Lesson 39 Molecular GeometryLesson 39 Molecular Geometry

Objectives:Objectives:

-         -         The student will explain why structure and function of The student will explain why structure and function of molecules are interrelated.molecules are interrelated.

-         -         The student will classify simple structures according The student will classify simple structures according to their shapes.to their shapes.

  

PA Science and Technology Standards: 3.1.10.BPA Science and Technology Standards: 3.1.10.B

I.I. Molecular shapesMolecular shapesa.      Up until now, we have molecular and a.      Up until now, we have molecular and empirical formulas. None of these indicates empirical formulas. None of these indicates

3 3 dimensional shape.dimensional shape.

b.      3D shape determines many of the b.      3D shape determines many of the properties of a chemical.properties of a chemical.

c.      This shape is so important, one change c.      This shape is so important, one change of one amino acid in a 141-146 amino acid of one amino acid in a 141-146 amino acid chain changes the shape of an large chain changes the shape of an large

hemoglobin molecule immensely, causing hemoglobin molecule immensely, causing sickle sickle cell anemia.cell anemia.

II.II. Shapes cannot be predicted from Shapes cannot be predicted from molecular formulasmolecular formulas

a.      Molecules of 2 atoms have simple a.      Molecules of 2 atoms have simple shapes which are easy to predict.shapes which are easy to predict.

b.      Shapes in molecules with more b.      Shapes in molecules with more then then 2 2 atoms become very complicated.atoms become very complicated.

c.      Molecular formulas which look c.      Molecular formulas which look similar similar can produce very different shapes.can produce very different shapes.

                                              i.      SOi.      SO2 2 - bent molecule- bent molecule

                                              ii.    COii.    CO22 – straight molecule – straight molecule

d.      To determine shapes, scientists have d.      To determine shapes, scientists have developed the VSEPR theory.developed the VSEPR theory.

e.      e.      VSEPR theoryVSEPR theory – (valence shell electron pair – (valence shell electron pair repulsion theory) – a system for predicting repulsion theory) – a system for predicting molecular shape based on the idea that pairs of molecular shape based on the idea that pairs of electrons orient themselves as far apart as electrons orient themselves as far apart as possible.possible.

f.      Even though this theory helps determine f.      Even though this theory helps determine molecular shape, it does not predict much about molecular shape, it does not predict much about the electron cloud behavior.the electron cloud behavior.

II.II. How to determine the molecular shape from the How to determine the molecular shape from the Lewis Lewis StructuresStructures

a.      a.      Draw the Lewis structure for the molecule.Draw the Lewis structure for the molecule.

b.      b.      Count the number of electron clouds Count the number of electron clouds surrounding the central atomsurrounding the central atom

                                                      i.      i.      Each single bond counts as an Each single bond counts as an electron electron cloudcloud

                                                      ii.ii. Each multiple bond (double or Each multiple bond (double or triple) triple) generally counts as a single electron generally counts as a single electron

cloudcloud

                                                      iii.iii. Each nonbonding electron pair must Each nonbonding electron pair must be be considered an electron cloud. These considered an electron cloud. These aid aid in determining the geometry, but in determining the geometry, but only only bonded atoms are included in the bonded atoms are included in the

shape.shape.

c.c. Apply the appropriate geometry based Apply the appropriate geometry based on the number of electron on the number of electron

clouds.clouds.

                  i.      i.      Two electron clouds – Two electron clouds – linearlinear

1. Example - CO1. Example - CO22

   ii. ii.  Three electron clouds – three bonding Three electron clouds – three bonding trigonal planartrigonal planar

1.      Example – BF1.      Example – BF33

iii.iii. Three electron clouds – two bonding – Three electron clouds – two bonding –

bentbent

1.      Example – GeF1.      Example – GeF22

iv.   Four electron clouds – four bonding – iv.   Four electron clouds – four bonding – tetrahedraltetrahedral

1.      Example - CH1.      Example - CH44

        v.v. Four electron clouds – three bonding – Four electron clouds – three bonding – trigonal pyramidaltrigonal pyramidal

1.      Example – NH1.      Example – NH33

  vi.vi. Four electron clouds – two bonding – Four electron clouds – two bonding – bentbent

1.      Example – H1.      Example – H22OO

d.d. Determine the approximate bond angles Determine the approximate bond angles in the moleculein the molecule

                                          i.i. Linear – 180Linear – 180oo

                                          ii.     ii.      Bent – less than 109.5Bent – less than 109.5oo

                           iii.iii. Trigonal planar – 120Trigonal planar – 120oo

                                     iv.iv. Tetrahedral – 109.5Tetrahedral – 109.5o o

v.v. Trigonal Pyramidal 107.3Trigonal Pyramidal 107.3o o

You Must Memorize These!!!!!!!!!!!!!!!!!!!!!!You Must Memorize These!!!!!!!!!!!!!!!!!!!!!!

III.III. RestrictionsRestrictions

a.a. The theory as presented here The theory as presented here doesn’t cover molecules other than doesn’t cover molecules other than

those from the main blocks.those from the main blocks.

b.      It also doesn’t consider geometry b.      It also doesn’t consider geometry from atoms which form bonds withfrom atoms which form bonds withmore than an octet of electrons.more than an octet of electrons.

c.c. It is useful as a basic discussion of It is useful as a basic discussion of this theory and for the problems this theory and for the problems presented in this text.presented in this text.

Questions:Questions:

  

1.1. Explain how the structure and function of Explain how the structure and function of a molecule are related.a molecule are related.

2.2. Tell what shape each of the following Tell what shape each of the following situations is:situations is:

a.a. 3 clouds, 2 bonding3 clouds, 2 bonding

b.b. 4 clouds, all are bonding4 clouds, all are bonding

c.c. 2 clouds, both bonding2 clouds, both bonding

d.d. 4 clouds, one unshared pair.4 clouds, one unshared pair.

e.e. 3 clouds, all bonding3 clouds, all bonding

f. 4 clouds, 2 bondsf. 4 clouds, 2 bonds

More Fun with ShapesMore Fun with ShapesDetermine the shape and bond angle of the Determine the shape and bond angle of the following compoundsfollowing compounds

1.1. NHNH33

2.2. CHCH44

3.3. SiOSiO22

4.4. NN22

5.5. BFBF33

6.6. CHCH22OO