1 Basic Concepts d&F-block Class 12

CLASS XII d&f-BLOCK ELEMENTS BASIC CONCEPTS/IMPORTANT FORMULA/EQUATIONS  “The elements which have partially p artially filled d-sub orbit or the elements in which the last  electron enters in (n-1) d-orbitals are called transition elements.”  The d -block -block element elementss are called called transition elements also becau because se they exhibi exhibitt transitional   s-block ock element elementss (electro (electropos positi itive ve behaviour   between highly reactive ionic-compound-forming  s-bl elements) elements) on one side, and mainly the covalent-com covalent-compoun pound-form d-forming ing  p-block  p-block elements (electronegative elements) on the other side.

Electronc Conf!"r#ton of Tr#n$ton Ele%ent$ From the point of view of electronic configuration, the elements which have partially filled d orbitals in their neutral atoms or in their common ions are called transition elements. elements. Thus, the outer  ('(* ('+ ()d ns , where n is the outermost shell, electronic configuration of the transition elements is n  ' ()d and n ' () stands for the penultimate shell. Ques:-,. Ques:-,. #re nc0 C#d%"% #nd Merc"r. not con$dered #$ te Tr#n$ton Ele%ent$1  ns: - n 2inc cadmium and mercury the last electron enters in s-orbital not in the (n-!) d-orbital, so these elements are not called transition elements. Their electronic configurations are (n ( n  " !) d !# ns$. %inc %ince, e, in thes thesee meta metals ls d -orb - orbit ital alss are comp comple letel tely y fille filled, d, henc hencee thes thesee do not not exhi exhibi bitt the the gene general ral charact characteris eristic tic proper propertie tiess of the transiti transition on elemen elements. ts. Theref Therefore ore,, these these metals metals are not consid considered ered as transition elements.

3ener#l 3ene r#l Trend$ n te t e Ce%$t Ce %$tr. r. of Fr$t F r$t Ro4 R o4 Tr#n$ton Tr#n$t on Ele%ent Ele %ent$$  5d-$ere$) (6 !lectronic (6 !lectronic "onfi#uration "onfi#uration &ll d -block -block elements exhibit 'd  ' d !"!# 4s!"$ electronic configuration. %ome characteristic features of  the electronic configuration configurationss of the transition transition elements are, &toms of all transition elements consist of  an inner core of electrons having noble gas configuration. For example,

Sc 7 8Ar9 5d  5d ( :s  :s+

; 7 8Kr9 :d (  n % The atomic radii of transition elements show the following characteristics. Ques.:-The atomic radii and atomic volumes of d-bloc% elements in any series decrease with increase in the atomic number. The decrease decrease however& is not re#ular. The atomic radii tend to reach minimum near at the middle of the series& and increase sli#htly towards the end of the series& why'  ns: - hen we go in any transition series from left to right, right, the nuclear charge increases increases gradually by one unit at each element. The added electrons enter the same penultimate shell, (inner d -shell). -shell). These

added electrons shield the outermost electrons from the attraction of the nuclear charge. The increased nuclear charge tries to reduce the atomic radii, while the added electron tries to increase the atomic radii. &t the beginning of the series, due to smaller number of electrons in the d -orbitals, the effect of  increased nuclear charge predominates, and the atomic radii decrease. n the middle of the series, the atomic radii tend to have a minimum value as observed ater in the series, when the number of d electrons increases, the increased shielding effect and the increased repulsion between the electrons tend to increase the atomic radii. Ques.:-The atomic radii increase while #oin# down in each #roup. owever& in the third transition series (d series) from hafnium (f) and onwards& the elements have atomic radii nearly e*ual to those of the second transition series elements& why'  ns: - The atomic radii increase while going down the group. This is due to the introduction of an additional shell at each new element down the group. & nearly e*ual radius of second (+-d series) and third transition series (d series) elements is due to a special effect called l#nt#nde contr#cton6 n the d - series of transitions elements, after lanthanum (a), the added !+ electrons go to the inner most + f orbitals (antepenultimate orbitals). The + f electrons have poor shielding effect. ut due to addition of  !+ extra protons in the nucleus the outermost electrons experience greater nuclear attraction. %o sie of  elements of -d series becomes smaller then +-d series.

