$ 5.1 Hybrid Orbital Theory and Atomic OrbitalHybridization1. Hybrid orbital theory-first proposed by Pauling in 1928Hybridization was introduced to explain molecular geometrywhen valencebond theoryfailed to correctlypredict themVALENCEBONDTHEORYValence electrons arelocalized betweentwo atoms (oraslone pairs).Half-filled atomic orbitals overlapto form bondsVB theorydemands orbital hybridization if necessary!MO THEORYSometimes Atomic orbital hybridization is naturally takenintoaccount!
VALENCE BOND THEORY • Valence electrons are localized between two atoms (or as lone pairs). • Half-filled atomic orbitals overlap to form bonds. • VB theory demands orbital hybridization if necessary! §5.1 Hybrid Orbital Theory and Atomic Orbital Hybridization 1. Hybrid orbital theory – first proposed by Pauling in 1928. • Hybridization was introduced to explain molecular geometry when valence bond theory failed to correctly predict them. MO THEORY • Sometimes Atomic orbital hybridization is naturally taken into account!
PleaserefertoWebtext:http://www.chem1.com/acad/webtext/chembond/cb06.htmlhttp://www.chem1.com/acad/webtext/chembond/cb07.htmlLinus Pauling (1901-1994) was the most famous American chemistof the 2Oth century and the author of the classic book The Nature ofthe Chemical Bond. He won the Nobel Prize in Chemistry in 1954and the Nobel Peace Prize in 1962
Please refer to Webtext: http://www.chem1.com/acad/webtext/chembond/cb06.html http://www.chem1.com/acad/webtext/chembond/cb07.html Linus Pauling (1901-1994) was the most famous American chemist of the 20th century and the author of the classic book The Nature of the Chemical Bond. He won the Nobel Prize in Chemistry in 1954 and the Nobel Peace Prize in 1962
a. Central Themes of Valence Bond TheoryBasic Principle of Valence Bond Theory: a covalent bondforms when the orbitals from two atoms overlap and a pair ofelectrons occupies theregion between the nuclei.1)Opposingspinsoftheelectronpair2)Maximum overlapof bonding orbitals.3)Hybridization of atomic orbitals,ifrequired.Pauling proposed that the valence atomic orbitals in a moleculeare different from those in the isolated atoms. We call thisHybridization!
a. Central Themes of Valence Bond Theory 1) Opposing spins of the electron pair. 2) Maximum overlap of bonding orbitals. 3) Hybridization of atomic orbitals, if required. Pauling proposed that the valence atomic orbitals in a molecule are different from those in the isolated atoms. We call this Hybridization! Basic Principle of Valence Bond Theory: a covalent bond forms when the orbitals from two atoms overlap and a pair of electrons occupies the region between the nuclei
b.Why do atomic orbitals need hybridization? e.g., H,O0 2s22p4·SupposeeachobondarisesfromtheoverlapofanH1sAOHwith one of the O2p AO.This model suggests that the H-O-Hbondangleshouldbe90°,muchsmallerthantheobservedone(104.5°)Evenworse,suchabondingmotifwouldposeUseofoverystrongrepulsionamongtheoccupied2s(O) AO and the two O-H bonds!p-orbitals
p - orbitals Use of O b. Why do atomic orbitals need hybridization? e.g., H2O • Suppose each bond arises from the overlap of an H1s AO with one of the O2p AO. Even worse, such a bonding motif would pose very strong repulsion among the occupied 2s (O) AO and the two O-H bonds! • This model suggests that the HO-H bond angle should be 90 , much smaller than the observed one (104.5 ). O 2s22p4
a) No hybridizationb)Hybridization-sp3PPX104.590°HBb1babInequivalentsp3H20hybridization ofO(2sp) AO's!
104.5 b1 b2 H2O Inequivalent sp3 hybridization of O(2sp) AOs! 90 a) No hybridization b) Hybridization –sp3