1 Basic Concepts d&F-block Class 12

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CLASS XII d&f-BLOCK ELEMENTSBASIC CONCEPTS/IMPORTANT FORMULA/EQUATIONS

“The elements which have partially 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 elements are called transition elements also because they exhibit transitional

behaviour between highly reactive ionic-compound-forming s-block elements (electropositive

elements) on one side, and mainly the covalent-compound-forming p-block elements (electronegative

elements) on the other side.

Electronc Conf!"r#ton of Tr#n$ton Ele%ent$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. Thus, the outer

electronic configuration of the transition elements is n ' ()d('(* ns('+, where n is the outermost shell,

and n ' () stands for the penultimate shell.

Ques:-,. #re nc0 C#d%"% #nd Merc"r. not con$dered #$ te Tr#n$ton 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 " !) d !# ns$.

%ince, in these metals d -orbitals are completely filled, hence these do not exhibit the general

characteristic properties of the transition elements. Therefore, these metals are not considered as

transition elements.

3ener#l Trend$ n te Ce%$tr. of Fr$t Ro4 Tr#n$ton Ele%ent$ 5d-$ere$)

(6 !lectronic "onfi#uration

&ll d -block elements exhibit 'd !"!# 4s!"$ electronic configuration. %ome characteristic features of

the electronic configurations of the 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 ( :s+ ; 7 8Kr9 :d ( <s+ L# 7 8Xe9 <d ( =s+

The half-filled and completely-filled d -orbitals gain extra-stability. %o, such con-figurations are

favoured wherever possible. For example

+6 tomic $adii The atomic radii of 'd -series of elements are compared with those of the neighbouring s- and p-

block elements.

(=: (:> (5< (+? (5> (+= (+< (+< (+@ (5> 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 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, the nuclear charge increases gradually by

one unit at each element. The added electrons enter the same penultimate shell, (inner d -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#nde contr#cton6 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 sie 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#iesThe ionisation energies (now called ionisation enthalpies, / H ) of the elements of first transition

series are given below0

The following generaliations 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 s-

and 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 increase 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 ,d-

and 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 sie of elements of -d series becomes smaller then +-d series. This leads to higher ionisation

energies for the d -series of transition elements.

<6 etallic "haracter &ll transition elements are metals. These are hard, and good conductor of heat and electricity.

&ll these metals are malleable, ductile and form alloys with other metals. These elements occur in three

types, e.g., face-centered cubic ( fcc), hexagonal closepacked (hcp) and body-centred cubic (bcc),

structures.

EXPLANATION6 The ionisation energies of the transition elements are not very high. The

outermost shell in their atoms have many vacant3partially filled orbitals. These characteristics make

these elements metallic in character.

The hardness of these metals, suggests the presence of covalent bonding in these metals. The

presence of unfilled d -orbitals favours covalent bonding. 4etallic bonding in these metals is indicated by the conducting nature of these metals. Therefore, it appears that there exists covalent and metallic

bonding in transition elements. The strength of inter atomic interactions becomes stronger as the

number of unpaired electrons increases. 5r, 4o and have maximum number of unpaired electrons so

these metals are very hard.

Ques.:- /hy is the ener#y of atomi0ation is very hi#h for d- bloc% elements'

=6 eltin# and oilin# 2ointsThe melting and boiling points of transition

elements except 5d and 6g are very high as compared

to the s-block and p-block elements. The melting and

boiling points first increase, pass through maxima and

then steadily decrease across any transition series. The

maximum occurs around middle of the series.

EXPLANATION6 &toms of the transition

elements are closely packed and held together by strong

metallic bonds which have appreciable covalent

character. This leads to high melting and boiling points

of the transition elements.

The strength of the metallic bonds depends upon

the number of unpaired electrons in the outermost shell

of the atom. Thus, greater is the number of unpairedelectrons stronger is the metallic bonding. n any

transition element series, the number of unpaired

electrons first increases from ! to and then decreases back to ero. The maximum five unpaired

electrons occur at 5r ('d series). &s a result, the melting and boiling points first increase and then

decrease showing maxima around the middle of the series.

The low meltin# points of 3n& "d& and # may be due to the absence of unpaired d-electrons

in their atoms6

4. 56idation 7tates4ost of the transition elements exhibit several oxidation states, i.e., they show variable valency

in their compounds. %ome common oxidation states of the first transition series elements are given below .