5 . +onic $adii  For ions having identical charges, the ionic radii decrease slowly with the increase in the atomic number across a given series of the transition elements.

EXPLANATION6  The gradual decrease in the values of ionic radius across the series of  transition elements is due to the increase in the effective nuclear charge.

: . +onisation !ner#ies The ionisation energies (now called ionisation enthalpies, / H   ) of the elements of first transition series are given below0

The following generaliations can be obtained from the ionisation energy values given above. Ques.:-The ionisation ener#ies of these elements are hi#h& and in most cases lie between those of sand p-bloc% elements. This indicates that the transition elements are less electropositive than s-bloc%  elements.  ns: - Transition metals have smaller atomic radii and higher nuclear charge as compared to the alkali metals. oth these factors tend to increase the ionisation energy, as observed. The ionisation energy in any transition series increases with atomic number1 the increase however is not smooth and as sharp as seen in the case of s- and p-block elements. EXPLANATION6 The ionisation energy increases due to the increa se in the nuclear charge with atomic number at the beginning of the series. 2radually, the shielding effect of the added electrons also increases. This shielding effect tends to decrease the attraction due to the nuclear charge. These two opposing factors lead to a rather gradual increase in the ionisation energies in any transition series.

Ques.:-The first ionisation ener#ies of d-series of elements are much hi#her than those of the ,dand d-series elements& why'.  ns: - n the d - series of transitions elements, after lanthanum (a), the added !+ electrons go to the inner most + f orbitals (antepenultimate orbitals). The + f electrons have poor shielding effect. ut due to addition of !+ extra protons in the nucleus the outermost electrons experience greater nuclear attraction. %o sie of elements of -d series becomes smaller then +-d series. This leads to higher ionisation energies for the d -series of transition elements.

 "  etc. & few typical complex ions are,

8FeCN)=9 :' 0 8C"N5):9+0 8;+O)=9+0 8NCO):90 8CoN5)=950 8FeF=95'  EXPLANATION6 This complex formation tendency is due to, (a) %mall sie of the transition metal cations. (b) 6igh positive charge density (c) The availability of vacant inner d -orbitals of suitable energy to accept lone pair of electrons.

(+6 >ormation of +nterstitial "ompounds Transition elements form a few interstitial compounds with elements having small atomic radii, such as hydrogen, boron, carbon and nitrogen. The small atoms of these elements get entrapped in  between the void spaces (called interstices) of the metal lattice. %ome characteristics of the interstitial compounds are, (a) These are non-stoichiometric compounds and cannot be given definite formulae. (b) These compounds show essentially the same chemical properties as the parent metals, but differ  in physical properties such as density and hardness. %teel and cast iron are hard due to the formation of interstitial compound with carbon. %ome nonstoichiometric compounds are, Se *6?@ (Ganadium selenide), Fe*6?:O, and titanium hydride T(6>.

 7ome properties !. nterstitial compounds are hard and dense. This is because1 the smaller atoms of lighter  elements occupy the interstices in the lattice, leading to a more closely packed structure. $. 4p are higher and '. They are chemically inert. Hue to greater electronic interactions, the strength of the metallic  bonds also increases.

(56 "atalytic 2roperties 4ost of the transition metals and their compounds particularly oxides have good catalytic  properties. Dlatinum, iron, vanadium pentoxide, nickel, etc., are important catalysts. Dlatinum is a general catalyst. >ickel powder is a good catalyst for hydrogenation of unsaturated organic compounds such as, hydrogenation of oils. %ome typical industrial catalysts are0 (a) Ganadium pentoxide (G $:) is used in the 5ontact process for the manufacture of sulphuric acid, (b) Finely divided iron is used in the 6aberIs process for the synthesis of ammonia. EXPLANATION6 4ost transition elements act as good catalyst because of, (a) The presence of vacant d -orbitals. (b) The tendency to exhibit variable oxidation states. (c) The tendency to form reaction intermediates with reactants. The presence of defects in their  crystal lattices.

