SESSION OBJECTIVES 1 fBlock elements 2 Introduction to

SESSION OBJECTIVES 1. f-Block elements 2. Introduction to lanthanides 3. Oxidation state 4. lanthanide contraction 5. Chemical reactivity 6. Introduction of Actinides 7. Comparison of actinides with lanthanides

INNER TRANSITION ELEMENTS The elements in which the additional electron enters (n-2)f orbitals are called inner transition elements. The valence shell electronic configuration of these elements can be represented as (n – 2)f 0 -14(n – 1)d 0 -1 ns 2. 4 f inner transition metals are known as lanthanides because they come immediately after lanthanum and 5 f inner transition metals are known as actinoids because they come immediately after actinium.

ELECTRONIC CONFIGURATION Element name Symbol Z Ln Ln 3+ Lanthanum Cerium Praesodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium [Xe]6 s 25 d 1 [Xe]4 f 16 s 25 d 1 [Xe]4 f 36 s 2 [Xe]4 f 46 s 2 [Xe]4 f 56 s 2 [Xe]4 f 66 s 2 [Xe]4 f 76 s 25 d 1 [Xe] 4 f 96 s 2 [Xe] 4 f 106 s 2 [Xe] 4 f 116 s 2 [Xe] 4 f 126 s 2 [Xe] 4 f 136 s 2 [Xe] 4 f 146 s 25 d 1 [Xe]4 f 0 [Xe]4 f 1 [Xe]4 f 2 [Xe]4 f 3 [Xe]4 f 4 [Xe]4 f 5 [Xe]4 f 6 [Xe]4 f 7 [Xe]4 f 8 [Xe]4 f 9 [Xe]4 f 10 [Xe]4 f 11 [Xe]4 f 12 [Xe]4 f 13 [Xe]4 f 14 La Ce Pr Nd Pm Sm Eu Eu Tb Dy Ho Er Tm Yb Lu 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Radius Ln 3+/ pm 116 114 113 111 109 108 107 105 104 103 102 100 99 99 98

ATOMIC AND IONIC SIZES: THE LANTHANIDE CONTRACTION As the atomic number increases, each succeeding element contains one more electron in the 4 f orbital and one proton in the nucleus. The 4 f electrons are ineffective in screening the outer electrons from the nucleus causing imperfect shielding. As a result, there is a gradual increase in the nucleus attraction for the outer electrons. Consequently gradual decrease in size occur. This is called lanthanide contraction.

ILLUSTRATIVE EXAMPLE Explain the cause and two consequences of lanthanoid contraction. Solution: The poor shielding effect of f-electrons is cause of lanthanoid contraction. Consequences There is close resemblance between 4 d and 5 d transition series. Ionization energy of 5 d transition series is higher than 3 d and 4 d transition series. Difficulty in separation of lanthanides

ILLUSTRATIVE EXAMPLE Why Zr and Hf have almost similar atomic radii? Solution Zr and Hf have almost similar atomic radii as a consequence of lanthanide contraction due to which their properties becomes similar.

ILLUSTRATIVE EXAMPLE Size of trivalent lanthanoid cation decreases with increase in atomic number. Explain. Solution It is due to poor shielding effect of f-electrons, valance electrons are strongly attracted towards nucleus, therefore, effective nuclear charge increases, hence ionic size decreases.

IONIZATION ENTHALPIES First ionization enthalpy is around 600 k. J mol-1, the second about 1200 k. J mol-1 comparable with those of calcium. Colours: Colours of these ions may be attributed to the presence of f electrons. Neither La 3+ nor Lu 3+ ion shows any colour but the rest do so. Absorption bands are narrow, probably because of the excitation within f level.

MAGNETIC PROPERTIES The lanthanoid ions other then the f 0 type (La 3+ and Ce 3+) and the f 14 type (Yb 2+ and Lu 3+) are all paramagnetic. The paramagnetism rises to the maximum in neodymium. Lanthanides have very high magnetic susceptibilities due to their large numbers of unpaired f-electrons. The strongest known magnets contain lanthanides (eg. Nd-Fe-B, Sm-Fe-N, and Sm. Co). Lanthanide complexes are used in MRI (medical resonance imaging), eg. [Gd(III)(dtpa)]2 -

OXIDATION STATES Predominantly +3 oxidation state. This irregularity arises mainly from the extra stability of empty, half filled or filled f subshell. Occasionally +2 and +4 ions in solution or in solid compounds are also obtained.

ILLUSTRATIVE EXAMPLE Why Sm 2+, Eu 2+, and Yb 2+ ions in solutions are good reducing agents but an aqueous solution of Ce 4+ is a good oxidizing agent? Solution The most stable oxidation state of lanthanides is +3. Hence the ions in +2 oxidation state tend to change +3 state by loss of electron acting as reducing agents whereas those in +4 oxidation state tend to change to +3 oxidation state by gain of electron acting as a good oxidising agent in aqueous solution.

