Newton regarded light as a stream of particles called corpuscles
of light
CLASS XII d&f-BLOCK ELEMENTS
BASIC 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.
Electronic Configuration of Transition ElementsFrom 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 1)d110 ns12, where n
is the outermost shell, and (n 1) stands for the penultimate
shell.
Ques:-Why are Zinc, Cadmium and Mercury not considered as the
Transition Elements?
Ans: - In zinc cadmium and mercury the last electron enters in
s-orbital not in the (n-1) d-orbital, so these elements are not
called transition elements. Their electronic configurations are (n
1) d10 ns2. Since, 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.General Trends in the
Chemistry of First Row Transition Elements (3d-series)1. Electronic
ConfigurationAll d-block elements exhibit 3d110 4s12 electronic
configuration. Some characteristic features of the electronic
configurations of the transition elements are, Atoms of all
transition elements consist of an inner core of electrons having
noble gas configuration. For example,
Sc : [Ar] 3d1 4s2 Y : [Kr] 4d1 5s2 La : [Xe] 5d1 6s2The
half-filled and completely-filled d-orbitals gain extra-stability.
So, such con-figurations are favoured wherever possible. For
example
2. Atomic RadiiThe atomic radii of 3d-series of elements are
compared with those of the neighbouring s- and p-block
elements.
164 147 135 129 137 126 125 125 128 137 in pm
The atomic radii of transition elements show the following
characteristics.
Ques.:-The atomic radii and atomic volumes of d-block elements
in any series decrease with increase in the atomic number. The
decrease however, is not regular. The atomic radii tend to reach
minimum near at the middle of the series, and increase slightly
towards the end of the series, why?
Ans: - When 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. At 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. In the middle of the series, the atomic radii tend to
have a minimum value as observed Later 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 going down in each group.
However, in the third transition series (5d series) from hafnium
(Hf) and onwards, the elements have atomic radii nearly equal to
those of the second transition series elements, why?Ans: - 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. A nearly equal radius of second (4-d series) and third
transition series (5d series) elements is due to a special effect
called lanthanide contraction. In the 5d- series of transitions
elements, after lanthanum (La), the added 14 electrons go to the
inner most 4f orbitals (antepenultimate orbitals). The 4f electrons
have poor shielding effect. But due to addition of 14 extra protons
in the nucleus the outermost electrons experience greater nuclear
attraction. So size of elements of 5-d series becomes smaller then
4-d series.3. Ionic RadiiFor ions having identical charges, the
ionic radii decrease slowly with the increase in the atomic number
across a given series of the transition elements.
EXPLANATION. 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.
4. Ionisation EnergiesThe ionisation energies (now called
ionisation enthalpies, IH) of the elements of first transition
series are given below:
The following generalizations can be obtained from the
ionisation energy values given above.Ques.:-The ionisation energies
of these elements are high, and in most cases lie between those of
s- and p-block elements. This indicates that the transition
elements are less electropositive than s-block elements.Ans: -
Transition metals have smaller atomic radii and higher nuclear
charge as compared to the alkali metals. Both these factors tend to
increase the ionisation energy, as observed. The ionisation energy
in any transition series increases with atomic number; the increase
however is not smooth and as sharp as seen in the case of s- and
p-block elements.
EXPLANATION. The ionisation energy increases due to the increase
in the nuclear charge with atomic number at the beginning of the
series. Gradually, 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 energies of 5d-series of elements
are much higher than those of the 3d- and 4d-series elements,
why?.
Ans: - In the 5d- series of transitions elements, after
lanthanum (La), the added 14 electrons go to the inner most 4f
orbitals (antepenultimate orbitals). The 4f electrons have poor
shielding effect. But due to addition of 14 extra protons in the
nucleus the outermost electrons experience greater nuclear
attraction. So size of elements of 5-d series becomes smaller then
4-d series. This leads to higher ionisation energies for the
5d-series of transition elements.5. Metallic CharacterAll
transition elements are metals. These are hard, and good conductor
of heat and electricity. All 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.EXPLANATION. The
ionisation energies of the transition elements are not very high.
The outermost shell in their atoms have many vacant/partially
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. Metallic 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. Cr, Mo and W have maximum number of unpaired electrons
so these metals are very hard.Ques.:- Why is the energy of
atomization is very high for d- block elements?6. Melting and
Boiling PointsThe melting and boiling points of transition elements
except Cd and Hg 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.
