Covalent Bonds With all humility and mildness, with patience, support one another in charity. Careful to keep the unity of the Spirit in the bond of peace Ephesians 4:2-3 Introduction A covalent bond is one in which two non-metals react and share electron pairs between them. These electron pairs are called bonding pairs or shared pairs. The number of bonds an atom will form is usually (but not always) the number of electrons needed to complete the octet. For example, carbon has 4 valence electrons and needs 4 more to complete an octet. Therefore, it may form 4 covalent bonds. On the other hand, nitrogen has 5 valence electrons, needs 3 more to complete an octet and therefore forms 3 covalent bonds. The sharing of electrons allows each atom to acquire Noble Gas electron configuration. The attachments are permanent, unless chemically acted upon and broken. Therefore, when placed in an aqueous solution, covalent compounds will not dissociate as ionic compounds do. Covalent compounds do not then conduct electricity. The melting points of covalent compounds tend to be much lower than ionic compounds (<300 o C). The physical structure of covalent compounds ranges from crystalline to amorphous. Some are gases and others liquids at room temperature. It is possible to draw a Lewis structure or a structural diagram of a molecule. The limitation of these diagrams is that they are two-dimensional representations. But, molecules are threedimensional. Knowing the three-dimensional structure of a molecule is crucial in understanding the properties and behavior or that molecule. The three-dimensional shape of molecules is predicted using the Valence Shell Electron Pair Repulsion (VSEPR) theory. In short, the idea of the theory is that atoms in a molecule will arrange themselves in order to maximize the distance between them and to minimize the repulsion of electron pairs. Electron pairs could be involved in bonding, or they could be nonbonding pairs. In this lab, we will explore the difference between a written structural diagram of a molecule and its true three-dimensional structure. Learning Objectives: Visualize in three dimensions the structure of covalently-bonded compounds Materials Required: From Chemistry Kit Protractor Student Supplied Periodic Table Scissors Marshmallow bag, small different colors Toothpicks Marker Preparation You will construct the three-dimensional model of a molecule using the marshmallows, toothpicks and Velcro in the following manner: Marshmallows represent atoms One toothpicks represents one pair of electrons, or a single covalent bond 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 1
Two toothpicks represent a double bond A piece of Velcro, one corner stuck into a marshmallow, represents a lone pair of electrons on that atom (marshmallow) You will type or write the structural diagram in Table 1. If you choose to type the structural diagram, you may use the following system: Two dashes for a horizontal line (--) Example: C -- H Angled lines a backslash ( \ ) or a forward slash ( / ) Example: H \ C Vertical lines the uppercase of the backslash key ( \ ) gives a vertical line ( ) Example: H C Electron pairs, horizontal two adjacent periods (.. ) Example:.. O Electron pairs, vertical colon ( : ) Example: : O Double Bonds an equal sign (=) Example: C=O Experiment 1. Methane has the chemical formula, CH4 Make a model of methane using the marshmallows and toothpicks Different colors for different atoms work best Choose one color for carbon and another color for hydrogen If there are not enough different colors, you can distinguish atoms by using dots with the permanent marker You may cut the marshmallows with the scissors to have more of one color Note: Make sure the model follows VSEPR theory rules. Use your text if you need assistance. Use the protractor to measure the angles formed by toothpicks in your model Did the angles in your model come close to the theoretical value? If not, move the toothpicks so that the angles match theoretical values Write the molecular shape and the bond angles in Table 1 2. Ammonia has the chemical formula, NH3 Model ammonia just as you did for methane in Step 1 Identify the molecular shape and the bond angle 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 2
3. Water has the chemical formula, H2O Model water just as you did for methane in Step 1 Identify the molecular shape and the bond angle 4. Ethane has the chemical formula, C2H6 Model ethane just as you did for methane in Step 1 5. Ethene has the chemical formula, C2H4 Model ethene, keeping VSEPR theory in mind Compare a Covalent Bond with an Ionic Bond Understanding the differences between ionic bonding and covalent bonding is critical. Covalent compounds share pairs electrons between atoms that bind those atoms in fixed orientations (angles) In ionic compounds, the less electronegative atom of the pair will lose one or more electrons to the more electronegative atom, creating positive and negative ions. The bond formed has no definite shape. Since the ionic compounds exist as pairs of ions, they are attracted to each other and will form lattice structures. Here you will form a lattice structure with Na + and Cl. This lattice structure should be at least 3 levels high so that you can understand the three-dimensional structure. Then you will compare it with a purely covalent compound, Cl2. 6. Compare covalent bonds with ionic bonds: Make one NaCl ionic bond: Take 6-8 marshmallows of one color and snip them in half o Place a + with a marker on 2-opposite-faces of each o These will be your Na + Take about 15 marshmallows of a second color o Place a - with a marker on 2-opposite-faces of each o These will be your Cl Slightly wet one edge of the Na + and stick it against the Cl to model electrostatic attraction between the ions Model NaCl solid: Make a second, third and fourth NaCl compound just as you did above How would these compounds stack together in a solid? How would they behave in an aqueous solution? 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 3