LUMINESCENCE OF LANTHANOID COMPLEXES Irradiation of some Lanthanide(III) complexes with UV light causes them to fluoresce The origin of fluorescence is 4 f-4 f transitions. –the excited state produced decays to the ground state with emission of energy. Some examples are Eu 3+ (red) and Tb 3+ (green) They can be used as phosphors in television sets and fluorescent lighting. These applications are specific to lanthanoid ions because of the sharp transitions observed.

PROPERTIES Ø Silvery white soft metals, tarnish in air rapidly Ø Hardness increases with increasing atomic number, samarium being steel hard. Ø Good conductor of heat and electricity.

CHEMICAL PROPERTIES Metal combines with hydrogen when gently heated in the gas. The carbides, Ln 3 C, Ln 2 C 3 and Ln. C 2 are formed when the metals are heated with carbon. They liberate hydrogen from dilute acids and burn in halogens to form halides. They form oxides and hydroxides, M 2 O 3 and M(OH)3, basic like alkaline earth metal oxides and hydroxides.

USES Best single use of the lanthanoids is for the production of alloy steels for plates and pipes. A well known alloy is misch metal which consists of a lanthanoid metal (~95%) and iron (~5%) and traces of S, C, Ca and Al. A good deal of misch metal is used in Mg based alloy to produce bullets, shell and lighter flint. Mixed oxides of lanthanoids are employed as catalysts in petroleum cracking.

THE ACTINIDES Result from the filling of the 5 f orbitals. All isotopes are radioactive, with only 238 U and 244 Pu having long half-lives. 232 Th, 235 U, Only Th and U occur naturally-both are more abundant in the earth’s crust than tin. The others must be made by nuclear processes.

SOME CHARACTERISTIC PROPERTIES OF ACTINIDES The dominant oxidation state of actinides is +3. Actinides also exhibit an oxidation state of +4. Some actinides such as uranium, neptunium and plutonium also exhibit an oxidation state of +6. The actinides show actinide contraction (like lanthanide contraction) due to poor shielding of the nuclear charge by 5 f electrons. All the actinides are radioactive. Actinides are radioactive in nature. So the study of their chemistry is difficult in the laboratory. Their chemistry is studied using tracer techniques.

ELECTRONIC CONFIGURATION Element name Symbol Z Ln Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium [Rn] 6 d 17 s 2 [Rn ]5 d 27 s 2 [Rn]5 f 26 d 17 s 2 [Rn]5 f 36 d 17 s 2 [Rn]5 f 46 d 17 s 2 [Rn]5 f 67 s 2 [Rn]5 f 76 d 17 s 2 [Rn]5 f 97 s 2 [Rn]5 f 107 s 2 [Rn]5 f 117 s 2 [Rn]5 f 127 s 2 [Rn]5 f 137 s 2 [Rn]5 f 146 d 17 s 2 Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Ln 3+ [Rn]4 f 0 111 [Rn]4 f 2 [Rn]4 f 3 103 [Rn]4 f 4 101 [Rn]4 f 5 100 [Rn]4 f 6 99 [Rn]4 f 7 99 [Rn]4 f 8 98 [Rn]4 f 9 98 [Rn]4 f 10 [Rn]4 f 11 [Rn]4 f 12 [Rn]4 f 13 [Rn]4 f 14 Radius Ln 3+/ pm

IONIC SIZES Actinide contraction of trivalent ions is similar to that of the lanthanides, and is caused again by the increasing nuclear charge. Magnetic properties are more complex than those of lanthanoids. Susceptibility is roughly parallel to the lanthanoids.

PHYSICAL AND CHEMICAL REACTIVITY Ø Silvery in appearance but display a variety of structures due to the irregularity in metallic radii which are far greater than are found in lanthanoids. Ø Highly reactive metals, especially when finely divided. Ø Action of boiling water gives mixture of oxides and hydroxides. Ø Combination with most moderate temperature. non-metals takes place at Ø HCl attacks all metals but most are slightly affected by HNO 3 owing to the formation of protective oxide layers. Ø Alkalies have no effect.

COMPARISON OF LANTHANIDES AND ACTINIDES Similarities Ø Lanthanides and actinides involve filling of f-orbitals and thus are similar in many respects. Ø The most common oxidation state is +3 for both lanthanides and actinides. Ø Both are electropositive in nature and thus very reactive. Ø Magnetic and spectral properties are exhibited by both lanthanides and actinides. Ø Actinides exhibit actinide contraction just like lanthanides.

DIFFERENCES Besides +3, lanthanides also show oxidation states of +2 and +4 while actinides show higher oxidation states of +4, +5, +6 and + 7 as well. Lanthanide ions are colourless while most of the actinide ions are coloured. Actinides have a greater tendency towards complex formation as compared to lanthanides.

Lanthanide compounds are less basic while actinide compounds have appreciable basicity Actinides form few important oxocations such as UO 22+, Pu. O 22+, etc, while such oxocations are not known for lanthanides. Almost all actinides are radioactive while lanthanides, except promethium, are non-radioactive. The magnetic properties of actinides can be easily explained while it is difficult to do so in the case of lanthanides.

ILLUSTRATIVE EXAMPLE Why do lanthanoids and actinoids have same physical and chemical properties? Solution They have similar electronic configuration and f-orbital is progressively filled.

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