d and f Block Elements
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Introduction to d-Block Elements
The d-block elements are defined as the elements from periodic table in which, last electron enters into the ‘d-orbital’ of the penultimate shell i.e. (n-1)d orbital. Where ‘n’ is the last shell.
The d-block elements are also called transition elements, because their properties are intermediate between the properties of highly electropositive s-block elements and highly electronegative p-block elements. Transition elements have partly or incompletely filled (n-1)d orbital in their elementary states.
Position in Periodic Table of d-Block Elements
- d-block elements are placed between s-block elements and p-block elements. The elements are placed between group 3 to 12 belonging to the period 4 to7.
- There are four series of transition elements i.e. 3d,4d,5d and 6d series
- The 3d series starts from Sc(z=21) to Zn(z=30) and belongs to 4th period.
- The 4d series includes all the elements from Y(z=39) to Cd(z=48) and belongs to 5th period.
- The 5th series starts with element La(z=57) and then includes all the elements from Hf(z=72) to Hg(z=80) and belongs to 6th period.
Thus 6d series starts from Ac(z=89) and then includes all the elements from Rf (z=104) to Unb(z=112) which belong to 7th period.
Electronic Configuration d-Block Elements
General electronic configuration of four series of d-block elements are given below.
- 3d series:[Ar] 3d1-10 4S1-2
- 4d series :[Kr] 4d1-10 5S0-2
- 5d series :[Xe] 5d1-10 6S2
- 6d series :[Rn] 6d1-10 7S2
Abnormal Electronic Configuration of Chromium And Copper among d Block Elements
- Chromium(24Cr) has electronic configuration
(Expected):[Ar] 3d4 4S2
(Observed):[Ar] 3d5 4s1
- Copper (29Cu) has electronic configuration
(Expected):[Ar] 3d9 4s1
(Observed):[Ar] 3d10 4s1
Explanation: due to more stability of half –filled and completely filled orbital.
Occurrence of d Block Elements
The soft elements of d-block or transition occur as sulphide minerals. Hard and electropositive elements occur as oxide.
General Characteristics of d-Block Elements
All the transition elements are metal having high melting as well as boiling point. They are good conductors of electricity and heat. They form complexes and colored compound.
Due to unpaired electrons most of the elements are paramagnetic while some exhibit diamagnetic character. They are good catalyst and they form alloys with different metals.
General Trend in Properties Of First Row Elements of d Block
- The ionization enthalpies of transition elements are quite high and lie between those of s-block and p-block elements. This is because the nuclear charge and atomic radii of transition elements lie between those of s-block and p-block elements.
- If IE1, IE2 and IE3 are the first, second and third ionization enthalpies of transition elements then IE1 <IE2<IE3.
- The ionization enthalpies value of transition metal can be used to predict thermodynamic stability of their compound.
- All the transition metals have high thermal as well as electrical conductivity and having very high melting and boiling points compared to those of representative elements due to their closed-packed structure.
- On going across the period melting points first increase, attain maximum value and then steadily decreases as atomic number.
- The strength of metallic bond depends upon the number of unpaired electrons, it increases up to middle i.e., up to (n-1)d5 hence accordingly melting and boiling points also increases.
- After (n-1) d5 configuration, the electrons starts pairing ,hence metallic strength, melting and boiling points decreases with increase in atomic number.
- The transition metal exhibit variable oxidation states and form compounds showing more than one (variable) oxidation states because energies levels of ns and (n-1)d are nearly similar. The oxidation state of 3d series is from +2 to +7 (except Cr and Cu).
- The lowest oxidation states of these elements are +1 or +2 which is due to their 4s electrons.
- As the number of unpaired electrons in 3d orbital increases the number of oxidation state also increases form Sc to Mn and after that electrons starts pairing in 3d orbital hence oxidation states decreases form Fe to Zn.
- The highest oxidation state of transition elements is +8 which shown by Os and Ru.
Atomic and Ionic Radii
- Atomic radii of the elements of 3d series gradually decreases up to Cr and then remain almost same for a few more elements and then increases slightly towards the end of end of the series.
- In 3d series the decreases in atomic radii is small from Sc to Cr because increase in nuclear charge at the centre of atom and added electrons filled up in vacant penultimate d orbital, atomic
- radii from Cr to Cu are almost similar and then slightly increases to Zn
- Ionic radii also follow similar trend as observed in the atomic radii values.
- Most of the compounds of transition metals are coloured in their solid or solution form. Colour of the compound of the transition metals may be attributed to the presence of incomplete (n-1)d orbital and the number of unpaired electrons.