EXPLANATION. Atoms 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 unpaired electrons stronger is the
metallic bonding. In any transition element series, the number of
unpaired electrons first increases from 1 to 5 and then decreases
back to zero. The maximum five unpaired electrons occur at Cr (3d
series). As a result, the melting and boiling points first increase
and then decrease showing maxima around the middle of the
series.
The low melting points of Zn, Cd, and Hg may be due to the
absence of unpaired d-electrons in their atoms. 7. Oxidation
States
Most of the transition elements exhibit several oxidation
states, i.e., they show variable valency in their compounds. Some
common oxidation states of the first transition series elements are
given below .Ques.:- Why do d-block elements show variable
oxidation states?
Ans.:- The outermost electronic configuration of the transition
elements is (n 1) d110 ns2. The energy of (n 1) d and ns- orbitals
are nearly same, so along with the ns-electrons (n 1) d-electrons
also involved in oxidation state so these elements shows variable
oxidation states. Also 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, Sc having a configuration 3d1 4s2 may show an oxidation
state of + 2 (only s-electrons are lost) and + 3 (when d-electron
is also lost). The highest oxidation state which an element of this
group might show is given by the total number of ns- and (n 1)
d-electrons.
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:-Why highest oxidation states are shown by oxide and
fluorides?The highest oxidation states are found in fluorides and
oxides because fluorine and oxygen are the most electronegative
elements.The highest oxidation state shown by any transition metal
is eight. The oxidation state of eight is shown by Ru and Os. An
examination of the common oxidation states reveals the following
conclusions:(a) The variable oxidation states shown by the
transition elements are due to the participation of outer ns- and
inner (n 1) d-electrons in bonding.
(b) Except scandium, the most common oxidation state shown by
the elements of first transition series is + 2. This oxidation
state arises from the loss of two 4s electrons. This means that
after scandium, d-orbitals become more stable than the
s-orbital.
(c) The greatest number of oxidation states is observed near
middle of the series. Eg:- Mn show +2 to +7 O.S. The highest
oxidation states are observed in fluorides and oxides. The highest
oxidation state shown by any transition element (by Ru and Os) is
+8.
(d) The transition elements in the + 2 and + 3 oxidation states
mostly form ionic bonds. In 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) Within a
group, the maximum oxidation state increases with atomic number.
For example, Iron shows the common oxidation state of + 2 and + 3,
but ruthenium and osmium in the same group form compounds in the +
4, + 6 and + 8 oxidation states.
(f) Transition metals also form compounds in low oxidation
states such as + 1 and 0. For example, nickel in nickel
tetracarbonyl, Ni(CO)4 has zero oxidation state. Fe(CO)5The bonding
in the compounds of transition metals in low oxidation states is
not always very simple.
8. Electrode Potentials (E)Standard electrode potentials of
half-cells involving 3d-series of transition elements are negative
except Cu.The negative values of E for the first series of
transition elements (except for Cu2+/Cu) indicate that:
These metals should liberate hydrogen from dilute acids, ,
M + 2H+ M2+ + H2(g)
2M + 6H+ 2M3+ + 3H2(g)
i.e., the reactions are favourable in the forward direction. In
actual practice however, most of these metals react with dilute
acids very slowly. Some of these metals get coated with a thin
protective layer of oxide. Such an oxide layer prevents the metal
to react further.
These metals should act as good reducing agents. There is no
regular trend in the E values. This is due to irregular variation
in the ionisation and sublimation energies across the series.
Relative stabilities of transition metal ions in different
oxidation states in aqueous medium can be predicted from the
electrode potential data. To illustrate this, let us consider the
following:
M(s) M(g)
H1 =Enthalpy of sublimation, subH
M(g) M+(g) + e H2 = Ionisation energy, IE
M+(g) M+(aq) H3 = Enthalpy of hydration, hydH
Adding these equations one gets,
M(s) M+ (aq) + e H = H1 + H2 + H3 = subH + IE + hydH
The H represents the enthalpy change required to bring the solid
metal M to the monovalent ion in aqueous medium, M+(aq).
The reaction, M(s) M+(aq) + e, will be favourable only if H is
negative. More negative is the value of H, more favourable will be
the formation of that cation from the metal. Thus, the oxidation
state for which H value is more negative will be more stable in the
solution.
