Lesson 39 Molecular Geometry Objectives: - The student will explain why structure and function of molecules are interrelated. - The student will classify simple structures according to their shapes. PA Science and Technology Standards: B

I. Molecular shapes a. Up until now, we have molecular and empirical formulas. None of these indicates 3 dimensional shape. b. 3D shape determines many of the properties of a chemical. c. This shape is so important, one change of one amino acid in a amino acid chain changes the shape of an large hemoglobin molecule immensely, causing sickle cell anemia.

II.Shapes cannot be predicted from molecular formulas a. Molecules of 2 atoms have simple shapes which are easy to predict. b. Shapes in molecules with more then 2 atoms become very complicated. c. Molecular formulas which look similar can produce very different shapes. i. SO 2 - bent molecule i. SO 2 - bent molecule ii. CO 2 – straight molecule ii. CO 2 – straight molecule

d. To determine shapes, scientists have developed the VSEPR theory. e. VSEPR theory – (valence shell electron pair repulsion theory) – a system for predicting molecular shape based on the idea that pairs of electrons orient themselves as far apart as possible. f. Even though this theory helps determine molecular shape, it does not predict much about the electron cloud behavior.

II.How to determine the molecular shape from the Lewis Structures a. Draw the Lewis structure for the molecule. b. Count the number of electron clouds surrounding the central atom i. Each single bond counts as an electron cloud i. Each single bond counts as an electron cloud ii.Each multiple bond (double or triple) generally counts as a single electron cloud ii.Each multiple bond (double or triple) generally counts as a single electron cloud iii.Each nonbonding electron pair must be considered an electron cloud. These aid in determining the geometry, but only bonded atoms are included in the shape. iii.Each nonbonding electron pair must be considered an electron cloud. These aid in determining the geometry, but only bonded atoms are included in the shape.

c.Apply the appropriate geometry based on the number of electron clouds. i. Two electron clouds – linear i. Two electron clouds – linear 1. Example - CO 2

ii. Three electron clouds – three bonding trigonal planar ii. Three electron clouds – three bonding trigonal planar 1. Example – BF 3 iii.Three electron clouds – two bonding – bent 1. Example – GeF 2

iv. Four electron clouds – four bonding – tetrahedral 1. Example - CH 4 v.Four electron clouds – three bonding – trigonal pyramidal v.Four electron clouds – three bonding – trigonal pyramidal 1. Example – NH 3

vi.Four electron clouds – two bonding – bent vi.Four electron clouds – two bonding – bent 1. Example – H 2 O

d.Determine the approximate bond angles in the molecule i.Linear – 180 o i.Linear – 180 o ii. Bent – less than o ii. Bent – less than o iii.Trigonal planar – 120 o iii.Trigonal planar – 120 o iv.Tetrahedral – o iv.Tetrahedral – o v.Trigonal Pyramidal o You Must Memorize These!!!!!!!!!!!!!!!!!!!!!!

III.Restrictions a.The theory as presented here doesn’t cover molecules other than those from the main blocks. b. It also doesn’t consider geometry from atoms which form bonds with more than an octet of electrons. c.It is useful as a basic discussion of this theory and for the problems presented in this text.

Questions: 1. Explain how the structure and function of a molecule are related. 2. Tell what shape each of the following situations is: a.3 clouds, 2 bonding b.4 clouds, all are bonding c.2 clouds, both bonding d.4 clouds, one unshared pair. e.3 clouds, all bonding f. 4 clouds, 2 bonds

More Fun with Shapes Determine the shape and bond angle of the following compounds 1. NH 3 2. CH 4 3. SiO 2 4. N 2 5. BF 3 6. CH 2 O