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Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

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Presentation on theme: "Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:"— Presentation transcript:

1 Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

2 CHEM1010/General Chemistry _________________________________________ Chapter 5. (L18)-Chemical Bonds Today’s Outline..Review of covalent bond..Review of how to write electron-dot formulas..Shapes of molecules..Molecular shapes and their properties: Polar and non-polar molecules

3 Chapter 5. (L18)-Chemical Bonds Building Your Chemical Vocabulary Phosphoric acid: H 3 PO 4, mild/weak acid Hydrocyanic acid: HCN, extemely toxic Boric acid: H 3 BO 3, mild/weak acid, antiseptic eye wash

4 Chapter 5. (L18)-Chemical Bonds Shape of molecules..We have learned the components of molecules (atoms) and how atoms can combine with each other to form molecules through ionic bonding or covalent bonding...Now, we have some new questions about molecules: Does a molecule have a shape? What shape can a molecule have? What determines the shapes of molecules? Can molecular shapes change? If so, under what conditions? How can I tell the shape of a molecule? Does the shape of a molecule affect its physical and chemical properties?

5 Chapter 5. (L18)-Chemical Bonds Shape of molecules..A molecule does have a shape, and it can be 1-D, 2-D, and 3-D..Common shapes of molecules: Molecular ShapeGeometryBond Angle Linear 1-D180º Bent 2-D<180º Triangular 2-D120º Pyramidal 3-Dvarious Tetrahedral 3-D109.5º

6 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Key concepts about molecular shape: -In most cases, in general speaking, covalent molecules have various shapes depending on their bonding situation -There is usually a center atom, which extends covalent bonds to combine with other atoms, which thus leads to arms along the bonds. -By looking at the molecular formula of a molecule, one usually may give an educated guess on its molecular shape. Key: Determining the center atoms and the “branch” atoms.

7 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Examples of molecular shapes Fig. 5.9 and Table 5.6 on textbook pp.141

8 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Theory about molecular shapes VSEPR Theory: The Valence Shell Electron Pair Repulsion Theory Electron pairs (covalent bonds) arrange themselves about a central atom in such a way that the repulsion between like-charged particles (atoms) is minimized. In other words, the “branch” atoms need to separate from each other as much as possible.

9 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Some simple rules to determine the shapes of molecules using the VSEPR Theory (1) Determine the central atom (2) Draw an electron-dot structure for the molecule, using a line for a shared pair (bonding pair, BP), and dots for non- bonding pair(s) (NBP) of electrons (3) Draw a tentative shape as if all the electron pairs are BPs to make sure the pairs as far away from each other as possible (4) If all electron pairs are BPs, then this tentative shape is the real shape (5) If there is one or more NBPs, then ignore the NBPs, the shape is close to the one as represented by the lines of BPs.

10 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Some simple rules to determine the shapes of molecules using the VSEPR Theory Note: The NBPs do have a secondary role in determination of the shape of a molecule, because the NBPs have some electrostatic effect on the other bonding pairs of electrons.

11 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Examples of determination of the shapes of molecules using the VSEPR Theory.... H H—O: H—N—H | | | H—C—H H H | H bent pyramidal tetrahedral

12 Chapter 5. (L18)-Chemical Bonds Shape of covalent molecules..Examples of determination of the shapes of molecules using the VSEPR Theory.... H—C N: :O=C=O: H—B—H | HCN CO 2 H BH [:O—N=O:] - NO 2 -..

13 Chapter 5. (L18)-Chemical Bonds Molecular shapes and their properties: Polar and non-polar molecules..Review: Polar covalent bond occurs to the molecules of atoms of somewhat different kinds, usually non- metals..How to determine if a molecule is polar It depends on the geometry as well as the polarity of each individual bond. A polar molecule has separate centers of positive and negative charge. The polarity of a molecule determines properties such as melting point, boiling point, and solubility.

14 Chapter 5. (L18)-Chemical Bonds Molecular shapes and their properties: Polar and non-polar molecules..Key to the polarity of molecule: Symmetry of molecules Examples of polarity of molecules Methane -A tetrahedral molecule, CH 4, -symmetrical, -the polarity of each bond cancelled out -non-polar

15 Chapter 5. (L18)-Chemical Bonds Molecular shapes and their properties: Polar and non-polar molecules Examples of polarity of molecules Ammonia -A pyramidal molecule, NH 3, -three BPs, one NBP, -the NBP occupies slightly larger space, pushing the three BPs closer to each other, actual bond angle = 107º, not 109.5º, -tripod analogy, -asymmetrical, -polar, partial negative charge on N, partial positive charge on H.