- Transition metal ions with no unpaired electrons are colourless because there is possibility of d-d transition. e.g. Sc+3(3d0), Ti+4(3d0), Cu+(3d10). Ions with unpaired electrons are coloured i.e. ions with electronic configuration 3d1 to 3d9 are coloured.
- In general, the colour of transition ions can be related to –
- Presence of unpaired d electrons.
- d-d transition.
- Nature of groups i.e. ligands linked to metal ions.
- Geometry of the complexes formed by the metal ion.
- Many transition metals are used as catalysts which influence the rate of chemical reaction.
- A catalytic substance is capable of forming an unstable intermediate compound which readily decomposes yielding the products and regenerating the catalyst.
A+B + C —–> [A-B-C] —–> A-B + C
(reactant) (catalyst) (intermediate) (product) (catalyst)
- The commonly used transition metals as a catalyst are Fe, Co, Pt, Cr, Mn etc.
- Mno2 acts as a catalyst for decomposition of KClo3 to O2.
- In manufacture of ammonia, Fe with Mo is used as catalyst.
- Nickel acts as a catalyst in hydrogenation of oils to fats.
- Due to presence of unpaired electrons in the (n-1)d orbital, most of the transition metal ions and their compounds are paramagnetic i.e. they are attracted by magnetic field.
- The transition element and their ions having all electrons paired are diamagnetic i.e. they are repelled by magnetic field.
- Metals like Fe, Co and Ni posses very high paramagnetic and acquire permanent magnetic moment hence, they are ferromagnetic.
- Alnico is an alloy of Al(12%), Ni(20%),Co(50%) and remaining Fe(18%). It is used to make permanent magnets.
- Bohr magneton (B.M) is unit of magnetic moment:
1B.M = eh/4πmec
- The effective magnetic moment µeff, of a paramagnetic substance is given by ‘spin only’ formula.
Where n is the number of unpaired electrons.
- The transition metals can form large number of alloys among themselves having high melting points.
- In the molten state, transition metals are miscible with one another, which forms solid alloy on cooling.
- Transition metals can form alloy with non transition metals such as brass (Cu-Zn) and bronze (Cu-Sn).
POTASSIUM DICHROMATE (K2Cr2O7)
Properties of Potassium Dichromate
Potassium Dichromate exists as orange red crystals and is soluble in water.
Action of Alkali
Alkali (KoH) reacts with potassium dichromate to give yellow coloured solution of potassium chromate.
K2Cr2O7 + 2KOH ——> 2K2CrO4 + H2O.
Potassium Dichromate is good oxidizing agent in acidic medium. Potassium dichromate (oxidation number of Cr = +6) is reduced to chromium sulphate (oxidation number of Cr = +3)
Cr2 O72- + 14H+ + 6e– ——> 2cr3+ + 7H2O.
By gaining six electrons dichromate ions acts as an oxidizing agent.
Structure of Chromate Ion and Dichromate Ion
Uses of Potassium Dichromate
- It is used as an oxidizing agent.
- Used in a dyeing.
- In manufacture of lead chromate and chrome alum.
- Used in the detect ion of chloride ion.
- In the tanning of leather.
- In manufacture of pigments and inks.
POTASSIUM PERMANGANATE (KMnO4)
Properties of KMnO4
- It is crystalline solid having deep purple colour. It is soluble in water at room temperature.
- When heated it decomposes giving oxygen at 473K.
2KMnO4 —–> K2MnO4 + MnO2 + O2
- At red heat, it further decomposes to K2MnO3 and oxygen.
- Heated solid KMnO4 gives KOH, MnO and water vapour in current of H2.
Structure of Manganate Ion and Permangate Ion
Uses of KMnO4
It is used as –
- An oxidizing agent.
- Baeyer’s reagent.
- For detecting halides in qualitative analysis.
Introduction to f-Block Elements
The elements in which differentiating electrons (last electrons) enters into pre-penultimate shell i.e. (n-2)f orbital are known as f block elements. f block elements are also known as inner transition elements.
There are two series of inner transition elements i.e. lanthanoid series (4f block elements) and actinoid series (5f block elements).
- This includes all the elements from cerium (z=58) to lutetium (z=71) which are group of 14 elements with differentiating electron occupying 4f sub shell.
- The name lanthanoid has been derived from lanthanum which is prototype of lanthanoid.
- Lanthanoid series are also called 4f series because last electron enters pre-penultimate in 4f orbital.
