Geometry of Molecules Significance? Enzymes (metabolic reactions) Drugs Eyesight Sense of smell

VSEPR Valence Shell Electron Pair Repulsion Based on the notion that electron groups will orient themselves as far away from one another as possible, making bond angles as large as possible.

Structure of water Water has two lone pairs. This causes a compression of the bond angle to 105. Water’s bonds are polar and is asymmetrical in shape so the bond polarities do not cancel. This also makes the water molecule polar, giving it a net positive charge on one side and a net negative charge on the other.

The Unusual Properties of Water Unusually high boiling point Compared to other compounds in Group 16

Two Types of Geometries Electron-Pair Geometry – based on the arrangement of electron groups around the central atom. (multiple bonds treated as one group) Molecular Geometry – based on the arrangement of atoms bonded to the central atom. (must differentiate between bonding and nonbonding electron groups)

Molecular shape determined by bond angles. Molecular size determined by bond length

Electron-Pair Geometries and Approximate Bond Angles

Nonbonding or “lone” pairs spread out more and can cause bond angles to compress. In ammonia, NH 3, the electron-pair geometry predicts a bond angle of but the lone pair causes a slight compression of the angle to 107.

Linear (straight line) Ball and stick model Space filling model EX. CO 2

Bent Ball and stick model Space filling model EX. H 2 O

Trigonal pyramid Ball and stick model Space filling model EX. NH 3

Tetrahedral Ball and stick model Space filling model Ex. CH 4

Molecular Polarity Molecules may be polar or nonpolar. Molecules that are polar are said to have a dipole or possess a dipole moment. Dipole Moment is a value that indicates the degree of polarity of the molecule.

Carbon dioxide is nonpolarWater is polar

Attractions between molecules van der Waals forces Weak attractive forces between non-polar molecules Hydrogen “bonding” Strong attraction between special polar molecules Intermolecular attractions

Intermolecular Forces of Attraction Attraction between a hydrogen atom on one molecule and a very electronegative atom on an adjacent molecule. Only occurs when hydrogen is bonded to F, O, or N. (HF, H 2 O, NH 3 ) Force of a attraction between nonpolar molecules caused by the motions of electrons and instantaneous dipoles. Forces of attraction between polar molecules. Forces of attraction between polar molecules and ions.

Dipole-dipole

Ion-dipole

Molecules will be polar if: 1. There are polar bonds in the molecule. 2. The molecule is not symmetrical.* *If the molecule is symmetrical, the bond polarities will cancel out leaving no net dipole in the molecule. In this case the molecule is nonpolar or does not possess a dipole moment.

Determining type of IFA

Hybridization Occurs when atomic orbitals mix to form a particular bonding set around a central atom. 2p 2 2s 2 1s 2 C H H H H Hybrid orbitals energy methane In methane the carbon is sp 3 hybrid.

Hybridization in Methane

‘sp’ Hybrid Orbitals

Sigma bonds/overlap Sigma overlap occurs on the internuclear axis and between the two bonded atoms. All single bonds are sigma bonds.

pi bonds Pi bonds occur when the overlap is on opposite sides of the internuclear axis. Only regular ‘p’ orbitals can form pi bonds. Double bonds are 1 sigma and 1 pi bondTriple bonds are 1 sigma and 2 pi bonds

Double bond = one sigma and one pi bond

Triple Bond One sigma and two pi bonds

How many sigma and pi bonds are in these molecules? 2-propanol Alpha amino acid

Do you know? 1. Are the bonds ionic or covalent in the following? (a) KCl (b) ZnS (c) PCl 3 (d) Na 2 O (e) HCl 2. Which of the following bonds would have the most ionic character? (a) F-F (b) F-O (c) F-N (d) F-C 3. How many valence electrons each of the following have? (a) C (b) Si (c) Mg (d) P 4. Which of the following has the greatest lattice energy? NaF or KCl Why? 5. Draw the Lewis structure for SO What is its e — pair geometry, molecular geometry, bond angles, and hybridization of the central atom?