1 basic concepts d&f-block class 12
TRANSCRIPT
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 1/16
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
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 2/16
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.
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 3/16
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'
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 4/16
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$#ton ener!.0 +!
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 5/16
M # ) G Ma*) H 5 Ent#l. of .dr#ton0 H.d
&dding these e*uations one gets,
Ms) G M a*) e ' H H ( H + H 5 H$"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.
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 6/16
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
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 7/16
(+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.
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 8/16
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 9/16
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
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 10/16
(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.
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 11/16
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 12/16
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 13/16
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 .
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 14/16
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 $c : : 1a disappears after the
lanthanides and the pairs of elements s"ch as, 5r – Hf, ;b – 0a, Mo – ', etc., have almost the
same si%e. 0he properties of these elements are also very similar. -t is th"s a direct conse!"ence
of lanthanide contraction that the elements of the second and third transition series resemble
each other m"ch more closely than do the elements of the first and second transition series.
8/20/2019 1 Basic Concepts d&F-block Class 12
http://slidepdf.com/reader/full/1-basic-concepts-df-block-class-12 15/16
(ii) <"e to lanthanide contraction, i.e., decrease of ionic si%e on moving from 1a23 to 1"23, the
covalent character in bonding increases in the direction 1a 23 : 1"23. 7s a res"lt, the basic
character of the lanthanide hydroxides (M(OH)2 ) decreases with increase in atomic n"mber.
0h"s, Fa(5), is the most basic& while Fu(5), is the least basic. This aspect has been
utili0ed in the separation of lanthanides from each other.
(B) >ormation of comple6 salts and ions. anthanide ions (4'7
) have high charge, butdue to their larger sie, these cannot polarie the neighbouring anion3molecule. &s a result, these
lanthanides do not show a strong tendency towards complex formation.
>) "olour of the salts and ions in solution 6 4ost of the lanthanide trivalent ions are
coloured in solid as well as in the solution phase. The ions containing x and (!+ " x) electrons show the
same colour. The colour of the salts or ions is due to the f " f transition of electrons.
Actnode$
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.