Ques.:- /hy do d-bloc% elements show variable o6idation states'



ns.:- The outermost electronic confi#uration of the transition elements is (n 8 1) d 1819 ns . The

ener#y of (n 8 1) d and ns- orbitals are nearly same& so alon# with the ns-electrons (n 8 1) d-electrons

also involved in o6idation state so these elements shows variable o6idation states. lso it arises due to

partially filled d-orbital.

Therefore, the number of oxidation states shown by these elements depends upon the number of

d -electrons it has. For example, %c having a configuration 'd ! + s$ may show an oxidation state of 7 $

(only s-electrons are lost) and 7 ' (when d -electron is also lost). Te !e$t od#ton $t#te 4c #nele%ent of t$ !ro" %!t $o4 $ !en D. te tot#l n"%Der of ns- #nd n ' () d -electron$6

The relative stability of the different oxidation states depends upon the factors such as,

electronic configuration, nature of bonding, stereochemistry, lattice energies and solvation energies.

Ques:-/hy hi#hest o6idation states are shown by o6ide and fluorides'

The highest oxidation states are found in fluorides and oxides because fluorine and oxygen are

the most electronegative elements.

Te !e$t od#ton $t#te $o4n D. #n. tr#n$ton %et#l $ e!t6 Te od#ton $t#te of

e!t $ $o4n D. R" #nd O$6

&n examination of the common oxidation states reveals the following conclusions0

(a) The variable oxidation states shown by the transition elements are due to the participation

of outer ns- and inner (n " !) d -electrons in bonding.(b) 8xcept scandium, the most common oxidation state shown by the elements of first

transition series is 7 $. This oxidation state arises from the loss of two + s electrons. This

means that #fter $c#nd"%0 d -orDt#l$ Deco%e %ore $t#Dle t#n te s-orDt#l6

(c) The greatest number of oxidation states is observed near middle of the series. 8g0- 4n

show 7$ to 79 :.%. The highest oxidation states are observed in fluorides and oxides. The

highest oxidation state shown by any transition element (by ;u and :s) is 7<.

(d) The transition elements in the 7 $ and 7 ' oxidation states mostly form ionic bonds. n

compounds of the higher oxidation states (compounds formed with fluorine or oxygen),

the bonds are essentially covalent. For example, in permanganate ion MnO4 – , all bonds

formed between manganese and oxygen are covalent .

(e) ithin a group, the maximum oxidation state increases with atomic number. For example,ron shows the common oxidation state of 7 $ and 7 ', but ruthenium and osmium in the

same group form compounds in the 7 +, 7 = and 7 < oxidation states.

(f) Transition metals also form compounds in low oxidation states such as 7 ! and #. For

example, nickel in nickel tetracarbonyl, >i(5:)+ has ero oxidation state. Fe(5:)

The bonding in the compounds of transition metals in low oxidation states is not always very simple.

@6 !lectrode 2otentials ( ! ;)%tandard electrode potentials of half-cells involving 'd -series of transition elements are negative

except 5u.The negative values of ? for the first series of transition elements (except for 5u$735u)

indicate that0These metals should liberate hydrogen from dilute acids, ,

M + G M+ + # )

+M = G +M5 5+ # )i.e., the reactions are favourable in the forward direction. n actual practice however, most of these

metals react with dilute acids very slowly. %ome of these metals get coated with a thin protective layer

of oxide. %uch an oxide layer prevents the metal to react further.

These metals should act as good reducing agents. There is no regular trend in the ? values. This

is due to irregular variation in the ionisation and sublimation energies across the series. ;elative

stabilities of transition metal ions in different oxidation states in a*ueous medium can be predicted from

the electrode potential data. To illustrate this, let us consider the following0

Ms) G M # ) H ( Ent#l. of $"Dl%#ton0 H$"D

M # ) G M # ) e ' H + Ion$#ton ener!.0 +!



M # ) G Ma*) H 5 Ent#l. of .dr#ton0 H.d

&dding these e*uations one gets,

Ms) G M a*) e ' H H ( H + H 5 H$"D +! H.d

The / H represents the enthalpy change re*uired to bring the solid metal 4 to the monovalent

ion in a*ueous medium, 47(a!).

The reaction, 4( s) @ 47(a!) 7 e ", will be favourable only if / H is negative. 4ore negative isthe value of / H , more favourable will be the formation of that cation from the metal. Thus, the

oxidation state for which / H val"e is more negative will be more stable in the sol"tion.

