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VSEPR and Chemical Polarity

VSEPR and Chemical Polarity. March 19, 2008. Molecular Geometry. 3-dimensional arrangement of atoms in a molecule: affects its physical and chemical properties, e.g., melting point, boiling point, density, and the types of reactions it undergoes.

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VSEPR and Chemical Polarity

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  1. VSEPR and Chemical Polarity March 19, 2008

  2. Molecular Geometry • 3-dimensional arrangement of atoms in a molecule: affects its physical and chemical properties, e.g., melting point, boiling point, density, and the types of reactions it undergoes. • Experiments needed to determined the precise structure of molecules • However, if we know the number of electrons surrounding a central atom in its Lewis structure, we can predict with considerable success the overall geometry.

  3. VSEPR Model • In a polyatomic molecule, where there are two of more bonds between the central atom and the surrounding atoms, the repulsion between electrons in different bonding pairs causes them to remain as far apart as possible. • The optimal geometry that a molecule resumes minimizes the repulsion. • Valance-Shell Electron-Pair Repulsion (VSEPR) model: accounts for the geometric arrangements of electron pairs around a central atom in terms of the electrostatic repulsion between electron pairs.

  4. Molecules in which central atom has NO lone pair • These molecules have the general formula ABx, where x = 2, 3, 4, 5, 6. • AB2 : Beryllium Chloride (BeCl2)  Linear

  5. AB3 : Boron Trifluoride (BFl3)  (Trigonal planar) • AB4 : Methane (CH4)  (Tetrahedral)

  6. AB5 : Phosphorus Pentachloride (PCl5)  (Trigonal bipyramidal) • AB6 : Sulfur Hexafluoride (SF6)  (Octahedral)

  7. Molecules in which central atom has one or more lone pairs • These molecules have the general formula ABxEy, where E is a lone pair on A, x = 2, 3, … and y = 1,2, … • Repulsive forces decrease in the following order: Lone-pair vs. lone-pair rep.>lone-pair vs. bonding-pair rep.> bonding-pair vs. bonding-pair rep.

  8. AB2E: Sulfur Dioxide (SO2)  (bent)

  9. AB3E: Ammonia (NH3)  (trigonal pyramidal) • AB2E2: Water (H2O)  (bent) • AB3E: Ammonia (NH3)  (trigonal pyramidal) • AB2E2: Water (H2O)  (bent)

  10. AB4E: Sulfur Tetrafluoride (SF4)  (see-saw or distorted tetrahedral) • AB3E2: Chlorine Trifluoride (ClF3)  (T-shape)

  11. AB2E3: Xenon Difluoride (XeF2)  (Linear) • AB5E1: Bromine Pentafluoride (BrF5)  (Square pyrimidal)

  12. AB4E2: Xenon Tetrafluoride (XeF4)  (Square planar)

  13. Guidelines for applying the VSEPR model • Step 1: Write the Lewis structure of the molecule, considering only the electron pairs around the central atom. If octet rule is not satisfied for the central atom, try adding double or triple bonds between the surrounding atoms and the central atom, using the lone pairs from the surrounding atoms. • Step 2: Count the number of electron pairs around the central atom (bonding pairs and lone pairs). Treat double and triple bonds as though they were single bonds. Refer to Table 10.1 to predict the overall arrangement of the electron pairs.

  14. Guidelines for applying the VSEPR model • Step 3: Use Tables 10.1 and 10.2 to predict the geometry of the molecule. • Note: In predicting the angles, note that a lone pair repels another lone pair or the bonding pair more strongly than a bonding pair repels another bonding pair.

  15. Chemical Polarity • In polar covalent molecules, there is a shift of electron density between the bonding atoms: electrons will concentrate near the more electronegative atom. (e.g., HF) • The consequent charge separation can be represented as: • A quantitative measure of polarity of a bond is its dipole moment ()

  16. Chemical Polarity • Diatomic molecules containing atoms of different elements (e.g., HCl, CO, and NO) have dipole moments and are called polar molecule. Diatomic molecules containing atoms of the same element (e.g., H2, O2, F2) are non-polar molecules because they do not have dipole moments. • For polyatomic molecules, both the polarity of the bonds and the molecular geometry determine whether there is a dipole moment. Even if polar bonds are present, the molecule will not necessarily have a dipole moment, e.g., CO2. O=C=O.

  17. Chemical Polarity (Examples) • NH3 molecule and NF3 molecule - + + -

  18. Chemical Polarity (Practice Questions) • IBr, • BF3 (trigonal planr), • CH2Cl2 (tetrahedral).

  19. Examples 2: Nitric acid (HNO3) • Step 1: The skeletal structure of HNO3: O N O H O • Step 2: The outer-shell electron configurations of N, O and H are 2s22p3 and 2s22p4, and 1s1 respectively. Thus there are 5 + (3 x 6) + 1, or 24, valence electrons to account for in HNO3. • Step 3: We draw a single covalent bond between N and each of the three O atoms, and between one O atom and the H atom. Then we fill in electrons to comply with the octet rule for the O atoms: • Step 4: We see that this structure satisfies the octet rule for all the O atoms but not for the N atom. Therefore we move a lone pair from one of the end ) atoms to form another bond with N. Now the octet rule is also satisfied for the N atom:

  20. Examples 3: Carbonate ion (CO32- ion) • Step 1: C is less electronegative than O. Therefore, it is most likely to occupy a central position as follows: O O C O • Step 2: The outer-shell electron configurations of C and O are 2s22p2 and 2s22p4, respectively, and the ion itself has two negative charges. Thus the total number of electrons are 4 + (3 x 6) + 2, or 24. • Step 3: We draw a single covalent bond between C and each O and comply with the octet rule for the O atoms: • Step 4: Although the octet rule is satisfied for the O atoms, it is not for the C atom. Therefore we move a lone pair from one of the O atoms to form another bond with C. Now the octet rule is also satisfied for the C atom:

  21. Exceptions to the Octet Rule • First category of exception – Incomplete Octet: • In some compound, the number of electrons surrounding the central atom in a stable molecule is fewer than eight; e.g., BeH2; BF3 • Second category of exception – Odd-Electron Molecules: • Some molecules contain an odd number of electrons. Among them are nitric oxide (NO) and nitrogen dioxide (NO2). • Third category of exception – Expanded Octet • Atoms of the second-period elements cannot have more than eight valence electrons; but atoms in and beyond the third period can have more than eight valence electrons because there are 3s, 3p and 3d subshells. • Examples: SF6. This molecule has 12 bonding electron pairs accommodated in 6 orbitals (one 3s, three 3p, and two of the five 3d orbitals). Sulfur can also form many compounds in which it obeys the octet rule, e.g., SCl2 (which is surrounded only by 8 electrons).

  22. Bonding Capacity Activity

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