Modeling Molecules Lab

Virtual Molecular Shapes INTRODUCTION: Individual atoms and molecules are far too small to be seen directly with anything but the most powerful electron microscopes. Despite this, we know much about the spatial arrangement of atoms in molecules. When x-rays are passed through a crystal, they produce a diffraction pattern as they interact with atoms in the crystal. This pattern can be mathematically deconstructed to produce a map showing where the atoms are, in space relative to each other. A good estimation of the shape of a molecule or ion can be determined qualitatively from the Lewis structure by figuring out where the bonding and non-bonding electron pairs are relative to each other, coupled with the knowledge that the electron pairs will repel each other such that they will be as far apart as possible from each other in space (VSEPR). Lone pairs take up more space than bonding electron pairs. Steps for Drawing Lewis Structures (a review): 1. 2. 3. 4. 5. Count the total number of electrons Put the most electronegative atom in the middle (never H) Put the other atoms around the central atom and draw a bond between them and the central atom Place lone pairs around the outer atoms to fill their octets. Count the electrons to see if you still have as many as in step 1. Check to see that the octet rule is in place for each atom. Note that atoms that are larger such as Cl, S, P, I, and As can expand their octet and hold more than 8 electrons. Small atoms from columns 1, 2, and 13 need 2, 4, and 6 electrons respectively. If the number of electrons is correct and every octet is satisfied, you are done. 6. If one of those two conditions is not fulfilled, try adding a double or triple bond to see if you can have the correct number of electrons and full octets. Remember that very rarely will the answer require a free radical (one electron not in a pair). None of these are in this lab. PROCEDURE Part I: Introduction to the molecular shapes Using the pictures below, create the following table. The names of the shapes you have to choose from are tetrahedral, octahedral, trigonal pyramidal, trigonal planar, square planar, square pyramidal, trigonal Virtual Molecular Shapes bipyramidal, seesaw, T-shaped, and bent. Bent has been done for you because there are two different ways to make a bent shape. There is a table in your book that may be helpful. Picture Shape # of atoms # of lone electron Steric number attached to central pairs (lone pairs + atom attached atoms) Bent (this example 2 2 4 is using H2O but you do not need to include an example compound) Triagonal Planar 3 None 3 Tetrahedral 4 None 4 Trigonal Pyramidal 3 1 4 Trigonal Bipyramidal 5 None 5 Seesaw 4 1 5 T-shaped 3 2 5 Octahedral 6 None 6 Square Pyramidal 5 1 6 Square Planar 4 2 6 Part II: Fill in the chart. Molecule Lewis Structure (Turn in a picture of your drawn shapes for full # of atoms attached to Steric number (lone pairs around Molecular Shape Virtual Molecular Shapes credit) CO32- NH3 the central atom 3 3 the central atom + number of attached atoms) 0+3= 3 1+3= 4 Trigonal Planar Trigonal Pyramidal H 2S 2 2+2= 4 Bent PCl4- 4 Square Planar IF5 5 ClF3 3 1+4= 5 1+4= 6 2+3= 5 ICl2- 2 2+2= 4 Linear AsF5 5 0+5= 5 Trigonal Bipyramidal Square Pyramidal T-Shaped Virtual Molecular Shapes Part III: ISOMERS Isomer and Example Geometric Isomers Question Why can’t the molecule in the picture rotate freely, turning the cis isomer into a trans isomer? Answer Because of the double bond Which of these C2H6O isomers is a gas at room temperature, and which is a liquid? Left one is a gas at room temp since oxygen is connected to carbon Right is a liquid oxygen is connected to carbon and a hydrogen Geometric Isomers must have a double bond. When the two green atoms are on the same side of the double bond (left) it is a cis isomer. When they are on opposite sides (right) it is a trans isomer. Structural Isomers Structural Isomers have the same formula but a different connectivity. Here on the left oxygen (red) is bonded to carbon (black), while on the right oxygen (red) is bonded to hydrogen (white) and carbon (black). Structural Isomers have different physical and chemical properties. Stereo Isomers Hint: The molecule on the left is called dimethyl ether. The molecule on the right is called ethyl alcohol because of the O-H group on the side. What is the relationship between these two isomers? Hint: The wicked queen in Snow White frequently consults her “Mirror, mirror on the wall”. Although stereoisomers may seem very similar they have very different properties. They are mirror images of each other or chiral. Like in class the example of our hands, when placed on top of each other they are different but next to each other are mirrored.