8lectrode potential for a 4n734 half-cell is a measure of the tendency for the reaction,

Mna*) n e ' G Ms)

Thus, this reduction reaction will take place if the electrode potential for 4n734 half-cell is

positive. The reverse reaction,

Ms) G Mna*) n e '

involving the formation of 4n7(a!) will occur if the electrode potential is negative, i.e., the tendency for

the formation of M n7(a!) from the metal M will be more if the corresponding # val"e is more negative .

n other words, te od#ton $t#te for 4c ! J #l"e $ %ore ne!#te or le$$ o$te) 4ll De

%ore $t#Dle n te $ol"ton6

When an element exists in more than one oxidation states, the standard electrode potential ( E °) values can be

used in predicting the relative stabilities of different oxidation states in aqueous solutions. The following rule isfound useful.

The oxidation state of a cation for which ΔH(= ΔsubH + IE + ΔhydH) or E ° is more negative (or lesspositive) will be more stable.

Trends in the < = < 7tandard !lectrode 2otentials The observed values of 8o of the solid metal atoms 4 to 47$ ions in solution and their

standard electrode potentials compared in Fig.The uni*ue behaviour of 5u, having a positive 8 o, accounts for its inability to liberate 6$ from

acids. :nly oxidising acids (nitric and hot

concentrated sulphuric) react with 5u, the acids

being reduced. The high energy to transform

5u(s) to 5u7$(a*) is not balanced by its

hydration enthalpy. The general trend towards

less negative 8o values across the series is

related to the general increase in the sum of the

first and second ionisation enthalpies. t is

interesting to note that the value of 8o for 4n,

>i and An are more negative than expected from

the trend.

The stability of the half-filled d sub-shell in

4n7$ and the complete filled d!# configuration in

An7$ are related to their 8? values, where 8o for

>i is related to the highest negative

Ques:-/hy is "r < reducin# and n<, o6idi0in# when both have d confi#uration'

Ques:-/hich is a stron#er reducin# a#ent "r < or >e < and why'

?6 >ormation of "oloured +ons7 - 4ost of the compounds of the transition elements arecoloured in the solid state and3or in the solution phase. The compounds of transition metals are coloured

due to the presence of unpaired electrons in their d -orbitals. This occurs as follows.



EXPLANATION6 n an isolated atom or ion of a

transition element, all the five d -orbitals are of the same

energy (they are said to be de#enerate). Bnder the

influence of the combining anion(s), or electron-rich

molecules, the five d -orbitals split into two (or some time

more than two) groups of different energies i.e. t$g and

eg-orbitals. The difference between the two energy levelsdepends upon the nature of the combining ions. 2enerally

this difference corresponds to the energy of the visible

region, (? @ ,A9 8 4B9 nm).

Typical splitting for octahedral and tetrahedral

geometries are shown in Fig. C.+.

$elationship between the colour of the absorbed radiation and that of the transmitted li#ht is #iven in

Table C..

(*6 a#netic 2roperties: - 4ost of the transition elements and their compounds show

parama#netism6 The paramagnetism first increases in any transition element series, and then decreases.

The maximum paramagnetism is seen around the middle of the series. The paramagnetism is described

in ohr a#neton (4) units. The paramagnetic moments of some common ions of first transitionseries are given below in Table C. on the next page.

EXPLANATION7 & substance which is attracted by magnetic field is called paramagnetic

s"bstance. The substances which are repelled by magnetic field are called diamagnetic s"bstances.

Daramagnetism is due to the presence of unpaired electrons in atoms, ions or molecules.

The magnetic moment of any transition element or its compound3ion is given by (assuming no

contribution from the orbital magnetic moment),

where, $ is the total spin (n E s) 0 n is the number of unpaired electrons and s is e*ual to !3$

(representing the spin of an unpaired electron).

((6 >ormation of "omple6 +onsTransition metals and their ions show strong tendency for complex formation. The cations of

transition elements (d -block elements) form complex ions with certain molecules containing one or

more lone-pairs of electrons, vi%., 5:, >:, >6' etc., or with anions such as, F " , 5l " , 5> " etc. & few

typical complex ions are,

8FeCN)=9:' 0 8C"N5):9+0 8;+O)=9+0 8NCO):90 8CoN5)=950 8FeF=95'

EXPLANATION6 This complex formation tendency is due to,

(a) %mall sie 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.

"olour of the "olour of the

absorbed li#ht transmitted li#ht absorbed li#ht transmitted li#ht ; hite green ;ed

;ed lue-green lue :range:range lue ndigo ellow

ellow ndigo Giolet ellow-green

ellow-green Giolet BG hite

2reen Durple



(+6 >ormation of +nterstitial "ompoundsTransition 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 non-

stoichiometric 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 2roperties4ost 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 >ormationTransition 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.

