The d and f Block Elements (f -Block Elements)

The d and f Block Elements f -Block Elements They were earlier called as rare earth metals as it was believed that they exist in earth’s crust to a very less extent for e.g. : Pm, does not exist in the earth’s crust. But this terminology is now not applicable as they exist in earth’s crust to a sufficient extent. all the fifteen elements together at one place. This has been done by placing the first element, lanthanum below yttrium and placing the remaining fourteen elements separately in the lower part of the periodic table. Lanthanide series (Z = 58 - 71) (Ce – Lu) INNER TRANSITION ELEMENTS Actinide series The elements in which the additional electron enters 2. in (n - 2)f orbitals are called inner transition elements. or f-block elements. 1. Position in the periodic table The lanthanides resemble yttrium in most of their properties. So it became necessary to accomodate 3. (Z = 90 - 103) (Th – Lw) Lanthanides (Lanthanones) Lanthanides are reactive elements so do not found in free state in nature. Most important minerals for lighter Lanthanides are - Monazite, cerites and orthite and for heavier lanthanides - Gadolinite and Xenotime Electronic configuration The general configuration of lanthanides may be given as 4f2-145s25p65d0/16s2. Lanthamide have outer three shells incomplete. Atomic Element Symbol Number Outer electronic configuration Atomic +3 ion 58 Cerium Ce 4f2 6s2 4f1 59 Praseodymium Pr 4f3 6s2 4f2 60. Neodymium Nd 4f4 6s2 4f3 61. Promethium Pm 4f5 6s2 4f4 62. Samarium Sm 4f6 6s2 4f5 63. Europium Eu 4f7 6s2 4f6 64. Gadolinium Gd 4f7 5d1 6s2 4f7 65. Terbium Tb 4f9 6s2 4f8 66. Dysprosium Dy 4f10 6s2 4f9 67. Holmium Ho 4f11 6s2 4f10 68. Erbium Er 4f12 6s2 4f11 69. Thulium Tm 4f13 6s2 4f12 70. Ytterbium Yb 4f14 6s2 4f13 71. Lutecium Lu 4f14 5d1 6s2 4f14 83 The d and f Block Elements Oxidation states + 2 and +4 occur particularly when they lead to It is to be noted here that filling of 4f orbitals in the atoms is not regular. A 5d electron appears in gadolinium (z = 64) with an outer electronic (i) A noble gas configuration e.g. Ce4+ (f0) configuration of 4f75d16s2 (and not 4f86s2). This is (ii) A half filled ‘f ‘ orbital e.g. Eu2+, Tb4+, (f7) because the 4f and 5d electrons are at about the 2+ 14 same potential energy and that the atoms have a (iii) A completely filled ‘f ‘ orbital e.g. Yb (f ) Therefore, in higher oxidation state, they act as tendency to retain stable half filled configuration. oxidising while in lower state as reducing agents. On the other hand, the filling of f orbitals is regular in 5. tripositive ions. In tripositive lanthanide ions the number of unpaired electrons regularly increases from lanthanum to Gadolinium (0 to 7) and then continuously decreases upto lutecium (7 to 0). So lanthanum and lutecium ions which are diamagnetic, all other tripositive lanthanide ions are Paramagnetic. After losing outer electrons, the f orbitals shrink in size and became more stable. Pm is the only synthetic radioactive lanthanide. Oxidation states 4. Magnetic properties Lanthanides Ce58 Oxidation States +3,+4 Pr59 +3, (+4) Nd60 +3 Pm61 +3 Sm62 Eu63 (+2), +3 +2,+3 Gd64 +3 Tb65 +3,+4 * Dy66 Ho67 +3; (+4) +3 Lanthanide ions having 4f0, 4f7,4f14 are colourless. Lanthanide ions 4f1 and 4d13 are also colourless. 6. Other Properties Er68 Tm69 Yb70 Lu71 (+2), +3 (+2), +3 +2,+3 +3 Colour - The lanthanide ions have unpaired electrons in their 4f orbitals. Thus these ions absorbs visible region of light and undergo f-f transition and hence exhibit colour. The colour exhibited depends on the number of unpaired electrons in the 4f orbitals. The ions often with 4fn configuration have similar colour to those ions having 4f14–n configuration. (a) Highly dense metals with high m.pts. (do not show any regular trend). (b) lonisation Energies - Lanthanides have fairly low ionisation energies comparable to alkaline earth metals. * Oxidation states in brackets are unstable states * The lanthanides contains two s electrons in the (c) Electro positive Character - High due to low I.P. outermost shell, they are therefore expected to (d) Complex formation - Do not have much tendency exhibit a characteristic oxidation state of +2. But for to form complexes due to low charge density the lanthanides, the +3 oxidation is common. because of their large size. This corresponds to the use of two outermost Lu+3 is smallest in size can only form complex. electrons (6s2) alongwith one inner electron. The inner electron used is a 5d electron (in La, Gd and (e) Reducing Agent - They readily lose electrones so are good reducing agent. Lu), or one of the 4f electron if no 5d electrons * In +3 oxidation states, nitrates, perchlorates and present. sulphates of lanthanides and actinides are water All the lanthanides attains +3 oxidation state and only soluble, while their hydroxides, fluorides and cerium, Praseodymium, and terbium exhibit higher carbonates are water insoluble. oxidation state (+4). * * 84 The d and f Block Elements * Alloys of lanthanides with Fe are called misch metals. (ii) Similarity of yttrium with lanthanides :* La(OH)3 is most basic in nature while Lu(OH)3 least The properties of yttrium are so similar to the basic. lanthanides that it is considered more a member of * Lanthanides form MC2 type carbide with carbon, the