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Topic 11: Chemical Bond Formation

Topic 11: Chemical Bond Formation. LECTURE SLIDES Valence electrons Ionic Bonding Covalent Bonding Lewis Structures Acid, Anion Relationships Resonance Structures Bond Length vs Bond Order Octet Violators Formal Charge. Kotz & Treichel, 9.1-9.6.

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Topic 11: Chemical Bond Formation

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  1. Topic 11: Chemical Bond Formation • LECTURE SLIDES • Valence electrons • Ionic Bonding • Covalent Bonding • Lewis Structures • Acid, Anion Relationships • Resonance Structures • Bond Length vs Bond Order • Octet Violators • Formal Charge Kotz & Treichel, 9.1-9.6

  2. CHAPTER 9: BONDING AND MOLECULAR STRUCTURE Now that we have examined the structure of the atom and the arrangement of its electrons, we are ready to turn to the molecules and compounds they form. Our next studies will center on the bonds that hold the atoms together in compounds, molecules and polyatomic ions. We will also examine the three dimensional shape of these species, and their polar or non polar nature.

  3. BOND FORMATION The interactions between atoms which lead to bond formation are all centered around the electrons in incomplete subshells and in incompleteouter shells: the valence electrons... The atoms of the elements lose, gain or share these electrons to achieve, where possible, the noble gas configurations we have met.

  4. For the “main group elements”, the s and p block members, the electrons available for bonding, the “valence electrons”are the outer shell sand p electrons (except those of the noble gases!) In forming compounds from these elements, only these electrons will be used, in two ways: They may be transferred to form ions so that incomplete subshells are completed or removed; They may be shared so that two atoms together have complete subshells

  5. For the transition metals, the valence electrons include both the s electrons from their outermost shell and also electrons from their inner, incomplete d subshell. The PT column number of the main group and transition metals gives the sum of valence electrons for each element in the family. Note that the column number indicates the maximum positive charge (or oxidation state) these metals can achieve in a compound through loss of e’s in a chemical reaction.

  6. For all main group elements (the columns 1A-8A), it is convenient for bonding purposes, to represent the elements as “Lewis Dot Symbols”, which include one dot for each of their valence electrons. The tendency of these elements to achieve an outer shell configuration of eight electrons (the “octet rule”) is easily visualized through use of these symbols. The valence electrons for transition elements (columns 3B-8B, 1B, 2B) are not represented by dot symbols.

  7. LEWIS DOT STRUCTURES FOR PERIOD 2 All elements, samecolumn: same dot structure

  8. The elements come together to form compounds so that each element can achieve a more satisfactory outer shell electronic configuration. Elements may lose or gain electrons resulting in cation and anion formation and the attraction between the two which we call the “ionic bond” Elements may share one or more pairs of electrons. The attraction of both nuclei for the same pair of electrons results in the force we call the “covalent bond”.

  9. The “ionic bond”: attraction of opposite charges when transfer of electrons cause formation of positive and negative species: cations and anions. The individual ions radiate charge in all directions and cluster in geometric patterns which are described as crystal lattices. Note in the following slide that each ion has many neighbors and the compound itself is not molecular in nature: no discrete “formula units” exist.

  10. Ionic compounds are all solids at room temperature with elevated melting points. Their melting points reflect the very high degree of attraction exhibited by these fully charged particles, which depends on the magnitude of their charge and the ionic size: The larger each charge and the smaller each ion, the greater the attraction. Energy = n(+) X n(-) d n= magnitude of charge d=distance between ions

  11. Ion formation and the resulting ionic bond occurs when metals of sufficiently low electronegativity(X) react with non-metals of sufficiently highX values. The most ionic of compounds are those formed between the active s block metals (X < 1)with the non-metals whose X values are 3.0 or larger. All compounds we have met containing “polyatomic” anions or ammonium are also of course truly ionictype compounds.

  12. Most active non-metals Most active metals

  13. ELECTRONEGATIVITY VALUES, MAIN GROUP ELEMENTS Non-metals METALS METALLOIDS

  14. THE COVALENT BOND The second mode of bond formation occurs when elements share one or more pairs of electrons to achieve where possible an outer shell octet. The attraction of both nuclei for the same pair of shared electrons is the basis of the covalent bond.