(<6 "hemical $eactivityThe d -block elements (transition elements) have lesser tendency to react, i.e., these are less

reactive as compared to s-block elements.EXPLANATION6 ow reactivity of transition elements is due to,

(i) their high ionisation energies,

(ii) low heats of hydration of their ions,

(iii) Their high heats of sublimation.

3ener#l Ce%c#l Proerte$ of Fr$t Ro4 Tr#n$ton Met#l Co%o"nd$The transition metals form a number of binary compounds with non-metals, e.g., carbon,

nitrogen, phosphorus, oxygen, sulphur and halogens. The chemical reactivity of transition elements may

be seen through the study of their oxides, sulphides and halides.

Ode$ of Fr$t Ro4 Tr#n$ton Ele%ent$Transition metals of first row ('d -series) generally react with oxygen at higher temperatures.

ecause of the tendency to exhibit variable oxidation states, these metals form a number of oxides of

different varieties.



The purple solution containing J4n:+ is evaporated under controlled conditions to get

crystalline sample of potassium permanganate.

P.$c#l roerte$6(i) J4n:+ crystallies as dark purple crystals with greenish luster (m.p. $' J).

(ii) t is soluble in water to an extent of =. g per !## g at room temperature. The

a*ueous solution of J4n:+ has a purple colour.

Ce%c#l roerte$6 %ome important chemical reactions of J4n:+ are given below0

(i) Acton of e#t6 J4n:+ is stable at room temperature, but decomposes to give oxygen at

higher temperature

(ii) Od$n! #cton6 J4n:+ is a powerful oxidising agent in neutral, acidic and alkaline media.

The nature of reaction is different in each medium. The oxidising character of J4n:+ (to be

more specific, of 4n:+ " ) is indicated by high positive reduction potentials for the following

reactions.

There are a large number of oxidation-reduction reactions involved in the chemistry of

manganese compounds. %ome typical reactions are

(a) n the presence of excess of reducing agent in acidic solutions permanganate ion gets

reduced to manganous ion, e.g.,

(b) &n excess of reducing agent in an alkaline solution reduces permanganate ion only to

manganese dioxide, e.g.,

(c) n faintly acidic and neutral solutions, manganous ion is oxidised to manganese dioxide

by permanganate.

(d ) n strongly basic solutions, permanganate oxidises manganese dioxide to manganate ion.

(e) n acidic medium, J4n:+ oxidises,

(i) Ferro"$ $#lt$ to ferrc $#lt$

This reaction forms the basis of volumetric estimation of Fe $7 in any solution by J4n:+.

(ii) O#lc #cd to c#rDon dode



(iii) S"lte$ to $"l#te$

(iv) Iodde$ to odne n #cdc %ed"%

Pot#$$"% cro%#te K +Cr+O>)

Dotassium dichromate is one of the most important compound of chromium, and also amongdichromates. n this compound 5r is in the hexavalent (7 =) state.

Pre#r#ton6 t can be prepared by any of the following methods0

(i) >rom potassium chromate7 Dotassium dichromate can be obtained by adding a calculated

amount of sulphuric acid to a saturated solution of potassium chromate.

J $5r $:9 crystals can be obtained by concentrating the solution and crystallisation.

(ii) anufacture from chromite ore7 J$5r$:9 is generally manufactured from chromite ore

(Fe5r$:+). The process involves the following steps.

(a) +reparation of sodi"m chromate. Finely powdered chromite ore is mixed with soda ash and

*uicklime. The mixture is then roasted in a reverberatory furnace in the presence of air. ellow

mass due to the formation of sodium chromate is obtained.

(b) onversion of chromate into dichromate. %odium chromate solution obtained in step (a) is

treated with concentrated sulphuric acid when it is converted into sodium dichromate.

:n concentration, the less soluble sodium sulphate, >a$%:+.!#6$: crystallies out. This isfiltered hot and allowed to cool when sodium dichromate, >a$5r $:9.$6$:, separates out on standing.



3ener#l C#r#cter$tc$ of L#nt#nde$

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 eDt : od#ton $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+f 9

) E" #nd ;D eDt + od#ton $t#te$6

8uropium and yetterbium get f 9 and f !+ configuration in 7$ oxidation state, as shown below0

8u$7 0 KLeM+f 9

b$7 0 KLeM+f !+

) L#0 3d0 #nd L" eDt onl. 5 od#ton $t#te$ d"e to e%t.0 #lf flled #nd f"lflled

:f-$"D orDt6

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< is stron# o6idi0in# a#ent

and it chan#es to <, state. +t is because the ! o value for "e<D "e<, is <1.4 E which su##ests that it

can o6idise water. ut its reaction rate is very slow so "e< is #ood analytical rea#ent .



