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George Mason University General Chemistry 212 Chapter 23 Transition Elements Acknowledgements

George Mason University General Chemistry 212 Chapter 23 Transition Elements Acknowledgements Course Text: Chemistry: the Molecular Nature of Matter and Change, 6 th ed , 2011, Martin S. Silberberg, McGraw-Hill

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George Mason University General Chemistry 212 Chapter 23 Transition Elements Acknowledgements

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  1. George Mason University General Chemistry 212 Chapter 23 Transition Elements Acknowledgements Course Text: Chemistry: the Molecular Nature of Matter and Change, 6thed, 2011, Martin S. Silberberg, McGraw-Hill The Chemistry 211/212 General Chemistry courses taught at George Mason are intended for those students enrolled in a science /engineering oriented curricula, with particular emphasis on chemistry, biochemistry, and biology The material on these slides is taken primarily from the course text but the instructor has modified, condensed, or otherwise reorganized selected material.Additional material from other sources may also be included. Interpretation of course material to clarify concepts and solutions to problems is the sole responsibility of this instructor.

  2. Transition Elements • Properties of the Transition Elements • The Inner Transition Elements • Highlights of Selected Transition Elements • Coordination Compounds • Theoretical Basis for the Bonding and Properties of Complexes

  3. Transition Elements • Main-Group vs Transition Elements • Most important uses of Main-Group elements involve the compounds made up of these elements • Transition Elements are highly useful in their elemental or uncombined form

  4. Transition Elements • Properties of Transition Elements • Recall: The “A” (Main Group) elements make up the “s” and “p” blocks • Transition Elements make up the • “d” block (B group) • “f” block elements (Inner Transition Elements) • As ions, transition metals (elements) provide fascinating insights into chemical bonding and structure • Transition metals play an important role in living organisms

  5. Transition Elements

  6. Transition Elements • Electron Configurations of the Transition Metals • In the Periodic Table, the Transition metals, designated “d-block (B-Group)” elements, are located in: • 40 elements in 4 series within Periods 4 -7 • Lie between the last ns-block elements in group [2A(2)] (Ca – Ra) and the first np-block elements in group [(3A(13)] (Ga & element 113 (unnamed) • Each series represents the filling of the 5 d orbitals l = 2  [ml = -2 -1 0 +1 +2] (5 orbitals per period x 2 electrons per orbital x 4 Periods = 40 Elements

  7. Transition Elements • Condensed d-block ground-state electron configuration: [noble gas] ns2(n-1)dx, with n = 4 -7; x= 1-10 (several aufbau build-up exceptions) • Partial (valence shell) electron configuration ns2(n-1)dx • Recall: Chromium (Cr) and Copper (Cu) are exceptions to the above aufbau configuration setup Expected: Cr [Ar] 4s23d4 Cu [Ar] 4s23d9 Actual: Cr [Ar] 4s13d5 Cu [Ar] 4s13d10 Reasons: change in relative energies of 4s & 3d orbitals and the unusual stability of ½ filled and filled sublevels (level 4 relative to level 3)

  8. Transition Elements Note Aufbau build up exceptions for “Cr” & “Cu”

  9. Transition Elements • The “Inner Transition” elements • Lie between the 1st and 2nd members of the “d-block” elements in Periods 6 & 7 (n=6 & n=7) • Condensed f-block ground-state electron configuration (Periods 6 & 7): [noble gas] ns2 (n-2)f14(n-1)dx, with n = 6 -7 • The 28 “f” orbitals are filled as follows: l = 3  [ml = -3 -2 -1 0 +1 +2 +3] 7 orbitals per period x 2 electrons per orbital x 2 periods = 28 Elements

  10. Transition Elements • Transition Metal Ions • Form through the loss of the “ns” electronsbefore the (n-1)d electrons Ex. Ti2+ [Ar] 3d2 4s2 → [Ar] 3d2 + 2e- (not [Ar] 4s2) (Ti2+ also called d2 ion) • Ions of different transition metals with the same electron configuration often have similar properties Ex. Mn2+ and Fe3+ are both d5 ions Mn2+ [Ar] 3d54s2→ [Ar] 3d5 + 2e- Fe3+ [Ar] 3d64s2→ [Ar] 3d5 + 3e- Both Ions have pale colors in aqueous solutions Both form complex ions with similar magnetic properties

