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3. Predicting Molecular Polarity The ability of an atom to attract electrons to itself is called electronegativity. When 2 atoms, having different electronegativities, form a covalent bond, the shared electrons that form the bond will be attracted by the more electronegative element. For example, oxygen is more electronegative that hydrogen so, when hydrogen and oxygen form a covalent bond, the bonding electrons are more likely to be found closer to the oxygen atom than the hydrogen atom. The electrons are shared unequally and the bond is distorted towards the oxygen. This results in a dipole; the oxygen atom has a partial negative charge (8-) and the hydrogen atom has a partial positive charge (8+). This type of bond is called a polar covalent bond: 8+ H-O 8- When a molecule contains more than one bond we can predict the molecular polarity by considering the polarity of the individual bonds, the symmetry of the molecule, and the presence or absence of lone pairs on the central atom. For a molecule to be polar, there must be a net dipole moment. In other words the polar bonds within the molecule must not cancel each other out. Consider carbon dioxide, CO₂: 6- 80=C=0 The oxygen atoms are more electronegative than the carbon atom, so the bonding electrons are attracted to the terminal oxygen atoms, away from the central carbon atom. This results in a partial negative charge at each end of the CO₂ molecule. Because of the symmetry of the CO₂ molecule, these dipoles cancel each other out and no net dipole results. Thus carbon dioxide is non-polar. Note: How did we know to choose carbon as the central atom in this structure? As a general rule, the least electronegative atom is chosen as the central atom. Of course, there are exceptions such as H₂O. Consider water, H₂O: H 8- 8- H 8- Because of the asymmetry of the water molecule, the two polar bonds within the water molecule do not cancel each other out. Thus, water is a polar molecule.
Polarity Statement An easy way to predict whether a simple molecule is polar or non-polar is to apply the following statement: "If all atoms bonded to the central atom are identical, and there are no lone-pairs on the central atom, the molecule is non-polar. In all other cases, the molecule is polar". Instructions for completing the lab worksheet 1. Using the methodology described in Table 1, draw Lewis structures of the covalent compounds and polyatomic ions given in the worksheet. 2. Predict the electron-pair geometry, molecular geometry and bond angles of the compounds and ions using VSEPR theory as detailed in Table 2 of this procedure. 3. Determine whether the compounds are polar or non-polar by applying the polarity statement given above. If the molecule is polar, write "P". If it is non-polar, write "NP".
MOLECULAR MODELING: A Study Assignment Pre-Lab Questions 1. How many valence electrons are available to draw the Lewis structures of CS₂ and H₂S? CS₂ 2. Draw the Lewis structures for CS₂ and H₂S. Refer to these structures when answering the following questions. 3. How many electron groups are on the central atoms of CS₂ and H₂S? How many lone-pairs are on the central atoms of each molecule? CS₂ CS₂ H₂S H₂S H₂S 4. What is the electron-pair geometry, molecular geometry and bond angle of CS₂ and H₂S? Molecular Geometry Electron Groups CS₂ Electron-pair Geometry 5. Determine if CS₂ and H₂S are polar or non-polar. Lone pairs H₂S Bond Angle