What s coming up??? Oct 25 The atmosphere, part 1 Ch. 8 Oct 27 Midterm No lecture Oct 29 The atmosphere, part 2 Ch. 8 Nov 1 Light, blackbodies, Bohr Ch. 9 Nov 3,5 Postulates of QM, p-in-a-box Ch. 9 Nov 8,10 ydrogen and multi e atoms Ch. 9 Nov 12 Multi-electron atoms Ch.9,10 Nov 15 Periodic properties Ch. 10 Nov 17 Periodic properties Ch. 10 Nov 19 Valence-bond; Lewis structures Ch. 11 Nov 22 VSEPR Ch. 11 Nov 24 ybrid orbitals; VSEPR Ch. 11, 12 Nov 26 ybrid orbitals; MO theory Ch. 12 Nov 29 MO theory Ch. 12 Dec 1 bonding wrapup Ch. 11,12 Dec 2 Review for exam
Describing the bonding in more complicated molecules... PROPYNE: C 3 CC 16 valence electrons Lewis Dot Structure.. C C C CENTRAL CARBON NEEDS OCTET
PROPYNE: C 3 CC has the structure C C C Tetrahedral Carbon: Linear Carbons ybridization? ybridization? sp 3 sp
PROPYNE: C 3 CC has the bonding C C C C sp 3 overlaps with 1s σ bond C sp 3 overlaps with C sp C p overlaps with C p π bonds C sp overlaps with C sp σ bond σ bond
These four atoms are in a straight line C C C FREE ROTATION of Methyl Group.
LACTIC ACID The bonding frame work of lactic acid O C C C O O
QUESTION O C C C O O TE BOND ANGLE SOWN IS 1 120 2 90 3 180 4 109
QUESTION O C C C O O TE BOND ANGLE SOWN IS 1 120 2 90 3 180 4 109
QUESTION O C C C O O TE BOND ANGLE SOWN IS 1 109 2 90 3 120 4 180
LACTIC ACID O O C C O C
OK, so we have it all figured out! or do we?? Let s think about oxygen, O 2 12 electrons Lewis picture gives :O::O:.... 3 electron groups around each sp 2 and 120 0
O O σ bond π bond All electrons paired diamagnetic BUT O 2 is paramagnetic unpaired electrons!!
MAGNETIC PROPERTIES Atoms, molecules or ions with at least one unpaired electron are paramagnetic. Paramagnetic materials are strongly attracted to magnetic fields. Nitrogen atom is PARAMAGNETIC N 1s 2 2s 2 2p 3 1s 2s 2p
Let s try another approach
Combining the two 1S orbitals from two hydrogen atoms will give another way to think about bonding
COMBINATION OF ORBITALS Remember, when we take linear combinations of orbitals we get out as many as we put in. ere, the sum of the 2 orbitals 1s A + 1s B = MO 1 builds up electron density between nuclei. 90% probability
COMBINATION OF ORBITALS 1s A 1s B = MO 2 results in low electron density between nuclei 1s A + 1s B = MO 1 builds up electron density between nuclei.
TE MO s FORMED BY TWO 1s ORBITALS
ANTI-BONDING σ 1s * 1s A 1s B = MO 2 BONDING Each orbital can hold up to two electrons. 1s A + 1s B = MO 1 σ 1s
COMBINING TWO 1s ORBITALS E Energy of a 1s orbital in a free atom A B Energy of a 1s orbital in a free atom
COMBINING TWO 1s ORBITALS E Energy of a 1s orbital in a free atom A σ 1s B Energy of a 1s orbital in a free atom Energy of 1s A +1s B MO ADDITION gives an Energy more negative than average of original orbitals
SUBTRACTION gives an. Energy more positive than average of original orbitals E Energy of a 1s orbital in a free atom A σ 1s * σ 1s B Energy of a 1s orbital in a free atom ADDITION gives an. Energy more negative than average of original orbitals
COMBINING TWO 1s ORBITALS σ 1s * E 1s A A B 1s B σ 1s
The bonding in 2 2 σ 1s * E 1s 1s σ 1s
2 σ 1s * E 1s 1s σ 1s 2 : (σ 1s ) 2
e 2 Atomic configuration of e 1s 2 e e 2 e σ 1s * E 1s 1s σ 1s One pair of electrons goes in σ 1s and the next pair in σ 1s *
e 2 : (σ 1s ) 2 (σ 1s *) 2 The e 2 molecule is not a stable species. e e 2 σ 1s * e E 1s 1s σ 1s The bonding effect of the (σ 1s ) 2 is cancelled by the antibonding effect of (σ 1s *) 2
Useful concept: BOND ORDER The net number of bonds existing after the cancellation of bonds by antibonds. the electronic configuration is. In e 2 (σ 1s) 2 (σ 1s*) 2 the two bonding electrons were cancelled out by the two antibonding electrons. There is no BOND! BOND ORDER = 0
BOND ORDER A measure of bond strength and molecular stability. If # of bonding electrons > # of antibonding electrons Bond order the molecule is predicted to be stable # of bonding 1/2 # of antibonding } electrons(n b ) electrons (n a ) = { = 1/2 (n b -n a ) A high bond order indicates high bond energy and short bond length. Consider 2+, 2,e 2+,e 2.
First row diatomic molecules and ions 2 2 + e 2 + e 2 E σ 1s * σ 1s Magnetism Para- Dia- Para- Bond order 1 ½ ½ 0 Bond energy (kj/mol) 436 225 251 Bond length (pm) 74 106 108