  11. Practice Problem Write condensed electron configurations for the following ions: Zr V3+ Mo3+ Vanadium (V) – Period 4 Zirconium (Zr) & Molybdenum (Mo) – Period 5 General Configuration: ns2(n-1)dx a. Zr is 2nd element in the 4d series: [Kr] 5s24d2 (d2 ion) b. V is the 3rd element in the 3d series: [Ar] 4s23d3 “ns” electrons lost first In forming V3+, 3 electrons lost – two 4s & one 3d  V3+ = [Ar] 4s23d3 → [Ar] 3d2 (d2 ion) + 3e- c. Mo lies below Cr in Period 5, Group 6B(6): [kr] 5s1 4d5 Note: Same electron configuration exception as Cr  Mo3+ = [Kr] 5s1 4d5→ [Kr] 4d3 (d3 ion) + 3 e-

  12. Transition Elements • Trends of Transition Elements Across a Period • Transition elements exhibit smaller, less regular changes in • Size • Electronegativity • First Ionization Energy than the Main Group Elements in the same group

  13. Transition Elements • Atomic Size • General overall decrease across a period for both Main group and Transition group elements • As the “d” orbitals are filled across a period, the change in atomic size within the transition elements evens out because the increased nuclear charge shields the outer electrons preventing them from spreading out Main group Transition Metals Main group

  14. Transition Elements • Electronegativity • Electronegativity generally increases across period • Change in electronegativity within a series (period) is relatively small in keeping with the relatively small change in size • Small electronegativity change in Transition Elements is in contrast with the steeper increase between the Main Group elements across a period • Magnitude of Electronegativity in Transition elements is similar to the larger main-group metals Transition Metals

  15. Transition Elements • Ionization Energy • Ionization Energy of Period 4 Main-group elements rise steeply from left to right as the electrons become more difficult to remove from the poorly shielded increasing nuclear charge, i.e., no “d” electrons • In the Transition metals, however, the first ionization energies increase relatively little because of the effective shielding by the inner “d” electrons reducing the effect of the increased nuclear charge Transition Metals

  16. Transition Elements • Trends Within (down) a Group (relative to main-group elements) • Vertical trends differ from those of the Main Group elements • Atomic Size • Increases, as expected, from Period 4 to 5 • No increase from Period 5 to 6 • Lanthanides, starting in period 6 with buried “4f” sublevel orbitals, appear between the 4d orbitals in period 5 and the 5d orbitals in period 6 – Aufbau buildup sequence • An element in Period 6 is separated from the one above it in Period 5 by 32 electrons (ten 4d, six 5p, two 6s, and fourteen 4f) • The extra shrinking that results from the increased nuclear charge due to the addition of the fourteen 4f electrons is called the: “Lanthanide Contraction”

  17. Transition Elements n=1 n=2 n=3 l=0 l=0 l=1 l=0 l=1 l=2 (1s) (2s) (2p) (3s) (3p) (3d) -1 0 +1 -1 0 +1 -2 -1 0 +1 +2 ml =0 0 0 n=4 Note: n > 7 & l > 3 Sublevels not utilized for any element in the current Period Table l=0 l=0 l=0 l=1 l=1 l=1 l=2 l=2 l=2 l=3 l=3 l=3 (6p,7p) (5p) (4p) (6s,7s) (5s) (4s) (6d) (4d) (5d) (5f) (6f) (4f) -1 0 +1 -1 0 +1 ml = 0 -1 0 +1 -2 -1 0 +1 +2 -3 -2 -1 0 +1 +2 +3 -2 -1 0 +1 +2 -2 -1 0 +1 +2 -3 -2 -1 0 +1 +2 +3 -3 -2 -1 0 +1 +2 +3 ml = 0 ml = 0 n=5 n=6,7

  18. Transition Elements Main Group Non-metals Main Group Metals Transition Metals Inner Transition Metals Order of Sublevel Orbital Filling

  19. Transition Elements • Trends Within a Group (relative to main-group elements) • Electronegativity (EN) – Relative ability of an atom in a covalent bond to attract shared electrons • EN of Main-group elements decreases down group • greater size means less attraction by nucleus • Greater Reactivity • EN in Transition elements is oppositethe trend in Main-group elements • EN increases from period 4 to period 5 • No change from period 5 to period 6, since the change in volume is small and Zeff increases (f orbital electrons) • Transition metals exhibit more covalent bonding and attract electrons more strongly than main-group metals • The EN values in the heavy metals exceed those of most metalloids, forming salt-like compounds, such as CsAu and the Au- ion

  20. Transition Elements • Trends Within a Group (relative to Main-group elements) • Ionization Energy – Energy required to remove an electron from a gaseous atom or ion • Main-group elements increase in size down a group, decreasing the Zeff , making it relatively easier to remove the outer electrons • The relatively small increase in size of transition metals, combined with the relatively large increase in nuclear charge (Zeff), makes it more difficult to remove a valence electron, resulting in a general increase in the first ionization energy down a group