(:6 lloy >ormation Transition metals form alloys among themselves. The alloys of transition metals are hard and high melting as compared to the host metal. Garious steels are the alloys of iron with metals such as chromium, vanadium, molybdenum, tungsten, manganese etc. EXPLANATION6  The atomic radii of the transition elements in any series are not much different from each other. &s a result, they can very easily replace each other in the lattice and form solid solutions over an appreciable composition range. %uch solid solutions are called alloys.

(a $%:+.!#6$: crystallies out. This is filtered hot and allowed to cool when sodium dichromate, >a $5r $:9.$6$:, separates out on standing.

(c) onversion of sodi"m dichromate to potassi"m dichromate. 6ot concentrated solution of sodium dichromate is treated with a calculated amount of potassium chloride, when potassium dichromate being less soluble crystallies out on cooling.

Ce%c#l roerte$6

(i) Acton of #l#le$6 ith alkalies, it gives chromates. For example, with J:6,

:n acidifying, the colour again changes to orange-red owing to the formation of dichromate.

&ctually, in dichromate solution, the 5r $:9 $"  ions are in e*uilibrium with 5r: + $"  ions. (iv) Od$n! n#t"re6 n neutral or in acidic solution, potassium dichromate acts as an excellent oxidising agent, and 5r $:9 $"  gets reduced to 5r '7. The standard electrode potential for the reaction,

is 7 !.'! G. This indicates that dichromate ion is a fairly strong oxidising agent, especially in strongly acidic solutions. That is why potassium dichromate is widely used as an oxidising agent, for  *uantitative estimation of the reducing agents such as, Fe $7. t oxidises, (a) erro"s salts to ferric salts  -onic e!"ation0 (b) $"lphites to s"lphates and arsenites to arsenates.  -onic e!"ation0 %imilarly, arsenites are oxidised to arsenates. (c)  Hydrogen halides to halogens.  -onic e!"ation0 (d)  -odides to iodine  -onic e!"ation0 Thus, when J is added to an acidified solution of J$5r$:9 iodine gets liberated. (e)  -t oxidises H $ to $ .  -onic e!"ation0 (i)

For%#ton of n$ol"Dle cro%#te$6 ith soluble salts of lead, barium etc., potassium dichromate gives insoluble chromates. ead chromate is an important yellow pigment.

(ii)

Cro%.l clorde te$t6 hen potassium dichromate is heated with conc. 6$%:+ in the  presence of a soluble chloride salt, the orange-red vapour of chromyl chloride (5r: $5l$) is formed.

5hromyl chloride vapour when passed through water give yellow-coloured solution containing chromic acid.

Str"ct"re of Cro%#te #nd cro%#te Ion$ .

 f-bloc% elements Inner-Tr#n$ton Ele%ent$7 L#nt#node$ #nd Actnod$ The elements which in their elemental or ionic form have partly filled  f -orbitals are called  f bloc/ elements. &s the  f - orbitals lie inner to the penultimate (second outermost) shell i.e. antepenultimate orbitals , therefore these elements having partially filled f -orbitals are also known as innertransition elements. There #eneral electronic confi#uration is (n-)f 1-1(n-1)d 9 or 1ns There are two series of inner-transition elements, each having !+ elements. The elements in which + f orbitals are progressively filled are called lanthanides. The elements in which  f orbitals are  progressively filled are termed actinides. anthanides thus belong to the first inner-transition series, while actinides belong to the second inner-transition series.

L#nt#node$ The fourteen elements (atomic no. < " 9!) after lanthanum are known as lanthanides or  lanthanons. &ll these elements closely resemble one another in their properties. ecause of their limited availability, these are also known as the rare earth elements.  >ames and the outer-electronic configurations of the lanthanides are given in Table C.!#.

