does ICl 2 have an overall net dipole moment (is it overall)? A) Yes, the electrons are found more towards the central iodine atom. B) Yes, the electrons are found more towards the outer chlorine atoms. C) No, the electron distribution is symmetrical around the central atom.

26 Geometry and Shapes non non

27 Clicker #5 Does SF 6 have an overall net dipole moment (is it overall)? A) Yes, the electrons are found more towards sulfur. B) Yes, the electrons are found more towards the fluorines. C) No, the electron distribution is symmetrical around the central atom.