  21. Transition Elements • Trends Within a Group (relative to Main-group elements) • Density • Atomic size (volume) is inversely related to density (As size increases density decreases) • Transition element density across a period initially increases, then levels off, finally dips at end of series • From Period 5 to Period 6 the density increases dramatically because atomic volumes change little while nuclear mass increases significantly • Period 6 series contains some of the densest elements known: Tungsten, Rhenium, Osmium, Iridium, Platinum, Gold (Density 20 times greater than water, 2 times more dense than lead)

  22. Transition Elements • Trends are unlike those for the Main-group elements in several ways • 2nd & 3rd members of a transition group are nearly same size • Electronegativity increases down a transition group • 1st ionization energies are highest at the bottom of transition group • Densities increase down a transition group (mass increases faster than density

  23. Transition Elements • Chemical Properties of the Transition Elements • Atomic & physical properties of Transitions elements are similar to Main group elements • Chemical properties of transition elements are very different from main group elements • Oxidation States • Main-group elements display one, or at most two, oxidation states • The ns & (n-1)d electrons in transition elements are very close in energy All or most can be used as valence electrons in bonding – Transition metals can have multiple oxidation states

  24. Transition Elements • Oxidation State (Number) • Magnitude of charge an atom in a covalent compound would have if its shared electrons were held completely by the atom that attracts them more strongly 4s 3d 4p Note: All 3 d5 Ex. MnO2; O.N. Mn +4 Ex. MnO4- ; O.N. Mn +7

  25. Transition Elements • Metallic Behavior • Atomic size and oxidation state have a major effect on the nature of bonding in transition metal compounds • Transition elements in their lower oxidation states behave more like metals – Oxides more basic • Transition elements in their higher oxidation states exhibit more covalent bonding – Oxides more acidic Ex. TiCl2 (Ti2+) is an ionic solid TiCl4 (Ti4+) is a molecular liquid

  26. Transition Elements • Metallic Behavior • In the higher oxidation states: • The atoms have fewer electrons • The nuclear charge pulls remaining electrons closer, decreasing the volume and increasing the density • The charge density (ratio of the ion’s charge to its volume) increases • The increase in charge density leads to more polarization of the electron clouds in non-metals • The bonding becomes more covalent • The stronger the covalent bond, the less metallic • The oxides, therefore, become less basic Ex. TiO (Ti2+) is weakly basic in water TiO2 (Ti4+) is amphoteric, reacting with both acid and base

  27. Transition Elements • Electronegativity, Oxidation State, Acidity/Basicity • Why does oxide acidity increase with oxidation state? • Metal with a higher oxidation state is more positively charged • Attraction of electrons is increased, i.e., electronegativity increases Effective Electronegativity = Valence State Electronegativity • EN Cr – 1.6 Al – 1.5 (basic oxide) Cr3+ – 1.7 Cr6+ – 2.3 P – 2.1 (acidic oxides)

  28. Transition Elements • Metallic Behavior • Reduction Strength (Redox) • Standard Electrode Potential, Eo ,generally decreases across a period • As the value of Eo becomes more negative, i.e., at the beginning of the series, the ability of the species to act as a reducing agent increases Thus, Ti2+, Eo = -01.63V, is a stronger reducing agent than Ni2+, Eo = -0.25V • All species with a negative value of Eo can reduce H+ • 2H+(aq) + 2e- H2(g) Eo = 0.0V) • Note: Cu2+ (Eo = +0.34 V) cannot reduce H+ • The magnitude of the Eo values between two species, and the relative degree of surface oxidation, determines the level of reactivity of the oxidation/reduction reaction in water, steam, or acid solution

  29. Transition Elements • Color in Transition Elements • Most Main-Group Ionic Compounds are colorless • Metal ions have a filled outer shell • With only much higher energy orbitals available to receive an “excited” electron, the ion does not absorb visible light • The partially filled “d” orbitals of the transition metals can absorb visible wavelengths and move to slightly higher energy “d” levels

  30. Transition Elements • Magnetism in Transition Elements • Magnetic properties are related to electron sublevel occupancy • A “Paramagnetic” substance has atoms or ions with “unpaired” electrons • A “Diamagnetic” substance has atoms or ions with only “paired” electrons • Most Main-Group metal ions are diamagnetic (filled outer shells) • Many Transition metal compounds are paramagnetic because of unpaired electron in the “d” subshells