  15. Covalent bonds are directed between two atoms, sharing together one or more pairs of electrons. This type of bonding leads to formation of discreet molecules, individual units made up of two or more atoms covalently bonded together. Any formula consisting solely of nonmetals and metalloids can assumed to molecular and covalent in nature. Covalent bonds hold together the atoms within a polyatomicion. Occasionally, metals with higher electronegativity values will form a compound more covalent than ionic in nature.

  16. Group Work, 11.1: Bond Type Ionic compounds,  en >1.6 or 1.7

  17. LEWIS DOT STRUCTURES: MOLECULES AND COMPOUNDS We are next going to use the Lewis Dot Symbols for the “main group elements” to represent the bonding and structure for various species, molecules, compounds and polyatomicions. • allvalence electrons for every atom will be included • allshared pairs of e’s will be indicated by a “dash” • all unshared pairs of e’s will be indicated by a dot • We will use the “octet rule” as our guiding principle.

  18. Diatomic Elements: H2 Cl2 N2 O2

  19. Chlorine, Cl2 (same for Br2, F2, I2) Check octets!

  20. N2, Nitrogen: NO OCTET STILL NO OCTET

  21. Covalent “triple bond”

  22. CORRECT LEWIS STRUCTURE, Covalent double bond OXYGEN ACTS LIKE THIS* *Required whole new bonding theory to explain...

  23. Lewis Structures: Compounds and Polyatomic IonsGUIDELINES First step: Decide on arrangement of atoms. For most species, the element written first in the molecule or ion is the central atom and the remainder of the atoms are grouped around it. Hydrogen is a problem in “oxo acids” where it is written first in the formula. Ignore H, start with the next atom in formula and place the H or H’s on the O or O’s.

  24. Second Step • Add up all available valence electrons. • If species is cation, subtract positive charge from total. • If species is anion,add negative charge to total. • Divide total by two to determine available number of • electron pairs Third Step Place a pair of electrons between each pair of bonded atoms to represent a single bond (use a “dash”!)

  25. Step 2 Step 1 Step 3

  26. Fourth Step Place leftover electron pairs around “terminal” atoms to achieve their octet (except H). Do central atomlast. Fifth Step Examine central atom to determine if a double or triple bond is required to achieve the central atom’s octet. Do so using unshared pairs, IF central atom is: C, N, P, O, S

  27. Step 4: H, duet N, octet No Step 5 needed

  28. Step 1 Step 2 Step 3

  29. Step 4

  30. GROUP WORK 11.2: Lewis Structures Use 5 steps: Arrange; adds up e’s; draw bonds; assign unshared pairs double bonds if needed to draw Lewis structures for following species: NBr3 CH2Cl2

  31. Now let’s examine situations requiring the double bond:

  32. No octet

  33. No octet Either one

  34. Be sure to include charge on finished product

  35. GROUP WORK, 11.3: Lewis Structures # 2 Use 5 steps: Arrange; adds up e’s; draw bonds; assign unshared pairs; double bonds if needed to draw Lewis structures for following species: H3PO4 NO21+ ClO41-

  36. Let’s explore the relationship between various “oxo” acids (H, Non metal element, O) and the charge and formula of their anion relative. Recall that acids, by definition, ionize in water to lose one or more H’s as H+. The anion left behind is named according to the name of its “parent” acid. In an acid/base reaction, as we met last unit, acids(H+) react with bases (OH-) to form water, leaving behind the anion of the acid and the cation of the base to form a salt.

  37. Recall that acids “ionize” in water, or react with a base to form water, in either case leaving behind some “anion”: Cl-, NO3-, SO42- etc... H2O H- “Anion” H+ + Anion- H2O + Na+An- H- “Anion” + NaOH Acid: HCl, HNO3 H2SO4 etc...

  38. Group Work, 11.4: Acids and Anions Give missing structures, next four slides! Base structure on accompanying acid or anion...

  39. Group Work 11.4a

  40. Group Work 11.4 b

  41. GW 11.4c

  42. Group Work, 11.4d

  43. RESONANCE THEORY: WHERE TO PLACE THE DOUBLE BOND... EQUIVALENT No OCTET EQUIVALENT

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