Position of Lanthanoids
- Lanthanoids belongs to group 3 of periodic table and are placed in the sixth period.
- Lanthanoids are shown at the bottom of the periodic table because it interrupts third transition series of d-block elements.
- Actual position of lanthanoids is in between (z=57) and hafnium (z=72).
Electronic Configuration of Lanthanoids
- The general electronic configuration of 4f block elements is
[xe] 4f1-14 5d0-1 6s2
- Elements La, Gd, Lu posses single electrons in 5d sub shell. In case of other lanthanoids 5d orbital is empty.
- Due to empty, half filled and completely filled orbital f0, f7 and f14 electronic configuration elements achieve extra stability.
Oxidation States of Lanthanoids
- The common oxidation state of all lanthanoids is +3. It is characteristics of series.
- Lanthanum, gadolinium and lutetium shows only +3 oxidation state by losing two 6s and one 5d electrons giving stable outer electronic configuration 4f0,4f7 and 4f14 respectively.
- In +4 oxidation state cerium, terbium attains 4f0 and 4f7 respectively.
- Europium (Eu) as well as ytterbium (Yb) attains 4f7 and 4f14 respectively in +2 oxidation state.
Chemical Reactivity of Lanthanoids
- Earlier members of lanthanoids are quite reactive, behaves like calcium but as atomic number increases, they behaves more like aluminium.
- Lanthanoids forms carbides, hydrides, oxides, nitrides, halides, etc.
Ln + C —-> Lanthanoid Carbide
2Ln + 3H2 —-> 2LnH3
2Ln + 3O2 —-> 2Ln2O3
2Ln + N2 —-> 2LnN
2Ln + 6HCl —-> 2LnCl3 + 3H2
2Ln + 3S —–> Ln2S3
The gradual decrease in atomic and ionic size of lanthanoids with increases in atomic number is called lanthanoid contraction. You read these notes under 12 class chemistry notes of chapter d and f block elements.
Causes of Lanthanoid Contraction
- Increases in atomic number of member of lanthanoids series, the positive charge on nucleus increases by +1 unit and one more electron enters in the pre-penultimate 4f sub shell. There is imperfect shielding of one electron by another electron in the same 4f sub shell.
- With increase in nuclear charge the valence shell is pulled slightly towards nucleus. As a result of the pull, the size of M3+ ions goes on decreasing with increasing in atomic number.
- In complete lanthanoid series atomic radii and ionic radii decreases with 10 pm and 21 pm respectively. Atomic radii shows some irregularities but ionic radii decreases steadily.
Effects of Lanthanoid Contraction
- Decrease in Basicity
Due to lanthanoid contraction the size of the tri-positive lanthanoid ion (M3+) regularly decreases with increase in atomic number i.e. from La3+ to La3+. It results into decrease of basic from La3+ to Lu3+.
- Ionic Radii of the Post Lanthanoids
The elements which follow the lanthanoids in the third transition series are known as post lanthanoid. As a result of lanthanoid contraction the atomic radii (size) of the elements which follow lanthanum (Hf, Ta, W, etc) are similar to that of the elements of 4d series. Since Zr-Hf, Nb-Ta, Mo-W and Tc-Re have almost identical sizes, similar number of valence electrons and similar properties these pairs of elements are called chemical twins.
- The series of fourteen elements 90Th to 103Lr which follows actinium (89Ac) and in which last electrons are progressively filled in 5f orbital in pre-penultimate shell are called actinoid series or 5f series.
- The elements which are synthetically or artificially prepared by man having atomic number higher than uranium (z=92) are called as trans-uranic elements (Np-93 to Uno-118).
Position of Actinoids
- The actinoids belongs to third group of the periodic table in the seventh period.
- Actinoids are placed at the bottom of the periodic table below the lanthanoid series because it interrupts fourth transition series of d-block elements.
- The general electronic configuration of 5f elements is represented as 5f1-14 6d0-1 7s2.
- The electronic configuration of actinoids is not definite.
- Actinium and thorium does not contain any 4f electron.
Oxidation States of Actinoids
- Actinoid show varied oxidation states like lanthanoids. The common oxidation states of actinoids is 3+. Ac, Th and Am shows +2 oxidation states. Th, Pa, U, Np, Pu, Am and Cm shows +4 oxidation states.
- +5 oxidation state is shown by element Th to Am. U, Np, Pu and Am show oxidation state +6.The highest oxidation state is +7 shown by Np and Pu.
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