Chem 105 Monday, 31 Oct 2011

Transcription

1 Chem 105 Monday, 31 Oct 2011 Ch 7: Ion sizes; Ionization Potential Ch 8: Drawing Lewis Formulas Formal charge Resonance 10/31/2011 1

2 Sizes of Ions Cations (remember ca + ion) always SMALLER than corresponding atom (you re removing electrons (-) usually a whole shell - without changing nuclear charge (+)) Anions Always LARGER than corresponding atom (you re adding electrons to complete a shell usually without increasing the nuclear charge.) 10/31/2011 2

3 Atom and Common Anion Size Comparison These 3 have 10 e- (isoelectronic) 10/31/2011 3

4 Ionization energy (= ionization potential) definition A (g) ---> A + (g) + 1e - E = 1 st ionization energy (kj/mol) A + (g) ---> A 2+ (g) + 1e - E = 2 nd ionization energy (kj/mol) A 2+ (g) ---> A 3+ (g) + 1e - E = 3 rd ionization energy (kj/mol) etc 10/31/2011 4

5 Ionization energies E are always positive, because you re separating e - from a cation (+). A (g) ---> A + (g) + 1e - E = 1 st ionization energy (kj/mol) 10/31/2011 5

6 IP DECREASES going down a group. (Outermost e - is further from nucleus less tightly held.) 10/31/2011 6

7 IP generally INCREASES going across a row. (Outermost e - is closer to the nucleus become MORE tightly held. Effective nuclear charge increases.) 10/31/ (Don t worry about slight variations.)

8 Ch. 8. Molecules and the Octet Rule 10/31/2011 8

9 10/31/ ydrogen atom: 1s 1 Carbon atom: 1s 2 2s 2 2p 2 Methane = 4 nuclei; C nucleus+core; 8 valence e- Solve Schrödinger Eq Core electrons Valence (outer shell) electrons

10 10/31/ Valence electrons = electrons in the shell (or two shells) outside the noble gas core [ ] d-electrons (if any) don t count here n [ ]ns 1 [ ]ns 2 [ ]ns 2 np x [ ](n-1)d x ns y See Table 8.1

11 10/31/ The Lewis Dot Symbol for a main group atom Place # dots = # valence electrons around the symbol of the element. Li C N F Ne Most smaller atoms can contain up to an octet of electrons in the valence shell noble gas configuration. (Some larger atoms can contain more than an octet). Cl Octet

12 10/31/ COVALENT Nonmetals form compounds by sharing electrons to obtain pseudo-octet. These are basis for covalent bonds in molecules.

13 10/31/ Formation of a covalent compound between 3 s, C, and F These are non-bonding electron pairs. alogens typically have 3 prs (6 total) C F C F Each line represents 2 e- shared between C and F. F contains an octet

14 10/31/ C F This is a Lewis formula - Elemental symbol for atom - 1 line for each covalent bond = 2 electrons shared - Extra dots for non-bonding (or lone pair ) valence electrons. The positions do not matter as long as electrons are in the correct locations.

15 Octet Rule: In molecules, smaller atoms like to be surrounded by 8 valence electrons. Either their own lone-pair electrons, or covalent bonds. 2 C 8 F 8 Exceptions: -- only has There are some smaller atoms with fewer than 8 -- Some larger atoms sometimes are surrounded by 10 or 12 valence electrons. 10/31/

16 10/31/ Covalent bonds place electron density between atoms. C 3 F F C F Mg F MgF 2 Ionic bonds have few, if any, electrons between atoms.

17 Drawing Lewis formulas 10/31/

18 Molecular formula (list of elements and charge) Connection map (what is connected to what) Lewis formula approximate geometry Computation: Geometry Optimization (nearly) exact geometry and polarity Single crystal X-ray diffraction The exact geometry 18

19 NB: Molecular formula is just a LIST. It does NOT necessarily indicate bond connections! 1. Count total no. valence electrons (+ or according to charge) 2. Connect atoms with single bonds (2 e- per bond) per map information. Chlorite ion ClO 2- (Cl is central) 7 + 2(6) + 1 = 20 valence e- Now there are 20 2(2) = 16 e- left 3. Starting with outer atoms, add remaining valence e- in pairs to make up octets. (still 4 e- left) 4. For ions, put bracket around formula and total charge on top, right. 10/31/ Each atom has an octet.

20 Do formaldehyde on the board. 10/31/

21 NB: Molecular formula is just a LIST. It does NOT necessarily indicate bond connections! Formaldehyde C 2 O (C is central) 1. Count total no. valence electrons (+ or according to charge) 4 + 2(1) + 6 = Connect atoms with single bonds (2 e - per bond) per map information. 3. Starting with outer atoms, add remaining valence e - in pairs to make up octets. 12 3(2) = 6 4. If a central atom is shy of octet, create a multiple bond by moving in an e - pair. Repeat as necessary. 10/31/

22 ow many valence electrons are in carbon dioxide? 3A 4A 5A 6A 7A 8A B

23 Counting valence electrons CO 2 : 1(4) + 2(6) = 16 10/31/