Electrode potential for a Mn+/M half-cell is a measure of the
tendency for the reaction,
Mn+(aq) + n e M(s)
Thus, this reduction reaction will take place if the electrode
potential for Mn+/M half-cell is positive. The reverse
reaction,
M(s)
Mn+(aq) + n einvolving the formation of Mn+(aq) will occur if
the electrode potential is negative, i.e., the tendency for the
formation of Mn+(aq) from the metal M will be more if the
corresponding E value is more negative. In other words, the
oxidation state for which E value is more negative (or less
positive) will be more stable in the solution. 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 is found useful.The oxidation state of a cation for
which H(= subH + IE + hydH) or E is more negative (or less
positive) will be more stable.
Trends in the M+2 / M+ Standard Electrode Potentials The
observed values of Eo of the solid metal atoms M to M+2 ions in
solution and their standard electrode potentials compared in
Fig.
The unique behaviour of Cu, having a positive Eo, accounts for
its inability to liberate H2 from acids. Only oxidising acids
(nitric and hot concentrated sulphuric) react with Cu, the acids
being reduced. The high energy to transform Cu(s) to Cu+2(aq) is
not balanced by its hydration enthalpy. The general trend towards
less negative Eo values across the series is related to the general
increase in the sum of the first and second ionisation enthalpies.
It is interesting to note that the value of Eo for Mn, Ni and Zn
are more negative than expected from the trend.The stability of the
half-filled d sub-shell in Mn+2 and the complete filled d10
configuration in Zn+2 are related to their E values, where Eo for
Ni is related to the highest negative
Ques:-Why is Cr+2 reducing and Mn+3 oxidizing when both have d4
configuration?Ques:-Which is a stronger reducing agent Cr+2 or Fe
+2 and why?
9. Formation of Coloured Ions: - Most of the compounds of the
transition elements are coloured in the solid state and/or 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.EXPLANATION. In an isolated atom or ion of a
transition element, all the five d-orbitals are of the same energy
(they are said to be degenerate). Under 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. t2g and eg-orbitals. The difference between the two
energy levels depends upon the nature of the combining ions.
Generally this difference corresponds to the energy of the visible
region, ( = 380 760 nm).Typical splitting for octahedral and
tetrahedral geometries are shown in Fig. 9.4. Relationship between
the colour of the absorbed radiation and that of the transmitted
light is given in Table 9.4.Colour of theColour of the
absorbed lighttransmitted lightabsorbed lighttransmitted
light
IRWhitegreenRed
RedBlue-greenBlueOrange
OrangeBlueIndigoYellow
YellowIndigoVioletYellow-green
Yellow-greenVioletUVWhite
GreenPurple
10. Magnetic Properties: - Most of the transition elements and
their compounds show paramagnetism. 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 Bohr Magneton (BM) units. The
paramagnetic moments of some common ions of first transition series
are given below in Table 9.5 on the next page.EXPLANATION: A
substance which is attracted by magnetic field is called
paramagnetic substance. The substances which are repelled by
magnetic field are called diamagnetic substances. Paramagnetism is
due to the presence of unpaired electrons in atoms, ions or
molecules.
The magnetic moment of any transition element or its
compound/ion is given by (assuming no contribution from the orbital
magnetic moment),
where, S is the total spin (n s) : n is the number of unpaired
electrons and s is equal to 1/2 (representing the spin of an
unpaired electron).
11. Formation of Complex IonsTransition 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,
viz., CO, NO, NH3 etc., or with anions such as, F, Cl, CN etc. A
few typical complex ions are,
[Fe(CN)6]4, [Cu(NH3)4]2+, [Y(H2O)6]2+, [Ni(CO)4], [Co(NH3)6]3+,
[FeF6]3EXPLANATION. This complex formation tendency is due to,
(a) Small size of the transition metal cations.(b) High positive
charge density(c) The availability of vacant inner d-orbitals of
suitable energy to accept lone pair of electrons.12. Formation of
Interstitial CompoundsTransition 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. Some 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.Steel and cast iron are hard due to
the formation of interstitial compound with carbon. Some
non-stoichiometric compounds are, VSe 0.98 (Vanadium selenide),
Fe0.94O, and titanium hydride TiH1.7.
