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6-2

6-2. Covalent Bonding. What is a MOLECULE ?. Why would nature favor forming a covalent bond?. Beaker Breaker. Based on the their locations on the periodic table are the following covalent bonds or ionic bonds MgCl 2 KCl NO 3 CO 2.

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6-2

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  1. 6-2 Covalent Bonding

  2. What is a MOLECULE ?

  3. Why would nature favor forming a covalent bond?

  4. Beaker Breaker Based on the their locations on the periodic table are the following covalent bonds or ionic bonds MgCl2 KCl NO3 CO2

  5. Attraction: nucleus of one atom to the electron cloud of the other atom • Repulsion:both nuclei repel…as do both electron clouds • As atoms approach, attraction increases and potential E goes down • Beyond a certain point, repulsion increases and E goes up ∴ bottom of valley on E curve is where there is a balance between attraction and repulsion

  6. MOLECULE: neutral group of atoms held together by covalent bonds • MOLECULAR COMPOUND: a cmpd that is made of molecules • MOLECULAR FORMULA: shows the kinds & #s of atoms making up a molecule i.e. H2O • DIATOMIC MOLECULE: molecule containing only 2 atoms i.e. H2

  7. Characteristics of the Covalent Bond • Bond Length: average distance between 2 bonded atoms (min potential E) In the H2 molecule, the 1s orbitals overlap and result in a increased electron density between the nuclei.

  8. Bond energy: • Energy required to break a chemical bond and form neutral isolated atoms • Measured in kJ/mol (kilojoules/mole) • Positive value • Bond energy and bond length values will vary depending on what atoms an element is bonded to…ave values found on tables P.168)

  9. Octet Rule • Chemical cmpds tend to form so that each atom, by gaining, losing or sharing electrons, has an octet (8) of electrons in its highest (outermost) energy level • Example of exceptions: • H: only has 2 valence e-s to reach stability • B: only has 6 valence e-s “ “ • Some elements have > 8 (occurs when atom bonds with highly electroneg elements & some “d” e-s are involved in bonding

  10. Electron Dot Diagrams • Electron configuration notation in which only the valence electrons of an atom of a particular element are shown, indicated by dots placed around the element’s symbol • F: 1s22s22p5

  11. “other” dot diagram format • Mg: [Ne]3s2 • “paired” electrons are shown as a pair

  12. Lewis Structures • Formulas in which…. • atomic symbols represent nuclei and inner-shell electrons • Dot-pairs or dashed between 2 atoms represent electron pairs in covalent bonds • Dots adjacent to only one atomic symbol represent unshared electrons

  13. Lewis structure Structural Formula: indicates The kind, number, arrangement, and bonds..but NOT the unshared Pairs of the atoms in a molecule

  14. Lewis structure Single bond: covalent bond produced by the sharing on ONE pair of electrons between 2 atoms

  15. Draw the Lewis structure for carbon tetrachloride • CCl4 • See page 171-172 for rules • Total # of valence electrons: • C: 1 x 4 e- = 4 e- • Cl: 4 x 7 e- = 28 e- • Total # = 32 e-

  16. or…..

  17. Draw the Lewis structure for the following molecules: • IBr • CH3Br • F2O • SiCl4 • C2HCl (????)

  18. Beaker Breaker • Draw the Lewis structures showing the bonds for: • HBr • CH4

  19. Multiple Covalent Bonds • DOUBLE BOND • sharing of TWO pairs of electrons between 2 atoms • TRIPLE BOND • sharing of THREE pairs of electrons between 2 atoms • Bond Energies: • triple > double > single • …..triple bonds are stronger and shorter

  20. Draw the Lewis structure for sulfur trioxide, SO3

  21. Draw the Lewis structure for sulfur trioxide, SO3 • S: 6 e- • O: 3 x 6 e- • Total # = 24 e- ?????

  22. Resonance Structures • bonding in molecules or ions that cannot be correctly represented by a single Lewis structure • the experimental properties (observed properties) do NOT support the paper-and-pen structure suggested by the Lewis structure

  23. Draw the resonance structure for sulfur trioxide, SO3 • How many valence electrons? • How are the atoms arranged relative to each other (skeleton structure)? • How must you distribute the valence electrons so all four atoms comply with the octet rule?

  24. Draw the resonance structure for sulfur trioxide, SO3 • S: 6 e- • O: 3 x 6 e- • Total # = 24 e- • (see next slide)

  25. - Lab evidence shows that all three S-O bonds are the same….not 1/3 double and 2/3 single bonding! - The arrow shows that the real structure is an average of all three resonance structures

  26. Covalent-network Bonding Covalent bonding exists throughout a large network of atoms…will be discussed in chapter 12 i.e. carbon atoms in a diamond

  27. Look at sample problems p.171-174 (if extra time at end of class)

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