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Properties of Molecular Substances

Properties of Molecular Substances. Low melting & boiling points Low heats of fusion & vaporization High vapor pressure May be soft, as wax May be crystalline, as sugar (weak lattice, based on dipole-dipole or H bonding) Molecules are neutral. NEVER conduct. Breaking .  Making.

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Properties of Molecular Substances

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  1. Properties of Molecular Substances Low melting & boiling points Low heats of fusion & vaporization High vapor pressure May be soft, as wax May be crystalline, as sugar (weak lattice, based on dipole-dipole or H bonding) Molecules are neutral. NEVER conduct.

  2. Breaking   Making Describe changes in chemical potential energy that accompany bond formation or bond breaking. Breaking a bond is endothermic. Making a bond is exothermic. Breaking a bond is endothermic. Making a bond is exothermic.

  3. Describe the relationship between stability & potential energy As PE , stability . As PE , stability .

  4. Exothermic - energy term is on product side - bond formation releases energy Does the above equation represent an endothermic or exothermic process?How do you know? H + H  H2 + energy

  5. Endothermic - energy term is on reactant side - bond breaking absorbs energy Does the above equation represent an endothermic or exothermic process?How do you know? H2 + energy  H + H

  6. Why do atoms form bonds? To achieve the electron configuration of the nearest noble gas!

  7. Electrons are shared Covalent Bonds

  8. How do you identify a covalent formula? All Nonmetals in formula

  9. Covalent bonds result from … The simultaneous attraction of electrons and two different nuclei.

  10. C6H6 CH What kinds of formulas do molecular substances have? H2O Molecular, which give exact composition of molecule Empirical, which give lowest whole number ratio of atoms in molecule Sometimes they are the same. Otherwise, molecular is a whole number multiple of empirical

  11. Structural Formula Shows which elements & how many atoms of each. Shows connectivity or how atoms are linked Shows type of bond – single, double or triple H-O-H O=C=O O-N=O

  12. Lewis Diagram Shows type & number of atoms. Shows connectivity Shows type of bonds Shows all nonbonding valence electrons, in addition to the bonding valence electrons Bonding electrons are in between two atoms!

  13. Single Bond Represented by 2 dots or 1 dash between atoms One electron pair or Two electrons Shared between atoms

  14. Double Bond Represented by 4 dots or 2 dashes between atoms Two electron pairs or Four electrons Shared between atoms

  15. Triple Bond Represented by 6 dots or 3 dashes between atoms Three electron pairs or Six electrons Shared between atoms

  16. Bond Energy Energy change that occurs when a bond is formed between two atoms. Symbol = D0

  17. Bond Energy As the number of electrons shared between 2 atoms increases, the attractive interactions increase, & the bond energy increases. Triple > Double > Single

  18. Bond Length Distance between two bonded nuclei. The more shared electrons between 2 nuclei, the greater the attractive interactions, the shorter the bond length.

  19.  Bond Overlap of two orbitals occurs on the line directly connecting the two nuclei. All single bonds are sigma bonds.

  20.  Bond Overlap of two orbitals occurs above and below the line that directly connects the two nuclei. Double bonds consist of one  & one  bond. Triple bonds consist of one & two  bonds.

  21. Procedure for writing a Lewis structure for a molecular substance Total up the valence electrons from all the atoms in the molecule. Draw the skeleton, including a single bond between every atom. Compare numbers: # of electrons needed for each atom in the skeleton to have an octet/duet # of electrons available after drawing skeleton Distribute electrons Verify by performing two validity checks

  22. Lewis Diagram of H2 Total # of valence electrons = 2 Skeleton: H – H Compare: 0 electrons needed & 0 electrons available No electrons to distribute Verify.

  23. Lewis Diagram of Cl2 # of valence electrons = 14 Skeleton: Cl – Cl Compare: Need 12, Have 12 Distribute Verify

  24. Bonding vs. Nonbonding electron pairs Bonding electrons are located BETWEEN two atoms. Nonbonding electrons are located on one atom only.

  25. Lewis Diagram of O2 # of valence electrons = 12 Skeleton: O - O A) Compare: Need 12, Have 10, deficient by 2, add 1 bond B) O  O. Compare: Need 8, have 8. Distribute Verify

  26. Lewis Diagram of N2 # of valence electrons = 10 Skeleton: N – N A) Compare: Need 12, Have 8, deficient by 4, add 2 bonds B) N  N. Compare: Need 4, have 4. Distribute Verify

  27. Lewis Diagram of H2O # of valence electrons = 8 Skeleton: H – O – H Compare: Need 4, Have 4 Distribute Verify

  28. H H – N – H - Lewis Diagram of NH3 # of valence electrons = 8 Skeleton: Compare: Need 2, Have 2 Distribute Verify

  29. H H - - Lewis Diagram of CH4 # of valence electrons = 8 Skeleton: H – C – H Compare: Need 0, Have 0 Distribute (nothing) Verify