Some properties1. Interstitial compounds are hard and dense.
This is because; the smaller atoms of lighter elements occupy the
interstices in the lattice, leading to a more closely packed
structure.2. Mp are higher and 3. They are chemically inert. Due to
greater electronic interactions, the strength of the metallic bonds
also increases.13. Catalytic PropertiesMost of the transition
metals and their compounds particularly oxides have good catalytic
properties. Platinum, iron, vanadium pentoxide, nickel, etc., are
important catalysts. Platinum is a general catalyst. Nickel powder
is a good catalyst for hydrogenation of unsaturated organic
compounds such as, hydrogenation of oils. Some typical industrial
catalysts are:(a) Vanadium pentoxide (V2O5) is used in the Contact
process for the manufacture of sulphuric acid,
(b) Finely divided iron is used in the Habers process for the
synthesis of ammonia.
EXPLANATION. Most 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.14. Alloy
FormationTransition metals form alloys among themselves. The alloys
of transition metals are hard and high melting as compared to the
host metal. Various steels are the alloys of iron with metals such
as chromium, vanadium, molybdenum, tungsten, manganese etc.
EXPLANATION. The atomic radii of the transition elements in any
series are not much different from each other. As a result, they
can very easily replace each other in the lattice and form solid
solutions over an appreciable composition range. Such solid
solutions are called alloys.15. Chemical ReactivityThe d-block
elements (transition elements) have lesser tendency to react, i.e.,
these are less reactive as compared to s-block elements.
EXPLANATION. Low 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.
General Chemical Properties of First Row Transition Metal
CompoundsThe 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.Oxides of First Row Transition Elements
Transition metals of first row (3d-series) generally react with
oxygen at higher temperatures. Because of the tendency to exhibit
variable oxidation states, these metals form a number of oxides of
different varieties.
Some general characteristics of the oxides of 3d-transition
series are given below.
(i) Formulae. The general formulae of the oxides of first row
transition metals (3d-series) are, MO, M2O3, M3O4, MO2, M2O5 and
MO3.
(ii) Acidic, basic or amphoteric character. The character of an
oxide depends upon the oxidation state of the metal in it. For
example,
The oxides of metals in low oxidation states are basic. For
example, TiO, VO, MnO, Cu2O etc., are basic in nature.
The oxides of metals in high oxidation states are acidic. For
example, V2O5, CrO3, Mn2O7 are acidic.
The oxides of metals in the intermediate oxidation states, are
generally amphoteric.
For example, CuO, Cr2O3, MnO2 etc., are amphoteric.
Important oxides of the first transition series elements are
given in Table 9.6.
SOME IMPORTANT COMPOUNDS OF TRANSITION ELEMENTSPotassium
Permanganate, (KMnO4)Potassium permanganate is a salt of an
unstable acid HMnO4 (permanganic acid). The Mn is in + 7 state in
this compound.
Preparation. Potassium permanganate is obtained from pyrolusite
as follows.
(i) Conversion of pyrolusite to potassium manganate. When
manganese dioxide is fused with potassium hydroxide in the presence
of air or an oxidising agent such as potassium nitrate or chlorate,
potassium manganate is formed, possibly via potassium
manganite.
(ii) Oxidation of potassium manganate to potassium permanganate.
The potassium manganate so obtained is oxidised to potassium
permanganate by either of the following methods.
(a) By chemical method: The fused dark-green mass is extracted
with a small quantity of water.
The filtrate is warmed and treated with a current of ozone,
chlorine or carbon dioxide. Potassium manganate gets oxidised to
potassium permanganate and the hydrated manganese dioxide
precipitates out. The reactions taking place are,
When CO2 is passed:
When chlorine or ozone is passed:
The purple solution so obtained is concentrated and dark purple,
needle-like crystals having metallic lustre are obtained.
(b) Electrolytic method: Presently, potassium manganate (K2MnO4)
is oxidised electrolytically.
The electrode reactions are,
The purple solution containing KMnO4 is evaporated under
controlled conditions to get crystalline sample of potassium
permanganate.Physical properties. (i) KMnO4 crystallizes as dark
purple crystals with greenish luster (m.p. 523 K).
(ii) It is soluble in water to an extent of 6.5 g per 100 g at
room temperature. The aqueous solution of KMnO4 has a purple
colour.