lanthanide series than a congener of scandium. which on hydrolysis gives C2H2. (iii) Anomalous behaviour of post-lanthanides :LANTHANIDE CONTRACTION The following anomalies may be observed in the * In the lanthanide series with increasing atomic behaviour of post-lanthanide elements. number, there is a progressive decrease in the size (a) Atomic size - The ionic radii of Zr+4 is about 9% from lanthanum to lutecium or from La+3 to Lu+3. more than Ti+4. Similar trend is not maintained on This contraction in size is known as lanthanide passing from the second to third transition series. contraction. The ionic radius of Hf+4, instead of increasing * The general electronic configuration of these elements (because of inclusion of one more electronic shell). is 4f0–145s2p6d0-16s2. In these elements the added decreases (or is virtually equal to Zr+4) as a electron enters the deep seated f-orbitals and therefore experiences considerable pull by the consequence of the lanthanide nucleus. contraction. * Such an electron cannot add to the size of the element This explains the close similarities between the and also because the intervening 5s2p6d1 electronic members of the second and third transition series shells, it is very little screening effect on the outermost than between the elements of the first and second 6s2 electrons. series. Hence with increasing atomic number, the enhanced * nuclear charge leads to contraction in the size of (b) lonisation potential and electronegativity :atoms and ions. The effect of lanthanide contraction is also seen in The atomic volumes of europium and ytterbium are the increase in the ionisation potential values and unexceptedly large. The large atomic size of Eu and electronegativities of the elements of the third Yb suggest weaker bonding in the solid elements. transition series, contrary to the general trend. Both these elements have only two electrons extra Because of the lanthanide contraction, the postthan the stable configurations (half filled, f7, and 14 completely filled, f ), hence they utilise two electrons lanthanide elements have stronger positive field and in metallic bonding as in the case with barium. thus the electrons are held more tightly. EFFECTS OF LANTHANIDE CONTRACTION The greater effective nuclear charge of the former make them more electronegative than the latter. (i) Close resembalace of Lanthanides :- The general (c) High density :- Because of lanthanide contraction the atomic sizes of the post lanthanide elements decrease in the sizes of the lanthanides with an increase in their nuclear charges result in a small become very small. consequently, the packing of increase in their ionisation energies. Hence their basic atoms in their metallic crystals become so much and ionic nature gradually decreases from La to Lu. compact that their densities are very high. This also explains the variations in properties such The densities of the third transition series elements as increased tendency for hydrolysis and formation are almost double to those of the second series of complex salts and decreased thermal stability. elements. solubility of their salts. 85 APPLICATION OF LANTHANIDES Cerium is most useful element in the lanthanides (a) Ceramic application - CeO2, La2O3' Nd2O3 and Pr2P3 are used as decolourizing agents for glasses. (b) CeS (m.p. - 2000°C) is used in the manufacture of a special type of crucibles and refractories. (c) Lanthanide compounds like cerium molybdate, cerium tungstate are used as paints and dyes. (d) In textile and leather industries (Ce salts). ACTINIDES (5f - BLOCK ELEMENTS) * The elements in which the extra electron enters 5forbitals of (n - 2)th main shell are known as actinides. * The man.made eleven elements Np93 - Lr103 are placed beyond uranium in the periodic table and are collectively called trans-uranic elements. * Th, Pa and U first three actinides are natural elements. 1. Electronic configuration :- 2. The d and f Block Elements Oxidation states In lanthanides-and actinides +3 oxidation is the most common for both of the series of elements. This oxidation state becomes increasingly more stable as the atomic number increases in the actinide series. Actinides Oxidation state* Th90 +4 Pa91 (+4), +5 U92 (+3), (+4), (+5), +6 Np93 (+3), (+4), +5, (+6), (+7) Pu94 (+3), +4, (+5), (+6), (+ 7) Am95 +2,(+3),(+4),(+5),(+6) Cm96 +3, (+4) Bk97 +3, (+4) Cf98 +3 Es99 +3 Fm100 +3 Md101 +3 No102 +3 Lw103 +3 Difference between Lathanides and Actinides The general configuration of actinides may be given as 5f1–14 6d0/1, 7s2. Atomic Elements Symbol Electronic No. 90 Thorium Th Configuration 6d27s2 91 Protactinium Pa 5f26d17s2 92 Uranium U 5f36d17s2 93 Neptunium Np 5f46d17s2 94 Plutonium Pu 5f67s2 95 Americium Am 5f77s2 96 Curium Cm 5f76d17s2 97 Berkellium Bk 5f97s2 98 Californium Cf 5f10 7s2 99 Einstenium Es 5f117s2 100 Fermium Fm 5f12 7s2 101 Mandelevnium Md 5f137s2 102 Nobellium No 5f147s2 103 Lawrencium Lw 5f146d17s2 Some important uses of actinides are as follows – Thorium : Thorium is used in atomic reactors as fuel rods and in the treatment of cancer. Uranium : Uranium is used as nuclear fuel. Its salts are used in glass industry (for imparting green colour). textile industry and also in medicines. Plutonium : Plutonium is used as fuel for atomic reactors as well as in atomic bombs. 86