Making Covalent Molecules 1. Purpose: Construct models of covalent compounds. 2. Materials: Molecular Model Kit 3. Introduction: A covalent bond is created when atoms share electrons. In covalent molecules, single, double, and triple bonds form between atoms. Let's look at how molecular formulas, electron dot diagrams and structural formulas are related to one another for several simple compounds. Molecular formulas show only the type and number of atoms in a molecule. Structural formulas show the atoms in their correct placement in the molecule. Electron-dot formulas resemble structural formulas but also include all the non-bonding outer electrons. First look at CH4 (also called methane). In the electron dot diagram you can see how the four valence electrons of carbon match up with the one valence electron of each of the four hydrogen atoms to give eight electrons around carbon. The electrons are arranged in four pairs representing four covalent bonds. The structural formula emphasizes the bonds rather than the electrons and shows one line for each of the bonds that connect the atoms together. Each line represents one shared pair of electrons (2 electrons). Molecular Formula Molecular Model Electron Dot Diagram Structural Formula CH4 Next you can see the same things for HC2H. However, in this case, the resulting structural formula is different in an important way. Since there are three pairs of electrons shared between the two carbon atoms, a triple line is drawn. Molecular Formula HC2H Molecular Model Electron Dot Diagram Structural Formula H:C:::C:H 4. Procedure: a) Use the ball-and-stick model kit to construct the model that represents the molecule, based on these guidelines: A. Non-bonding electrons: Each unshared pair of electrons ("lone pair") will be represented by 1 short, rigid connector, with no ball at the other end of it. B. Bonding electrons: i. Single bonds will be represented by 1 connector--use the short, rigid connectors for bonds involving hydrogen atoms and use the longer, more flexible connectors for all other bonds. ii. Double bonds will be represented by 2 longer, more flexible connectors iii. Triple bonds will be represented by 3 long, flexible connectors. Name: ______________________________ Block: _____ Date: ____________________ Making Covalent Molecules ATOM NUMBER OF BONDING SITES (holes) Hydrogen and alkali metals 1 Oxygen family 2 Carbon family 4 Halogens 1 Nitrogen family 3 COLOR Sometimes atoms form 2 bonds with another atom. This is called a double bond. Triple bonds are 3 bonds (for a total of 6 electrons) between the same two atoms. *Note: the structural formula should be written so that it matches, as much as possible, the same shape of the molecule that you constructed (see the examples). Molecular Formula Name HF hydrogen monoflouride O2 molecular oxygen HCl hydrogen monochloride BrCl H2O dihydrogen monoxide CO2 carbon dioxide SiCl4 Electron-dot formula Structural formula Molecular Formula Name Electron-dot formula Structural formula H2S C2H2 dicarbon dihydride NH3 (also called ammonia) NBr3 N2H4 C2H6 (also called ethane) C4H10 tetracarbon decahydride Analysis 1. How many electrons are shared in a… a) single bond ______ b) double bond ______ c) triple bond ______ 2. Explain how molecular formulas, electron dot diagrams and structural formulas are related to one another __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ 3. What is a dipole? Give an example of a molecule with a dipole and draw the dipole on its Lewis structure. 4. What factors determine whether a molecule is polar or non-polar? Explain. 5. Both water and carbon dioxide are triatomic molecules. Explain why one is polar and the other is nonpolar. 6. Both hydrogen monofluoride and nitrogen gas were two-atom molecules AND linear. Explain why one of these is polar and the other is nonpolar. 7. Both carbon tetrachloride and methyl chloride were tetrahedral shaped. Explain why one of these is polar and the other is nonpolar. 8. Why did we not make NaCl, BaO, Al2O3 or Li3N in this activity? 9. Why are there four holes in the model for the atom nitrogen? 10. Compare the molecules of N2 and H2. Which molecule has the longest bond? Why? 11. If you have a molecule with a double or triple bond in it that molecule could be what two shapes? What determines which of those shapes the molecule is? 12. When you make the diatomic molecule for nitrogen, which way are the open holes facing and why. 13. a. If a molecule has two atoms its shape is _________________________. b. If a molecule has all single bonds and four atoms it takes the shape of _______________________ or _________________________. 14. What shape is the molecule SiS2?