  30. Cl Cl - - Lewis Diagram of CCl4 # of valence electrons = 32 Skeleton: Cl – C – Cl Compare: Need 24, Have 24 Distribute Verify

  31. H H - - Lewis Diagram of C2H4 - H H B) Compare: Need 0, Have 0 - H – C  C - H # of valence electrons = 12 Skeleton: H – C – C - H A) Compare: Need 4, Have 2. Add 1 bond. Distribute (nothing) Verify

  32. Lewis Diagram of C2H2 H – C  C - H # of valence electrons = 10 Skeleton: H – C – C - H A) Compare: Need 8, Have 4. Add 2 bonds. B) Compare: Need 0, Have 0 Distribute (nothing) Verify

  33. Lewis Diagram of CO2 # of valence electrons = 16 Skeleton: O – C - O A) Compare: Need 16, Have 12, deficient by 4, add 2 bonds B) O  C  O. Compare: Need 8, have 8. Distribute Verify

  34. What are Resonance Structures? <--> <--> Sometimes, more than one valid Lewis structure can be written for a molecule. For CO2:

  35. Resonance Structures The atoms are in the same location. The electrons are distributed differently.

  36. What are the three general ways the octet rule breaks down? Molecules with an odd # of electrons can never satisfy the octet rule for all their atoms. (NO, NO2, ClO2) Some molecules have an atom with less than an octet. (BF3, BeH2) Some molecules have an atom with more than an octet. (PCl5, SF6)

  37. Molecules with odd # of electrons • Consider NO 5 + 6 = 11 valence electrons • Skeleton: N – O • A) Compare: the N needs 6 & the O needs 6 for a total of 12. Have only 9 available. Add one bond. B) N = O now each atom needs 4 for a total of 8. Have only 7 available. • Distribute: :N = O: • Verify . .. The N atom has only 7 valence e-

  38. Molecules with odd # of electrons • Consider NO2. 5 + 2(6) = 17 electrons • Skeleton: O – N – O • A) Compare: each O needs 6 & N needs 4. Need 16 total. Have 13 available. Add 1 bond. B) O = N – O Compare: Need 4 + 2 + 6 or 12 electrons. Have 11 available. • Distribute: :O = N – O: • Verify .. . .. .. The N atom has only 7 e-

  39. Molecules with odd # of electrons • Consider ClO2 7 + 2(6) = 19 valence e- • Skeleton is O – Cl – O • Compare: Need 6 + 4 + 6 = 16 e- Have: 19 – 4 = 15. Deficient by 1 e-. Can’t fix. • Distribute :O – Cl – O: • Verify .. . .. The Cl atom has only 7 valence e- ! .. .. ..

  40. Molecules with atoms that have less than an octet Occurs in molecules with Be and B. Be likes to have 4 valence electrons in molecules. B likes to have 6 valence electrons in molecules.

  41. Molecules with atoms that contain more than an octet • Only the central atom can have more than an octet. • And only if it belongs in rows 3-7 of the PT. • Consider PF5. 5 + 5(7) = 40 valence e-. • Since P is in row 3 it has empty d orbitals available which can be used for bonding. • Skeleton: F F F Distribute the remaining 30 e- by placing 6 e- on each of the 5 F atoms. F P F The P has > than an octet.

  42. Polyatomic Ions A group of covalently bonded atoms that has gained or lost electrons and hence acquired a charge

  43. Lewis Diagrams of Polyatomic Ions • When calculating the total # of valence electrons, you must adjust for the charge of the ion. • Total up the electrons contributed from each atom. • Add 1 electron for each negative charge. Or • Subtract 1 electron for each positive charge.

  44. +1 H H - H – N – H - Lewis Diagram of NH4+1 # of valence electrons = 9 – 1 = 8 Skeleton: Compare: Need 0, Have 0 Distribute (nothing) Verify

  45. Lewis Diagram of OH-1 : : # of valence electrons = 6 + 1 + 1 = 8 Skeleton: [O – H]-1 Compare: Need 6, Have 6 Distribute: [:O - H]-1 Verify

  46. Determining Molecular Shape from the Lewis Structure • Count up the number of electron domains on the central atom. • Single, double, & triple bonds each count as ONE domain. • Lone electron pairs (or even a lone singleton) counts as ONE domain. • Count up the number of atoms that are bonded to the central atom. • Compare these two numbers to get the shape.

  47. Shapes on the Regents Exam

  48. Additional Shapes: Know *reds

  49. Tetrahedral Molecule

  50. Trigonal Pyramidal Molecule

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