1. Molecular Geometry and Polarity Part I: Molecular Geometry – Valence Bond Theory (A) Dr. Chin Chu River Dell Regional High School

3. Molecular Geometry • Lewis structures tell us how the atoms are connected to each other, a.k.a. bonding between atoms. • They do not address how those bonds are formed. • They also don’t tell us anything about shape. • The shape of a molecule can greatly affect its properties.

4. Molecular Geometry Three theories: • VSEPR: Valence shell electron pair repulsion theory • Does not explain in detail how those covalent bonds are formed between atoms. • Valence Bond Theory: • Does provide insights into details of covalent bond formation and associated geometry. • Molecular Orbital Theory: advanced course

5. Valence Bond Theory Combines bonding with geometry. Addresses the equality of bonds in a molecule as far as length and energy are concerned.

6. Valence Bond Theory • Explains the structures of covalently bonded molecules • ‘how’ bonding occurs • Principles of VB Theory: • Bonds form from overlapping atomic orbitals and electron pairs are shared between two atoms • A new set of hybridized orbitals may form out of necessity. • Lone pairs of electrons are localized on one atom

7. http://csi.chemie.tu-darmstadt.de/ak/immel/script/redirect.cgi?filename=http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/orbitals/vrml.htmlhttp://csi.chemie.tu-darmstadt.de/ak/immel/script/redirect.cgi?filename=http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/orbitals/vrml.html

8. How Do Covalent Bonds Form? • Formation of any chemical bonds is driven by the desire of atoms to achieve valence electron configurations with lower energy (more stable) state, a.k.a. Octet Rule. • For example: chlorine gas, Cl2 Each chlorine atom has 7 valence e-, one short of the desirable 8 e-. Where will the extra e- coming from? OK, you don’t have to give me an attitude. How about we share one e- so each of us can get 8? Hey, the other chlorine atom. You got some valence e-, right? How about give me one? Cl Cl That is win-win. Everyone is happy. Hence the formation of a single covalent bond between two Cl atoms. Cl Cl

9. How Do Covalent Bonds Form? • Wait! According to Schrodinger’s quantum mechanical theory, electrons must be on specific orbitals, and there are maximum two electrons occupying the same orbital spinning in opposite directions (Pauli exclusion principle). • What actually happened when the Cl-Cl bond is formed? Cl Cl z ground state valence shell e- conf. Cl 3p y Cl atom x Cl 3s Cl

10. How Do Covalent Bonds Form? • In VBT, a bond will be formed if there is overlap of appropriate orbitals on two atoms and these orbitals are populated by a maximum of two electrons. • The two 3p orbitals occupied by 2 electrons are not going to be involved in bonding due to limit imposed by the Pauli Exclusion Principle. • But the one 3p orbital occupied by 1 electron is capable of forming a strong bond with the other 3p orbital from the 2ndCl atom.

11. VBT: Orbital Overlap to Form Sigma Bonds

12. Sigma Bonds  bonds: symmetric about the internuclear axis s-s overlap s-p overlap p-p overlap: Sideways overlap

13. Valence Bond Theory: Pi Bond • Bonding in Oxygen Molecule (O2) • Bonding in Nitrogen Molecule (N2)

14. Valence Bond Theory: Pi Bond  bonds: form by vertical overlap. Exist in double and triple bonds Atomic axis

15. Formation of Pi Bond Forms by a vertical overlap of two p orbitals (p-p vertical overlap) Exists only when there are multiple covalent bonds Example H2 -C=C-H2: the double bonds contains one sigma and one pi bond

16. Types of Overlaps • Sigma bond (σ ) bond • s-s overlap • p-p overlap • s-p overlap • sp overlap • sp2 • sp3 • sp3d • sp3d2 • Pi (π) bond