  31. Transition Elements • Chemical Behavior Within a Group • Main_Group • The decrease in Ionization Energy (IE) going down a group results in “increased reactivtiy” • Transition metals • Ionization Energy increases down group • The Standard Electrode Potential (Eo) also increases (becomes more positive) • Chromium is stronger reducing agent

  32. Transition Elements • The Inner Transition Elements • Lanthanides (Rare Earth Elements) (Cerium (Ce); Z = 58 – Lutetium (Lu); Z = 71) • Silvery, high melting point (800 – 1600oC) metals • Small variations in chemical properties makes them difficult to separate • Occur naturally in the +3 oxidation state as M3+ ions of very similar radii • Most lanthanides have the ground-state electron configuration filling the “f”subshell level [Xe] 6s2 4fx 5d0 x varies across series (Period) • Exceptions – Ce, Gd, Lu have single e- in 5d orbital

  33. Sample Problem Finding the Number of Unpaired Electrons The alloy SmCo5 forms a permanent magnet because both Samarium and Cobalt have unpaired electrons How many unpaired electrons are in the Sm atom (Z=62)? Ans: Samarium is the eighth element after Xe (Noble Shell) [Xe] 6s2 4f6 Two (2) electrons go in the 6s sublevel In general, the 4f sublevel fills before the 5d sublevel (slide 17) Recall previous slide - only Ce, Gd, Lu have 5d electrons  Remaining 6 electrons go into the 4f orbitals 6s 4f 5d 6p Six unpaired electrons

  34. Transition Elements • The Actinides: (Thorium (Th); Z=90 - Lawrencium; Z=103) • All Actinides are Radioactive (Alpha (4He2) Decay • Only Thorium & Uranium occur in nature • Share very similar chemical & physical properties • Silvery and chemically reactive • Principal oxidation state is +3, similar to lanthanides

  35. Transition Elements • Highlights of Selected Transition Metals • Period 4 – Chromium & Manganese • Chromium • Silvery, shiny metal with many colorful compounds • Cr2O3 acts as protective coating on easily corroded (oxidized) metals, such as iron • “Stainless” steels contain as much as 18 % Cr, making them highly resistant to corrosion • Electron Configuration ([Ar] 4s1 3d5) with 6 valence electrons occurs in all possible positive oxidation states • Important ions Cr2+, Cr3+, Cr6+ • Non-metallic character and oxide acidity increase with metal oxidation state • Cr2+ potential reducing agent (Cr loses additional electrons) • Cr6+ potential oxidizing agent (Cr gains electrons)

  36. Transition Elements • Highlights of Selected Transition Metals • Chromium • Chromium (II) – Cr2+ • CrO is basic and largely ionic • Forms insoluble hydroxide in neutral or basic solution • Dissolves in acid to yield Cr2+ ion and water CrO(s) + 2H+→ Cr2+ (aq) + H2O(l) • Chromium(III) – Cr3+ • Cr2O3 is amphoteric, similar properties as Aluminum • Dissolves in acid to yield violet Cr3+ ion Cr2O3(s) + 6H+(aq) → 2Cr3+(aq) + 3H2O(l) • Reacts with base to form the green Cr(OH)4- ion Cr2O3(s) + 3H2O + OH-→ 2Cr(OH)4-(aq)

  37. Transition Elements • Highlights of Selected Transition Metals • Chromium (con’t) • Chromium (VI) - Cr6+ (Deep Red) • CrO3 is covalent and acidic • Dissolves in water to form Chromic Acid (H2CrO4) CrO3(s) + H2O(l) → H2CrO4(aq) • H2CrO4 yields yellow Chromate ion (CrO42-) in base H2CrO4(aq) + 2OH(l) → CrO42-(aq) + 2H2O(l) • Chromate ion forms orange dichromate (Cr2O72-) ion in acid 2CrO42-(aq) + 2H+(aq) ⇆ Cr2O72-(aq) H2O(l)

  38. Transition Elements • Highlights of Selected Transition Metals • Manganese • Hard and Shiny • Like Vanadium & Chromium used to make steel alloys • Chemistry of Manganese is similar to Chromium • Metal reduces H+ from acids to form Mn2+ ion Mn(s) + 2H+(aq) → Mn2+(aq) + H2(g) Eo = 1.18 V • Manganese can use all its valence electrons (several oxidation states) to form compounds • Mn2+ Mn4+ Mn7+ most important • As oxidation state rises from +2 to +7, the valence state electronegativity increases and the oxides of Mn change from basic to acidic • Mn(II)O (basic) Mn(III)2O3 (amphoteric) • Mn(IV)O2 (insoluble) Mn(VII)2O7 (acidic)

