Name Molecular Models Lab Day Introduction: The atom and molecule are truly small; 6 10 23 carbon atoms occupy about 4 cm 3, or each atom has a volume of about 6 10 24 cm 3. Molecules, though larger than their constituent atoms, are still staggeringly small. Some of the molecules in nature composed of tens of thousands of atoms are visible only with sophisticated tools such as the electron microscope. It is a wonder that so much is known about the sizes, shapes, and arrangements of atoms in molecules. The behavior of a large number of the same molecules is dependent upon the structure of the individual molecule so that much can be learned from studying the macroscopic properties of a compound. A technique called x-ray crystallography has allowed the chemist to determine which atoms are joined to which others and to determine the distance between them. Spectrophotometric techniques (such as nuclear magnetic resonance and infrared and ultraviolet absorption spectroscopy) and more classical techniques (such as elemental analysis and density determination) have given insights into the structure of molecules. Still, many of these molecular structures are so complicated and delicate that even the most sophisticated chemist has difficulty envisioning them. It is only natural that what sometimes the mind cannot see, the hands and eyes can. The ideal situation would be to have models of the atoms that we could assemble into structures having features reflecting the bonding and electronic characteristics of the elements. Some of these characteristics might be: size of the atom when bonded; angles at which bonds are formed; nature of electrons not directly involved in bonding (i.e., unshared pairs of electrons); positions of the nuclei in relationship to the electrons and their relative flexibility in position, and so on. What would be required would be a model that has these characteristics. In practice, the materials from which models are constructed vary in their ability to represent these qualities and we are constricted by the very nature of the model. One of the simplest models is the stick-and-ball scheme. A solid sphere, not truly representing the nucleus but a compromise in approximating the size of atoms as a whole, has holes drilled at angles depicting the usual bonding angles. Plastic sticks of various lengths are used to represent the pairs of electrons forming the bond and they fit into the drilled holes connecting the spheres together. Sometimes, flexible sticks are used as bonds to connect spheres when the bonding angles are substantially different than the drilled holes.
2 Experimental: Obtain a box containing a collection of sticks and balls. The black colored balls with four holes drilled at angles of approximately 109.5º represent carbon. The blue balls represent nitrogen; there are 3 drilled holes on one side of the ball drilled at 120º representing the bonding angles (the other hole can be used to represent the lone pair of electrons on nitrogen). The white spheres with a single hole represent hydrogen. The red balls with two holes drilled at about 120º are oxygen atoms. The green, orange, and purple spheres with single drilled holes represent chlorine, bromine, and iodine respectively. The plastic sticks represent the bonds between atoms (spheres). The shorter white sticks connect hydrogen to other elements. The short grey plastic sticks serve as bonds between the other elements excluding hydrogen. The flexible long grey plastic sticks are used to connect atoms together in multiple bonding situations. Activities: A. Before starting the lab, draw the Lewis dot structures for the elements listed below: carbon nitrogen oxygen fluorine chlorine iodine hydrogen Next, fill in the table below: element # unpaired electrons # pairs of electrons (2 e = 1 pair) C N O F Cl I H
3 Draw the Lewis dot structures for the molecules below: H2 F2 O2 N2 Fill in the table below: molecule # covalent bonds # unshared electron pairs single, double, or triple bond type? H2 F2 O2 N2 B. In lab, complete this section by (1) drawing the Lewis dot structure for the indicated compound, (2) construct a stick-and-ball model, and (3) using colored pencils, draw a replica for your model next to the dot structure. compound dot structure stick-and-ball replica C2H6 C2H4 C2H2
4 Compare the three compounds. Note that C2H6 has four single bonds to carbon. Applying the Valence Shell Electron Pair Repulsion Model (VSEPR), what is the molecular geometry around each of the carbon atoms? (choose from: tetrahedral, trigonal pyramidal, trigonal planar, bent, or linear). What is the bond angle between the H C H atoms? (choose from 109.5º, 107º, 120º, 105º, or 180º). Is the molecule as a whole polar or is it non-polar?. The second compound, C2H4 has a double bond between the carbon atoms. What is the molecular geometry around each of the carbon atoms?. What is the bond angle between the H C H atoms?. Is the molecule polar or is it non-polar?. The third compound, C2H2 has a triple bond between the carbon atoms. What is the molecular geometry around each of the carbon atoms?. What is the bond angle between the H C H atoms?. Is the molecule polar or is it non-polar? compound dot structure stick-and-ball replica NH3 CO2 both O s are attached to the C, not each other H2CO the H s are attached to C as is the O NH3 is similar in shape to CH4 but in place of one of the hydrogens, nitrogen has a pair of electrons. What is the molecular geometry around the nitrogen atom?. What is the bond angle between the H N H atoms?. Is NH3 polar or non-polar?.
5 How many atoms are attached to carbon in CO2?. How many atoms are attached to carbon in H2CO?. Considering the shapes and number of atoms attached to carbon, what is molecular geometry of CO2?. Of H2CO?. What is the O C O bond angle in CO2?. What is the H C O bond angle in H2CO?. In the Lewis dot formulation of a compound, it is sometimes necessary for one atom to bond to another s unused pair of electrons. This bond is called a coordinate covalent or dative bond. Use this concept in constructing models below: compound dot structure stick-and-ball replica H2SO4 use a black sphere. the H s are attached to individual O s, the O s are attached to S HNO3 the H is attached to an O, the O s are attached to N H2CO3 use the flexible long grey plastic sticks to attach each O to C, the H s are attached to O s What is the shape about the sulfur atom in H2SO4?. What is the shape about the nitrogen atom in HNO3?. How many atoms are directly attached to the sulfur in H2SO4?. How many atoms are directly attached to the nitrogen in HNO3?. What is the O S O bond angle in H2SO4?. What is the O N O bond angle in HNO3?.
6 Now let s have some fun with H2CO3! Remove the hydrogens, but not the connecting flexible grey plastic sticks. Draw a Lewis dot structure below and a pencil sketch of the anion CO3 2 (carbonate) that you just made. Lewis dot structure stick-and-ball replica See how you could re-attach the flexible grey plastic sticks to the carbon (remember carbon can make only four bonds). Draw the three possibilities (try numbering each oxygen atom to tell them apart). In detaching and re-attaching the flexible grey plastic sticks, the atoms do not change their relative positions. The flexible grey plastic sticks symbolize pairs of electrons as they move about the molecule. What is this called?.