SECTION II: BUILDING MODELS Lesson 9 New Smells, New Ideas Lesson 10 Two s Company Lesson 11 Let s Build It Lesson 12 What Shape Is That Smell? Lesson 13 Sorting It Out Lesson 14 How Does the Nose Know?
LEARNING OBJECTIVES Apply VSEPR to predict electronic geometry and shapes of simple molecules Distinguish between polar and nonpolar bonds in molecules Predict polarity of simple molecules from bond polarity and molecular shape
LESSON 9: NEW SMELLS, NEW IDEAS Ball-and-Stick Models A ball-and-stick model is a three-dimensional representation of a molecule that shows us how the atoms are arranged in space in relationship to one another.
TOTAL NUMBER OF GROUPS DICTATES ELECTRONIC GEOMETRY Octet rule: Two linear Three trigonal planar Four tetrahedral Additional possibilities (expand octet): Five trigonal bipyramidal Six - octahedral
Electronic geometry considers bonded atoms only. Molecular geometry considers unbonded pairs as well
LESSON 10: TWO S COMPANY Electron Domains
YOU WILL BE ABLE TO: determine the shapes of small molecules explain how lone pairs of electrons influence molecular shape describe electron domain theory and how it relates to molecular shape
KEY QUESTION How do electrons affect the shape of a molecule?
MOLECULAR GEOMETRY Molecular geometry is the three-dimensional arrangement of a molecule s atoms in space. Linear Bent Trigonal-planar Tetrahedral Trigonalpyramidal Trigonal-bipyramidal Octahedral
Electron domain: The space occupied by valence electrons in a molecule, either a bonded pair(s) or a lone pair. Electron domains affect the overall shape of a molecule. Electron domain theory: The idea that every electron domain in a molecule is as far as possible from every other electron domain in that molecule.
VALENCE SHELL ELECTRON PAIR REPULSION THEORY
Molecular Shape VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs. VSEPR theory states that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible.
VSEPR THEORY VSEPR (pronounced vesper ) stands for Valence Shell Electron Pair Repulsion Based on Electron Dot (Lewis structures) Theory predicts shapes of compounds based on electron pairs repelling (in bonds or by themselves) Electrons around central nucleus repel each other. So, structures have atoms maximally spread out
METHANE AS A MODEL Examine a simple hydrocarbon such as methane. Methane s chemical formula: CH 4 What do you predict for it s molecular structure? What do you predict for it s geometric shape? (remember VSEPR) 15
The overall geometric shape of a methane model is tetrahedral. The H atoms are at the vertices of a tetrahedron. Bonded pairs of electrons take up space. This space is called an electron domain Incorrect models electron pairs are not equally distant. Correct models All angles between bonds are the same.
Tetrahedral shape: The shape around an atom with four bonded pairs of electrons. This is the shape of a methane molecule.
ANOTHER EXAMPLE Now examine another simple molecule such as ammonia. Ammonia s chemical formula: NH 3 An electron domain describes the area occupied by a set of electrons in a bond or a lone pair. What do you predict for it s molecular structure? What do you predict for it s geometric shape? 18
Unshared electron pairs repel other electron pairs more strongly than bonding pairs do. This is why the bond angles in ammonia and water are somewhat less than the 109.5 o bond angles of a perfectly tetrahedral molecule.
Trigonal pyramidal one atom at the apex and three atoms at the corners of a trigonal base, resembling a tetrahedron, However, the three hydrogen atoms are repelled by the electron lone pair in a way that the geometry is distorted to a trigonal pyramid. 20
Use VSEPR theory to predict the molecular geometry of boron trichloride, BCl 3. First write the Lewis structure. Boron is in Group 13 and has 3 valence electrons. Chlorine is in Group 17 so each chlorine atom has 7 valence electrons. The three B-Cl bonds stay farthest apart by pointing to the corners of an equilateral triangle, giving 120 o angles between the bonds. This would be trigonal-planar geometry.