  39. Transition Elements • All Manganese species with oxidation states greater than +2 act as oxidizing agents (gaining the electrons lost by the atoms being oxidized) Mn7+O4-(aq) + 4H+ + 3e-→ Mn4+O2(s) + 2H2O(l) Eo = 1.68 Mn7+O4-(aq) + 2H2O + 3e- → Mn4+O2(s) + 4OH- Eo = 0.59 (Mn7+O4- is a much stronger oxidizing agent in acid solution than in basic solution – note difference in Eo values) 4s 3d 4p

  40. Transition Elements • Manganese • Unlike Cr2+ & Fe2+, the Mn2+ (3d5) ion resists oxidation in air • Recall: half-filled (-1/2 spin electrons missing) & filled sublevels are more stable than partially filled sublevels • Cr2+ is a d4 species and readily loses a 3d electron to form the d3 ion Cr3+, which is more stable • Fe2+ is a d6 species and removing a 3d electron yields the stable, half-filled d5 configuration of Fe3+ • Removing an electron from Mn2+ disrupts the more stable d5 configuration

  41. Transition Elements & TheirCoordination Compounds • Coordination Compounds (Complexes) • Most distinctive aspect of transition metal chemistry • Complex – Substances that contain at least one complex ion • Complex ion – Species consisting of a “central metal cation” (either a main-group or transition metal) that is bonded to molecules and/or anions called “Ligands” • The Complex ion is typically associated with other(counter) ions to maintain neutrality • A coordination compound behaves like an electrolyte in water • Complex ion and counter ion separate • Complex ion behaves like a polyatomic ion – the ligands and central atom remain attached

  42. Transition Elements & TheirCoordination Compounds • Components of Coordination Compound • When solid complex dissolves in water, the complex ion and the counter ions separate, but ligands remain bound to central atom [Co(NH3)6]Cl3(s) Octahedral Geometry Central Atom Ligands Counter Ions

  43. Transition Elements & TheirCoordination Compounds • Complex ions • A complex ion is described by the metal ion and the number and types of ligands attached to it • The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs • The metal-ligand bonding can range from covalent to more ionic • Furthermore, the metal-ligand bond order can range from one to three (single, double, triple bonds) • Ligands are viewed as Lewis Bases (donate electron pairs), although rare cases are known involving Lewis acidic ligands

  44. Transition Elements & TheirCoordination Compounds • Complex ions • The complex ion structure is related to three characteristics: • Coordination Numbers • The number of ligand atoms that are bonded directly to the central metal ion • Coordination number is specific for a given metal ion in a particular oxidation state and compound • Coordination number in [Co(NH3)6]3+ is 6 • The most common coordination number in complex ions is 6, but 2 and 4 are common, with a few higher

  45. Transition Elements & TheirCoordination Compounds • Complex ions • Geometry – Depends on Coordination No. & Nature of Metal Ion d1 d8 d3 d9 d5 d10 d6

  46. Transition Elements & TheirCoordination Compounds • Complex Ions • Donor Atoms per Ligand • The Ligands of complex ions are “molecules” or “anions” with one or more donor atoms that each donate a lone pair of electrons to the metal ion to form a covalent bond • Atoms with lone pairs of electrons often come from Groups 5A, 6A, or 7A (main-group elements)

  47. Transition Elements & TheirCoordination Compounds • Complex Ions • Ligands are classified in terms of the number of donor atoms (teeth) that each uses to bond to the central metal ion • Monodentate Ligands use a “single” donor atom • Bidentate Ligands have two donor atoms • Polydentate Ligands have more than two donor atoms

  48. Transition Elements & TheirCoordination Compounds Donor Atom The Ligands contains one or more Donor atoms that have electron pairs to donate to the Central Atom

  49. Transition Elements & TheirCoordination Compounds • Complex Ions • Chelates (Greek “chela” – crab’s claw) • Bidentate and Polydentate ligands give rise to “rings” in the complex ion • Ex: Ethylene Diamine (abbreviated (en) in formulas) (:N – C – C – N:) forms a 5-member ring, with the two electron donatingN atoms bonding to the metal atom Such ligands seem to grab the metal ion like claws Ethylenediaminetetraacetate (EDTA) Used in treating heavy-metal poisoning, by acting as a scavenger of lead and other heavy-metal ions, removing them from blood and other body fluids

  50. Transition Elements & TheirCoordination Compounds • Formulas and Names of Coordination Compounds • Important rules for writing formulas of coordinate compounds • The cation is written before the anion • The charge of the cation(s) is balanced by the charge of the anions • In the complex ion, neutral ligands are written before anionic ligands • The entire ion is placed in brackets, i.e., [ ]

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