Chemical properties. Some important chemical reactions of KMnO4
are given below:
(i) Action of heat. KMnO4 is stable at room temperature, but
decomposes to give oxygen at higher temperature
(ii) Oxidising action. KMnO4 is a powerful oxidising agent in
neutral, acidic and alkaline media. The nature of reaction is
different in each medium. The oxidising character of KMnO4 (to be
more specific, of MnO4) 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. Some typical
reactions are
(a) In the presence of excess of reducing agent in acidic
solutions permanganate ion gets
reduced to manganous ion, e.g.,
(b) An excess of reducing agent in an alkaline solution reduces
permanganate ion only to manganese dioxide, e.g.,
(c) In faintly acidic and neutral solutions, manganous ion is
oxidised to manganese dioxide
by permanganate.
(d) In strongly basic solutions, permanganate oxidises manganese
dioxide to manganate ion.
(e) In acidic medium, KMnO4 oxidises,
(i) Ferrous salts to ferric salts
This reaction forms the basis of volumetric estimation of Fe2+
in any solution by KMnO4.
(ii) Oxalic acid to carbon dioxide
(iii) Sulphites to sulphates
(iv) Iodides to iodine in acidic medium
Potassium Dichromate (K2Cr2O7)Potassium dichromate is one of the
most important compound of chromium, and also among dichromates. In
this compound Cr is in the hexavalent (+ 6) state.
Preparation. It can be prepared by any of the following
methods:
(i) From potassium chromate: Potassium dichromate can be
obtained by adding a calculated amount of sulphuric acid to a
saturated solution of potassium chromate.
K2Cr2O7 crystals can be obtained by concentrating the solution
and crystallisation.
(ii) Manufacture from chromite ore: K2Cr2O7 is generally
manufactured from chromite ore (FeCr2O4). The process involves the
following steps.
(a) Preparation of sodium chromate. Finely powdered chromite ore
is mixed with soda ash and quicklime. The mixture is then roasted
in a reverberatory furnace in the presence of air. Yellow mass due
to the formation of sodium chromate is obtained.
(b) Conversion of chromate into dichromate. Sodium chromate
solution obtained in step (a) is treated with concentrated
sulphuric acid when it is converted into sodium dichromate.
On concentration, the less soluble sodium sulphate, Na2SO4.10H2O
crystallizes out. This is filtered hot and allowed to cool when
sodium dichromate, Na2Cr2O7.2H2O, separates out on standing.
(c) Conversion of sodium dichromate to potassium dichromate. Hot
concentrated solution of sodium dichromate is treated with a
calculated amount of potassium chloride, when potassium dichromate
being less soluble crystallizes out on cooling.
Chemical properties. (i) Action of alkalies. With alkalies, it
gives chromates. For example, with KOH,
On acidifying, the colour again changes to orange-red owing to
the formation of dichromate.
Actually, in dichromate solution, the Cr2O7 2 ions are in
equilibrium with CrO4 2 ions.
(iv) Oxidising nature. In neutral or in acidic solution,
potassium dichromate acts as an excellent oxidising agent, and
Cr2O7 2 gets reduced to Cr3+. The standard electrode potential for
the reaction,
is + 1.31 V. 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 quantitative estimation of the reducing agents such as,
Fe2+.
It oxidises,(a) Ferrous salts to ferric salts
Ionic equation:
(b) Sulphites to sulphates and arsenites to arsenates.
Ionic equation:
Similarly, arsenites are oxidised to arsenates.
(c) Hydrogen halides to halogens.
Ionic equation:
(d) Iodides to iodine
Ionic equation:
Thus, when KI is added to an acidified solution of K2Cr2O7
iodine gets liberated.(e) It oxidises H2S to S.
Ionic equation:
(i) Formation of insoluble chromates. With soluble salts of
lead, barium etc., potassium dichromate gives insoluble chromates.
Lead chromate is an important yellow pigment.
(ii) Chromyl chloride test. When potassium dichromate is heated
with conc. H2SO4 in the presence of a soluble chloride salt, the
orange-red vapour of chromyl chloride (CrO2Cl2) is formed.
Chromyl chloride vapour when passed through water give
yellow-coloured solution containing chromic acid.
