Chem 105 Monday, 31 Oct 2011 Ch 7: Ion sizes; Ionization Potential Ch 8: Drawing Lewis Formulas Formal charge Resonance 10/31/2011 1
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
Atom and Common Anion Size Comparison These 3 have 10 e- (isoelectronic) 10/31/2011 3
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
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
IP DECREASES going down a group. (Outermost e - is further from nucleus less tightly held.) 10/31/2011 6
IP generally INCREASES going across a row. (Outermost e - is closer to the nucleus become MORE tightly held. Effective nuclear charge increases.) 10/31/2011 7 (Don t worry about slight variations.)
Ch. 8. Molecules and the Octet Rule 10/31/2011 8
10/31/2011 9 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
10/31/2011 10 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
10/31/2011 11 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
10/31/2011 12 COVALENT Nonmetals form compounds by sharing electrons to obtain pseudo-octet. These are basis for covalent bonds in molecules.
10/31/2011 13 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
10/31/2011 14 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.
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 2. -- There are some smaller atoms with fewer than 8 -- Some larger atoms sometimes are surrounded by 10 or 12 valence electrons. 10/31/2011 15
10/31/2011 16 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.
Drawing Lewis formulas 10/31/2011 17
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
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. 10/31/2011 19 Each atom has an octet.
Do formaldehyde on the board. 10/31/2011 20
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 = 12 2. 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. 10/31/2011 21
ow many valence electrons are in carbon dioxide? 3A 4A 5A 6A 7A 8A 1. 8 2. 10 3. 12 4. 16 5. 20 17 16 10 92 4 2B 8 10 12 16 20 1 2 3 4 5 6 7 8 9 10 11 12 13 1 14 27 40 53 66 79 92 105 118 131 144 157 170 183 196
Counting valence electrons CO 2 : 1(4) + 2(6) = 16 10/31/2011 23