Molecular Geometry I. Lewis Structures Localized Electron Bonding Model

1. Molecular Geometry • I. Lewis Structures • Localized Electron Bonding Model • A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms. • Pairs of e- are either • Shared in a bond (bonding pair) • Localized on a single atom (lone pair) • Duet and Octet Rules are in effect: most stable form of any atom • Lewis Structures • Uses : and – to show arrangement of valence electrons in a molecule • Ionic Compounds are straightforward:

2. Covalent Compounds • Shared electrons count for both atoms • A line stands for one pair of electrons • Rules for drawing Lewis structures • Add up total valence e- for all atoms: CH4 = 4x1 + 4 = 8 e- • Use one pair for each bond • Arrange leftover e- to satisfy octet/duet • Examples: CCl4, CO2, H2O and N2, NH3, CF4, NO+

3. 4. Exceptions to the Octet Rule a. 2nd row elements C, N, O, F always observe the octet rule. b. 2nd row elements B and Be (have very small size) often have fewer than 8 electrons around themselves - they are very reactive. c. 3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals. d. When writing Lewis structures, satisfy octetsfirst,then place electrons around elements having available d orbitals. e. PCl5, ClF3, RnCl2

4. 5. Formal Charge • a. The difference between the number of valence electrons (VE) on the free atom and the number assigned to the atom in the molecule. • b. We need: • a. # VE on free neutral atom—from its group in periodic table • b. # VE “belonging” to the atom in the molecule • One e- for each bond • All the lone pairs electrons • c. Examples: • i. NO+ • ii. NO2- • iii. CO2 Counting Three Ways: 1. Total Electrons: sum valence number of each atom + (- charges) – (+ charges) 2. Octet Counting: lone pair electrons + 2 electrons for each shared bond 3. Formal Charge: valence number – bonds – lone pair electrons

5. IV. VSEPR = Valence Shell Electron-Pair Repulsion • Electron pairs want to be as far apart from one another as possible • This applies to bonding pairs and lone pairs alike • Steps in Applying VSEPR • Draw the Lewis Structure • Count atoms and lone pairs and arrange them as far apart as possible (counting a 4th way) • Determine the name of the geometry based only on where atoms are

6. B. Complications to VSEPR • Lone pairs count for arranging electrons, but not for naming geometry • Example: NH3 (ammonia) • Lone pairs are larger than bonding pairs, resulting in adjusted geometries • Bond angles are “squeezed” to accommodate lone pairs Trigonal pyramidal Tetrahedral

7. b. Lone pairs must be as far from each other as possible • 4. Double and triple bonds are treated as only 1 pair of electrons

8. 5. Names for and examples of complicated VSEPR Geometries linear trigonal planar bent trigonal pyramidal tetrahedral bent T-shaped linear trigonal bipyramidal. see-saw octahedral square pyramidal square planar

9. Incident: General Lab Cleanliness and Order