The z plane 3-D
Each shape has a name (you will have to memorize these) tetrahedral trigonal pyramidal bent linear trigonal planar
Note: that lone pairs of electrons effect the shape but are ignored in the name of the geometry
The geometry of a molecule refers to the positions of atoms only. 25
VSEPR AND MOLECULAR GEOMETRY
VSEPR AND MOLECULAR GEOMETRY
methane, CH 4 Tetrahedral 109.5 Bonds are all evenly spaced apart
.. ammonia NH 3 Trigonal Pyramidal Less repulsion between the bonding pairs of electrons surprise: the lone pairs occupy more space than the bonded atoms (with very few exceptions)
H O H.. water H 2 O bent Two unbonded pairs of electrons make bond angles slightly less than tetrahedral due to greater repulsion
.. Carbon dioxide CO 2 linear Bonded electrons can take maximum position apart 180
.. Barium floride BF 3 Trigonal planar No lone pairs of electrons allows maximum bond angle in 1-D plane
LESSON 11: LET S BUILD IT Molecular Shape
CHEMCATALYST 1. What is the Lewis dot structure of formaldehyde, CH 2 O? 2. Draw formaldehyde s structural formula. 3. How many electron domains do you think this molecule has? Explain your reasoning.
KEY QUESTION How can you predict the shape of a molecule?
YOU WILL BE ABLE TO: predict and explain molecular shape, including in molecules with multiple bonds
PREPARE FOR THE ACTIVITY Work in groups of four. Using the gumdrop, marshmallow, and toothpick kits, build a model of formaldehyde, CH 2 O.
DISCUSSION NOTES Double or triple bonding changes the number of electron domains around an atom, affecting the overall shape of a molecule. Trigonal planar shape: A flat triangular shape found in small molecules with three electron domains surrounding the central atom.
DISCUSSION NOTES (CONT.) Linear shape: A geometric shape found in small molecules with two electron domains surrounding the central atom. The number of electron domains is more important in determining the structure of a molecule than is the number of atoms.
DISCUSSION NOTES (CONT.) The more atoms in a molecule, the more combinations of shapes you might see together.
WRAP UP How can you predict the shape of a molecule? Drawing the Lewis dot structure of a molecule allows us to predict its three dimensional shape. The presence of double or triple bonds changes the number of electron domains around an atom, which in turn affects the overall shape of the molecule. The shape of large molecules is determined by the smaller shapes around individual atoms.
Water, H 2 O, has two unshared pairs, and its molecular geometry takes the shape of a bent or angular molecule. Bent
Molecule Lewis Structure Number of electron pairs SHAPE CH 4 4 Tetrahedral NH 3 4 Trigonal Pyramidal (3 shared 1 lone pair)
Molecule Lewis Structure Number of electron pairs SHAPE H 2 O 4 (2 shared 2 lone pairs) Bent or V CO 2 2 Linear
Molecule Lewis Structure Number of electron pairs SHAPE BeCl 2 2 Linear BF 3 3 Trigonal Planar
LESSON 14: HOW DOES THE NOSE KNOW? Receptor Site Theory
CHEMCATALYST 1. Suppose you needed to separate coins but could not see them. Explain how you would make a machine that detects and sorts coins. 2. How do you think your nose detects a smell?
KEY QUESTION How does the nose detect and identify different smells?
YOU WILL BE ABLE TO: come up with a plausible model to explain how smell works in the nose, based on the evidence thus far describe the receptor site model
PREPARE FOR THE ACTIVITY Work in groups of four.
DISCUSSION NOTES Scientists have proposed many theories about how smell works and created models corresponding to these theories.
DISCUSSION NOTES (CONT.) Receptor site theory: The currently accepted model explaining how smells are detected in the nose. Molecules fit into receptor sites that correspond to the overall shape of the molecule. This stimulates a response in the body.
WRAP UP How does the nose detect and identify different smells? The currently accepted model for smell describes smell molecules landing in receptor sites that fit the shape of the smell molecules. In the receptor site model, each receptor site has a specific shape that corresponds to the shape of just a few smell molecules.
CHECK-IN One of the molecules that gives coffee its smell is 2- furylmethanethiol. 1. Write down everything you know about how this molecule is detected by the nose. 2. Draw a possible receptor site for this molecule.
MOLECULAR POLARITY AND DIPOLE-DIPOLE FORCES The strongest intermolecular forces exist between polar molecules. Polar molecules act as tiny dipoles. A dipole is created by equal but opposite charges that are separated by a short distance. The direction of a dipole is from the dipole s positive pole to its negative pole. A dipole is represented by an arrow with its head pointing toward the negative pole and a crossed tail at the positive pole. The dipole created by a hydrogen chloride molecule is represented below: H Cl
The forces of attraction between polar molecules are known as dipoledipole forces. The negative region in one polar molecule attracts the positive region in adjacent molecules. So the molecules all attract each other from opposite sides.
Which atom attracts e- more? electronegativities 2.1 3.0 H Cl δ + δ - 2.1 2.1 H H 2.5 3.5 C = O O = C = O