Chem 162A 1/3/2007 Please Review Chapter 13. General Concepts of Bonding in Molecules Types of chemical bonds: ionic, covalent, etc. Bond energy, bond length, bond polarity, etc. Localized electron (LE) model of bonding A molecule is a collection of atoms bound together by sharing electrons btw atomic orbitals. Arrangement of valence electrons through the Lewis structure. — methane CH4: — ammonia NH3: .. The valence shell electron-pair repulsion (VSEPR) model Prediction of molecular geometry Minimize electron-pair repulsions around atoms.

.. 109.5o C methane CH4: H H tetrahedral geometry N ammonia NH3: H What types of orbitals are used to share electrons, i.e. form covalent bonds? Addressed in Chapter 14.

Figure 14.1: (a) Lewis structure of the methane molecule (b) the tetrahedral molecular geometry of the methane molecule. Copyright © Houghton Mifflin Company. All rights reserved.

14.1 Hybridization and the LE Model of Bonding — Assume bonding involves only valence orbitals — Methane, CH4: Isolated atoms Valence orbitals H 1s1 C 2s22p2 (2p: 2px, 2py, 2pz) H atoms in CH4 will use 1s orbitals Of the two types of orbitals (2s and 2p) Which will C atoms use for bonding in CH4? — If both are used: 2 different types of C-H bonds (Contrary to experimental facts) — Neither of the “native” atomic orbitals of C atoms are used; instead, new hybrid orbitals are used.

Hybridization of atomic orbitals The mixing of the “native” atomic orbitals to form special orbitals for bonding is called hybridization. The 4 new equivalent orbitals formed by mixing the one 2s and three 2p orbitals are called sp3 orbitals. The carbon atom is said to undergo sp3 hybridization, i.e. is sp3 hybridized. Energy-level diagram showing the sp3 hybridization Energy 2p hybridization sp3 2s Orbitals in C in CH4 molecule Orbitals in isolated C atom

Figure 14.3: native 2s and three 2p atomic orbitals characteristic of a free carbon atom are combined to form a new set of four sp3 orbitals. Copyright © Houghton Mifflin Company. All rights reserved.

Figure 14.5: Energy-level diagram showing the formation of four sp3 orbitals Copyright © Houghton Mifflin Company. All rights reserved.

Figure 14.6: Tetrahedral set of four sp3 orbitals on the carbon atom Copyright © Houghton Mifflin Company. All rights reserved.

SP3 Hybridization by Other Atoms Other atoms requiring 4 equivalent tetrahedral atomic orbitals for bonding are also sp3 hybridized. E.g. ammonia, NH3: Lewis structure — 4 effective electron pairs around the nitrogen atom — from VSEPR model we predict a tetrahedral geometry N: 2s22p3 The hybrid atomic orbitals used by N atom in NH3 are sp3, i.e. N is sp3 hybridized. ..

Figure 14.7: The nitrogen atom in ammonia is sp3 hybridized. Copyright © Houghton Mifflin Company. All rights reserved.