Structure of Chromate and Dichromate Ions. f-block
elementsInner-Transition Elements: Lanthanoides and ActinoidsThe
elements which in their elemental or ionic form have partly filled
f-orbitals are called f-block elements. As 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 general electronic configuration is
(n-2)f1-14(n-1)d0 or 1ns2There are two series of
inner-transition elements, each having 14 elements. The elements in
which 4f orbitals are progressively filled are called lanthanides.
The elements in which 5f orbitals are progressively filled are
termed actinides. Lanthanides thus belong to the first
inner-transition series, while actinides belong to the second
inner-transition series.LanthanoidesThe fourteen elements (atomic
no. 58 71) after lanthanum are known as lanthanides or lanthanons.
All these elements closely resemble one another in their
properties. Because of their limited availability, these are also
known as the rare earth elements.
Names and the outer-electronic configurations of the lanthanides
are given in Table 9.10.
General Characteristics of Lanthanides
General physical characteristics of lanthanides are described
below:
(1) Electronic configuration. The outer-electronic
configurations of lanthanides are given in Table 11.9. There is
however, some uncertainty about the correctness of these
configurations. The 5d and 4f energy levels are very close-by. It
is not always possible to decide with certainty whether the
electron has entered 5d or 4f level. Due to the extra-stability of
half-filled and completely filled orbitals, there is a tendency to
acquire f7 and f14 configurations wherever possible. The general
electronic configuration of lanthanides may be described as 4f114
5d 01 6s2.
(2) Oxidation states. All Lanthanoides exhibit a common stable
oxidation state of +3. in addition some lanthaniodes shows +2 and
+4 oxidation state also. These are shown by those elements which by
doing so attain the stable f0, f7 and f14 configurations. For
example:
(i) Ce and Tb exhibit +4 oxidation states.
Cerium (Ce) and terbium (Tb) attain f0 and f7 configuration
respectively when they get +4 oxidation state, as shown below:
Ce4+ :[Xe]4f0
Tb4+:[Xe]4f7(ii) Eu and Yb exhibit +2 oxidation states.
Europium and yetterbium get f7 and f14 configuration in +2
oxidation state, as shown below:
Eu2+ :[Xe]4f7
Yb2+:[Xe]4f14(iii) La, Gd, and Lu exhibit only +3 oxidation
states due to empty, half filled and fulfilled 4f-sub orbit.The
stability of different oxidation state has strong effect on the
properties of those elements. For example, Ce(IV) is favoured
because of its noble gas configuration. But it is strong oxidant
changing to common +3 oxidation state.
Similarly, Eu2+ is stable because of its half filled 4f7
configuration. However, it is a strong reducing agent changing to
Eu3+ (common oxidation state.) Similarly, Yb2+ having the
configuration 4f14 is a reductant. Samarium also behaves like
europium exhibiting both +2 and +3 oxidation states.
Important note: - Irrespective of noble gas configuration 4f0
Ce+4 is strong oxidizing agent and it changes to +3 state. It is
because the Eo value for Ce+4| Ce+3 is +1.74 V which suggests that
it can oxidise water. But its reaction rate is very slow so Ce+4 is
good analytical reagent.Similarly Eu+2 is stable with half filled
4f7 configuration but Eu+2 is strong reducing agent and it changes
to +3 state. It is because the Eo value for Eu+3 | Eu+2 is
negative.(3) Magnetic properties. La3+ and Lu3+ 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.(4) Reduction potentials
and metallic character. The standard electrode (reduction)
potentials of the lanthanide ions become less negative across the
series. Thus, their reducing power decreases in going from Ce to
Lu. The highly negative E values indicate these elements to be
highly electropositive metals capable of displacing hydrogen from
water.
The M(OH)3 are ionic and basic in character. These hydroxides
are stronger than Al(OH)3 and weaker than Ca(OH)2. The basic
strength decreases in going from La to Lu.
(5) Atomic and ionic size: Lantha-nide contraction. The atomic
and ionic sizes decrease steadily in going from Ce to Lu. This
decrease can be explained as follows.
EXPLANATION. In the atoms of lanthanides, the nuclear charge
increases with
atomic number, and the added electrons go to the inner 4f
orbitals. The shielding effect of 4f electrons from the increased
nuclear charge, is poor. Thus, as the atomic number increases, the
effective nuclear charge experienced by each 4f electron increases.