7imilarly 8u< is stable with half filled f 4 confi#uration but 8u< is stron# reducin# a#ent and

it chan#es to <, state. +t is because the ! o value for 8u<, D 8u< is ne#ative.

(,) a#netic properties6 a'7 and u'7 are diamagnetic, while the trivalent ions of the rest of

the lanthanides are paramagnetic in nature. The paramagnetic moment values of the lanthanide ions are

higher than those expected on the basis of the number of unpaired electrons. This occurs due to an

appreciable contribution from orbital angular momentum.

() $eduction potentials and metallic character 6 The standard electrode (reduction)

potentials of the lanthanide ions become less negative across the series. Thus, their reducing power

decreases in going from 5e to u. The highly negative ? values indicate these elements to be highly

electropositive metals capable of displacing hydrogen from water.

The 4(:6)' are ionic and basic in character. These hydroxides are stronger than &l(:6) ' and

weaker than 5a(:6)$. Te D#$c $tren!t decre#$e$ n !on! fro% L# to L"6

() tomic and ionic si0e: Fantha-nide contraction6 The atomic and ionic sies

decrease steadily in going from 5e to u. This decrease can be explained as follows.8LD&>&T:>. n the atoms of lanthanides, the nuclear charge increases with

atomic number, and the added electrons go to the inner + f orbitals. The shielding effect of + f electrons

from the increased nuclear charge, is poor. Thus, as the atomic number increases, the effective nuclear

charge experienced by each + f electron increases. This causes a slight reduction in the entire + f shell.

The successive contractions accumulate and the total effect for all the lanthanides is called l#nt#nde

contr#cton6

The variation of ionic radii of lanthanide ions is shown in Fig. C.!=.

The + f electrons also shield the valence shell from contracting appreciably. n lanthanides, the

decrease of radius for fourteen elements (5e to u) is ! pm.

This may be compared with the second period decrease of <!

pm in the radii for 9 elements (i to F) and with that of the

third period elements (>a to 5l), <= pm. Con$e"ence$ of

l#nt#nde contr#cton6 The lanthanide contraction has a

highly significant effect on the relative properties of the

elements which precede and follow lanthanides in the

periodic table. %ome important conse*uences of lanthanide

contraction are0

(i) 0he radi"s of 1a23 ion, for example, is pm larger

than that of 23 ion which lies immediately above it in

the periodic table. On this basis, if the fo"rteen

lanthanides had not intervened, the radi"s of Hf43

sho"ld have been greater than that of 5r 43 (which

lies immediately above it) by abo"t 6 pm. &"t, the

lanthanide contraction of abo"t the same magnit"de

almost cancels the expected increase. 7s a res"lt, Hf 43 and 5r 43 have almost e!"al radii, being 86 and 89

pm respectively.

-t is seen that the normal increase in si%e from $c : : 1a disappears after the

lanthanides and the pairs of elements s"ch as, 5r – Hf, ;b – 0a, Mo – ', etc., have almost the

same si%e. 0he properties of these elements are also very similar. -t is th"s a direct conse!"ence

of lanthanide contraction that the elements of the second and third transition series resemble

each other m"ch more closely than do the elements of the first and second transition series.



(ii) <"e to lanthanide contraction, i.e., decrease of ionic si%e on moving from 1a23 to 1"23, the

covalent character in bonding increases in the direction 1a 23 : 1"23. 7s a res"lt, the basic

character of the lanthanide hydroxides (M(OH)2 ) decreases with increase in atomic n"mber.

0h"s, Fa(5), is the most basic& while Fu(5), is the least basic. This aspect has been

utili0ed in the separation of lanthanides from each other.

(B) >ormation of comple6 salts and ions. anthanide ions (4'7

) have high charge, butdue to their larger sie, these cannot polarie the neighbouring anion3molecule. &s a result, these

lanthanides do not show a strong tendency towards complex formation.

>) "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.

Actnode$

The fourteen elements (atomic number C#"!#') after actinium are called actinides. These are also called

second series of innertransition 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$tc$ of Actnde$

(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 #ctnde contr#cton6

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 solution6 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.

1 Basic Concepts d&F-block Class 12

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Transcript of 1 Basic Concepts d&F-block Class 12