3ener#l C#r#cter$tc$ of L#nt#nde$ 2eneral physical characteristics of lanthanides are described below0

(1) !lectronic confi#uration6 The outer-electronic configurations of lanthanides are given in Table !!.C. There is however, some uncertainty about the correctness of these configurations. The  d  and + f energy levels are very close-by. t is not always possible to decide with certainty whether the electron has entered d or + f level. Hue to the extra-stability of half-filled and completely filled orbitals, there is a tendency to ac*uire  f 9  and  f !+ configurations wherever possible. The general electronic configuration of lanthanides may be described as f 181 d  981 Bs. () 56idation states6 &ll anthanoides exhibit a common stable oxidation state of 7'. in addition some lanthaniodes shows 7$ and 7+ oxidation state also. These are shown by those elements which by doing so attain the stable  f 9 & f 4  and f 1 configurations. For example0 ) Ce #nd TD eDt : od#ton $t#te$6 5erium (5e) and terbium (Tb) attain f #  and f 9  configuration respectively when they get 7+ oxidation state, as shown below0 5e+7 0 KLeM+f#   Tb+7 0 KLeM+f9   ) E" #nd ;D eDt + od#ton $t#te$6 8uropium and yetterbium get f 9 and f !+ configuration in 7$ oxidation state, as shown below0 8u$7 0 KLeM+f9   b$7 0 KLeM+f!+  ) L#0 3d0 #nd L" eDt onl. 5 od#ton $t#te$ d"e to e%t.0 #lf flled #nd f"lflled :f-$"D orDt6 The stability of different oxidation state has strong effect on the properties of those elements. For example, 5e(G) is favoured because of its noble gas configuration. ut it is strong oxidant changing to common 7' oxidation state. %imilarly, 8u$7 is stable because of its half filled +f 9 configuration. 6owever, it is a strong reducing agent changing to 8u '7 (common oxidation state.) %imilarly, b $7 having the configuration +f !+ is a reductant. %amarium also behaves like europium exhibiting both 7$ and 7' oxidation states.  +mportant note: - +rrespective of noble #as confi#uration f 9 "e) "olour of the salts and ions in solution 6 4ost of the lanthanide trivalent ions are coloured in solid as well as in the solution phase. The ions containing x and (!+ " x) electrons show the same colour. The colour of the salts or ions is due to the f " f transition of electrons.

Actnode$ The fourteen elements (atomic number C#"!#') after actinium are called actinides. These are also called  second series of innertransition elements. The general electronic configuration of actinides is  f !"!+ =d  #"! 9 s$. >ames and the outer-electronic configurations of actinides are given below in Table C.!!. 3ener#l C#r#cter$tc$ of Actnde$ (1) 56idation states6 The oxidation states commonly exhibited by actinides are given in Table C.!$. The most stable state is indicated by Dold letter. The 7 ' state becomes more stable as the atomic number increases.

() tomic and +onic radii. The radii for tripositive (4'7) and tetrapositive (4+7) ions decrease in going from Th to 5m. This steady decrease is similar to that observed in lanthanides and is called #ctnde contr#cton6 F#ct7 The actinide contraction is lar#er than lanthanide contraction. ;eason0 because in lanthanoids electrons are filled in +f orbital whose screening effect is more stronger than f orbitals of actinoid elements. (,) "olour of salts and ions in solution 6 4ost of the salts of actinides having 4'7 or 4+7 ions are coloured. ons having  f ?,  f ! and  f 9 configurations are colourless, while those containing  f  $ ,  f ',  f +,  f  and  f = configurations are coloured.

U$e$ of L#nt#nde$ Hue to their alloy-forming tendencies, actinides and lanthanides form many alloys particularly with iron. These elements improve the workability of steel. & well known alloy is %$c-%et#l which consists of a rare earth element (C+ " CN), iron (up to N) and traces of sulphur, carbon, calcium and aluminium. The pyrophoric alloys containing rare-earth metals are used in the preparation of ignition devices, e.g., tracer bullets and shells and flints for lighters. This alloy has normally the composition0 cerium +#.N, lanthanum 7 neodymium ++N, iron +.N, aluminium #.N and the remainder calcium, silicon and carbon. (i) "erium constitute about ,9-9G of the alloys of lanthanides. They are used fro scaven#in# o6y#en and sulphur. (ii) Thorium is used in the manufacture of fine rods of atomic reactor. (iii) Thorium salts are also used in treatment of cancer. (iv) Hranium and 2lutonium are used for production of nuclear ener#y by the nuclear fission.