Because of the electrostatic charge, the Na + in one compound is attracted to the Cl of another o Therefore, what you will NOT observe is what appears in the figure to the right. o This will not occur when ionic compounds are in the solid form o This not occur when ionic compounds are dissolved in aqueous solutions What IS observed is a three-dimensional stacking With all the Na + and Cl marshmallows you have cut, model a salt crystal The figure should resemble the following: Model NaCl dissociation: Dissociation into Na + and Cl occurs when NaCl is placed in water Model how the NaCl Ionic Bond Lattice is pulled apart by water molecules Model how the NaCl would appear if we could view the tiniest particles in solution Make a Cl2 bond: Gather about 10 marshmallows of one color you may use the same ones you designated for Chlorine above How many valence electrons does Chlorine have? The Cl2 molecule is a covalent bond Make 5 Cl2 molecules using a toothpick to model one electron pair shared between two atoms Compare these molecules with the NaCl in the ionic bond crystal What are the differences and similarities How may the differences in the bonds account for the differences in physical properties between covalent and ionic bonds? 7. Data Analyses and Conclusions 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 4
NAME Table 1 Structure of Covalent Molecules CHEMICAL FORMULA STRUCTURAL (Lewis) DIAGRAM MOLECULAR SHAPE BOND ANGLE Methane Ammonia Water Ethane ------- ------- Ethene ------- ------- Data Analysis 1. In today s experiment, and in Chemistry in general, we model something unseen (atoms and compounds) with something seen. Describe what is being modeled nexed to each of the listed items below: One marshmallow Two marshmallows connected with one toothpick Two marshmallows connected with two toothpicks One toothpick (or the straight line between C O for example) 2. Compare the molecular shapes and bond angles of methane, ammonia and water. a. How many total domains are around each central atom? Of the domains, how many consist of lone pairs and how many are bonding pairs? Methane: Ammonia: Water b. What effect does one or more lone pairs have on the bond angles and molecular shapes? 3. Compare the models for ethane and ethene side by side. Note: Spellcheckers will most likely change ethene to ethane when you type in your answers. Double check the names of the molecules you typed in prior to submitting the lab report. 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 5
a. Which molecule is flat in shape? b. Which molecule can rotate freely around all its covalent bonds? c. Which molecule is rigid, with no free rotation around its bonds? 4. The formula for ethyne is C2H2. a. What kind of covalent bond joins the carbons in ethyne? b. What shape is the ethyne molecule? 5. Molecules are polar when atoms of different electronegativity values do not balance each other within the molecule. Based on the molecular structures you modeled in this lab: 1) Classify each molecule as polar or nonpolar, and 2) Explain your answers. a. Methane b. Ammonia c. Water d. Ethane e. Ethene 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 6
6. Choose a molecule that has at least 3 atoms in it. Answer the following questions about this molecule: Remember: Only covalent molecules are composed of covalent bonds. a. Give the name and chemical formula b. Type the structural diagram for the molecule c. Make a model of the molecule and name its shape 7. Compare ionic compounds and covalent compounds a. Compare the arrangement of electrons between two atoms involved in a covalent bond and two atoms involved in an ionic bond b. How does the structure of the ionic bond influence the structure of the ionic compound in solid form? Use your model today in your explanation. c. How does the structure of the ionic bond influence the structure of the ionic compound in aqueous form? Use your model today in your explanation. d. Covalent molecules share electron pairs in fixed orientations or angles. Based on what you have learned today, summarize how the shapes of molecules determined? Explain your answer. e. The Cl2 is gas at room temperature. But, suppose you have a covalent molecule that is solid at room temperature, such as sucrose (common table sugar). How do covalent compounds such as sucrose (table sugar) behave when dissolved in solution? Do they behave the same or differently as we modeled today with sodium chloride (table salt)? 2016 Catholic Initiatives in Math and Science, LLC All Rights Reserved 7