This causes a slight reduction in the entire 4f shell. The
successive contractions accumulate and the total effect for all the
lanthanides is called lanthanide contraction.The variation of ionic
radii of lanthanide ions is shown in Fig. 9.16.The 4f electrons
also shield the valence shell from contracting appreciably. In
lanthanides, the decrease of radius for fourteen elements (Ce to
Lu) is 15 pm. This may be compared with the second period decrease
of 81 pm in the radii for 7 elements (Li to F) and with that of the
third period elements (Na to Cl), 86 pm. Consequences of lanthanide
contraction. The lanthanide contraction has a highly significant
effect on the relative properties of the elements which precede and
follow lanthanides in the periodic table. Some important
consequences of lanthanide contraction are:
(i) The radius of La3+ ion, for example, is 22 pm larger than
that of Y3+ ion which lies immediately above it in the periodic
table. On this basis, if the fourteen lanthanides had not
intervened, the radius of Hf 4+ should have been greater than that
of Zr 4+ (which lies immediately above it) by about 20 pm. But, the
lanthanide contraction of about the same magnitude almost cancels
the expected increase. As a result, Hf 4+ and Zr 4+ have almost
equal radii, being 80 and 81 pm respectively.
It is seen that the normal increase in size from Sc Y La
disappears after the lanthanides and the pairs of elements such as,
Zr Hf, Nb Ta, Mo W, etc., have almost the same size. The properties
of these elements are also very similar. It is thus a direct
consequence of lanthanide contraction that the elements of the
second and third transition series resemble each other much more
closely than do the elements of the first and second transition
series.
(ii) Due to lanthanide contraction, i.e., decrease of ionic size
on moving from La3+ to Lu3+, the covalent character in bonding
increases in the direction La3+ Lu3+. As a result, the basic
character of the lanthanide hydroxides (M(OH)3) decreases with
increase in atomic number.
Thus, La(OH)3 is the most basic, while Lu(OH)3 is the least
basic. This aspect has been utilized in the separation of
lanthanides from each other.
(6) Formation of complex salts and ions. Lanthanide ions (M3+)
have high charge, but due to their larger size, these cannot
polarize the neighbouring anion/molecule. As a result, these
lanthanides do not show a strong tendency towards complex
formation.
(7) Colour of the salts and ions in solution. Most of the
lanthanide trivalent ions are coloured in solid as well as in the
solution phase. The ions containing x and (14 x) electrons show the
same colour. The colour of the salts or ions is due to the f f
transition of electrons.
ActinoidesThe fourteen elements (atomic number 90103) after
actinium are called actinides. These are also called second series
of inner-transition elements. The general electronic configuration
of actinides is 5f 114 6d 01 7s2. Names and the outer-electronic
configurations of actinides are given below in Table 9.11.
General Characteristics of Actinides(1) Oxidation states. The
oxidation states commonly exhibited by actinides are given in Table
9.12. The most stable state is indicated by bold letter. The + 3
state becomes more stable as the atomic number increases.(2) Atomic
and Ionic radii. The radii for tripositive (M3+) and tetrapositive
(M4+) ions decrease in going from Th to Cm. This steady decrease is
similar to that observed in lanthanides and is called actinide
contraction. Fact: The actinide contraction is larger than
lanthanide contraction.Reason: because in lanthanoids electrons are
filled in 4f orbital whose screening effect is more stronger than
5f orbitals of actinoid elements.
(3) Colour of salts and ions in solution. Most of the salts of
actinides having M3+ or M4+ ions are coloured. Ions having 5f , 5f
1 and 5f 7 configurations are colourless, while those containing 5f
2, 5f 3, 5f 4, 5f 5 and 5f 6 configurations are coloured.
Uses of Lanthanides Due to their alloy-forming tendencies,
actinides and lanthanides form many alloys particularly with iron.
These elements improve the workability of steel. A well known alloy
is misch-metal which consists of a rare earth element (94 95%),
iron (up to 5%) 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
composition: cerium 40.5%, lanthanum + neodymium 44%, iron 4.5%,
aluminium 0.5% and the remainder calcium, silicon and carbon. (i)
Cerium constitute about 30-50% of the alloys of lanthanides. They
are used fro scavenging oxygen 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) Uranium and Plutonium are used for production of nuclear
energy by the nuclear fission.