16 Chapter 5. (L18)-Chemical Bonds Molecular shapes and their properties: Polar and non-polar molecules Examples of polarity of molecules Water -A bent molecule, H 2 O -two BPs, two NBPs, -the two NBPs occupy slightly larger space, pushing the two BPs closer to each other, actual bond angle = 104.5º, not 109.5º, -unsymmetrical, -polar, partial negative charge on O, partial positive charge on H, a dipole, + — -. Polarity of water molecules have profound effects on many aspects of chemistry

17 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of a covalent bonding molecule is determined primarily by (a) non-bonding pairs of electrons; (b) inner shell electrons; (c) mass of the molecule; (d) bonding pairs of valence electrons of the molecule.

18 Chapter 5. (L18)-Chemical Bonds Quiz Time The non-bonding pairs of valence electrons have (a) no effect on the shape of a molecule at all; (b) have the primary effect on the shape of a molecule; (c) have some effect on the shape of a molecule because they usually occupy larger space in the molecule; (d) none of above is right.

19 Chapter 5. (L18)-Chemical Bonds Quiz Time The VSEPR theory is (a) the Very Special Electron Pop Reputation theory; (b) the Very Spicy Exciting Potato Roll theory; (c) the Valence Shell Electron Pair Repulsion theory; (d) the Very Secret Evening Pizza Recipe theory.

20 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of the molecule, H 2 O, is (a) pyramidal; (b) tetrahedral; (c) triangle; (d) bent with the bonding angle of 104.5°.

21 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of the molecule, CH 4, is (a) pyramidal; (b) tetrahedral with the bonding angle of 109.5°; (c) triangular; (d) bent with the bonding angle of 104.5°.

22 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of the molecule, H 2, is (a) pyramidal; (b) tetrahedral with the bonding angle of 109.5°; (c) linear; (d) bent with the bonding angle of 104.5°.

23 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of the molecule, BF 3, is (a) pyramidal; (b) tetrahedral with the bonding angle of 109.5°; (c) triangular; (d) bent with the bonding angle of 104.5°.

24 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of the molecule, NH 3, is (a) pyramidal; (b) tetrahedral with the bonding angle of 109.5°; (c) triangular; (d) bent with the bonding angle of 104.5°.

25 Chapter 5. (L18)-Chemical Bonds Quiz Time The shape of the molecule, NH 3, is pyramidal because (a) it has a nasty smell; (b) it has three H atoms; (c) it has one pair non-bonding valence electrons, which occupy slightly more space; (d) it has a nitrogen atom.

26 Chapter 5. (L18)-Chemical Bonds Quiz Time Which of the following molecules is a polar molecule? Is it (a) CO 2 ; (b) BF 3 ; (c) Cl 2 ; (d) H 2 O.

27 Chapter 5. (L18)-Chemical Bonds Quiz Time Which of the following molecules is a non-polar molecule? Is it (a) NH 3 ; (b) HCl; (c) H 2 S; (d) O 2.

28 Chapter 5. (L18)-Chemical Bonds Quiz Time The VSEPR theory predicts the molecule of ammonia, NH 3, should have a bond angle of 109.5º, but the angle is actually 107º. This is because (a) nitrogen atom has more mass; (b) ammonia has three H atoms; (c) nitrogen atom has a non-bonding pair of valence electrons; (d) nitrogen does not have a non-bonding pair of valence electrons.

29 Chapter 5. (L18)-Chemical Bonds Quiz Time The VSEPR theory predicts the molecule of water, H 2 O, should have a bond angle of 109.5º, but the angle is actually 104.5º. This is because (a) nitrogen atom has more mass; (b) water has two H atoms; (c) oxygen atom has two non-bonding pairs of valence electrons; (d) oxygen has one non-bonding pair of valence electrons.

30 Chapter 5. (L18)-Chemical Bonds Quiz Time Water molecule, H 2 O, is polar because (a) it is symmetrical; (b) the mass of oxygen is larger than that of hydrogen; (c) the H-O bonding is polar and the molecule is asymmetrical; (d) hydrogen has a non-bonding pair of electrons.

31 Chapter 5. (L18)-Chemical Bonds Quiz Time A molecule with polar bonds is (a) absolutely polar; (b) absolutely non-polar; (c) generally polar if it is asymmetrical and non-polar if it is symmetrical; (d) arbitrary in its polarity.


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