Chemistry of Elements of First Transition Series
There are four types of orbital i.e. s, p, d and f. On the basis of electronic configuration-
· s-block elements : last electron goes in s orbital.
· p-block elements : last electron goes in p orbital.
· d-block elements : last electron goes in d orbital.
· f-block elements : last electron goes in f orbital.
Transition Elements: -The elements of d-block which act as bridge or transit point between s and p-block elements is known as transition elements.
d-block elements have four series :
1. First transition series :-
This series contains elements from atomic number 21 (scandium) to atomic number 30 (Zinc). It is called 3d series because last electron goes in 3d orbital. And it is present in fourth period of periodic table.
2. Second transition series :-
This series contains elements from atomic number 39 (Yttrium) to atomic number 48 (cadmium). It is called 4d series because last electron goes in 4d orbital. And it is present in fifth period of periodic table.
3. Third transition series :-
This series contains elements 57 (Lanthanum) and from atomic number 72 (Hafnium) to atomic number 80 (Mercury). It is called 5d series because last electron goes in 5d orbital. And it is present in sixth period of periodic table.
4. Fourth transition series :-
This series contains Actinium (89) and element with atomic number 104 (Rutherfordium) and all above series elements. It is called 6d series. And it is present in sixth period of periodic table. All the elements other then Actinium in this series are synthesised.
Characteristic Properties of d-block elements :Main characteristics of d-block elements are as follow:
Physical state and metallic properties :
All d-block elements are solid except Mercury. Mercury is present in liquid state. Atoms of d-block elements have maximum 2 electrons in outermost shell so they show metallic character. Unlike s-block elements these are rigid, malleable and ductile. D-block elements are good conductor of heat and electricity. and have metallic lustre.
Melting point and Boiling point :
These have high Melting and high Boiling point due to stron bond between elements. Zn, Cd and Hg have low Melting and low Boiling point due to completed sub-orbit.
Atomic radius :
In periods of d-block elements, atomic radius generally decreases with increase in atomic no. It is due to increase in nuclear charge of atoms in a period.
Ionic radius :
d-block elements form cations which are smaller than its corresponding atoms. Ionic radius generally decreases across a period with increase in atomic no.
Atomic volume :
Atomic volume of the d-block elements is low as compared to near s and p block elements. Atomic volume decreases with increase in atomic number in a period, but after acquiring a minimum volume it increases due to increased screening effect.
In a period of d block elements gradual increase in density take place with increase in atomic no. And density decreases after acquiring maximum value due to increase in atomic radius and atomic volume.
Standard Electrode Potential :
Standard Electrode Potential of hydrogen is assumed to be zero. Standard Electrode Potential of other electrodes is determined relative to Standard Electrode Potential of hydrogen.
Ionization Potential :
Ionization Potential of d-block elements is intermediate between s and p block elements. Ionization Potential value increases with increase in atomic number across a period.
Electropositive character :
d-block elements are electropositive, but as compared to s-block elements these are less electropositive. So form electrovalent compound with more difficulties as compared to s-block elements.
Electronegativity of transition elements increases with increase in nuclear charge. Last element of each period have complete d sub-shell so have low electronegativity value because screening effect of complete d sub-shell is more than incomplete d sub-shell.
Oxidation states :
d-block elements show variable oxidation state. Electrons in d orbital are responsible for variable oxidation state. +2 oxidation state occur on removal of two s electron from outermost shell. Other oxidation state (i.e. more than +2) require removal of electron of d orbital of penultimate shell.
Complex formation :
Due to incomplete d sub-shell of d-block elements they are able to form complex compounds. Central ion is capable to accept lone pair of electron donated by ligands to form complex. Empty orbital in a atom adjust these lone pair of electrons. According to Pauling, Transition elements either have empty orbitals or they produced empty orbital when surrounded by the ligands.
Catalytic character :
d-block elements or their compounds are used as a catalyst in many chemical reactions. Generally Fe, Cr, Pt, Ni, V2O5, Mn etc., are used as a catalyst. A essential property of a catalyst is to form an unstable intermediate. Due to various oxidation state transition elements form intermediate easily. Good catalyst have free valency on its surface. Catalyst are used in finely divided form to increase surface area to attain increased number of free valancy.
Alloy formation :
Due to approximately equal size of d-block elements they form alloy.
d-block elements are less reactive due to higher ionization potential (because of smaller atomic size), so hydration of cations of d-block elements is difficult and have high heat of sublimation.
Formation of Interstitial or non-stoichiometric compounds :
Compounds which do not follow valency rule are known as non-stoichiometric compounds. These type of compound are formed due to entrance of the non-metallic atoms into the interatomic spaces of metal atoms. Eg. TiH1.7, VSe0.98, FeO0.94, etc.
Coloured Ions :
Compounds or Ions of transition elements appear to be coloured due to these reasons-
(i) d-d transition
(ii) charge transfer
(i) d-d transition :
In a compound of d-block element colour depend on transition of electron from lower energy level to higher energy level and in ion of d-block elements, due to unpaired electron in d orbital, it get splitted into two parts at the time of complex formation. These two orbital differ in their energies. Electron absorb radiations in visible region and transition of electron occur from lower energy level to higher. So colour of transition elements depends on d-d transition. Also If no of d-d transition is higher then colour of ion is darker.
(ii) charge transfer :
d-d transition is not possible in PbO2, MnO4-, Cr2O72-, Sn2+ and Sn4+ etc. No unpaired d electrons are found in them. In these ions transition of electrons occur from orbital of one atom to orbital of another atom by absorbing radiation to produced dark colour. This transition occur in UV region (1800 - 4000 ) is known as charge transfer transition.
Magnetic Properties :
On the basis of magnetic behaviour they are classified into five categories
The type of substance which when placed in a magnetic field the intensity of magnetic field decreases as compared to the intensity in vacuum the substances are known as diamagnetic substances and this property is called diamagnetism.
Diamagnetism is due to the presence of paired electrons hence found in all substances except hydrogen. Magnetic lines of force tends to move away from the substances so these substances are repelled by magnetic field and these substances align themselves at right angle to magnetic field. Diamagnetism is occurs due to the presence of paired electrons. In this type of substances magnetic moment produced by one electron is cancelled by another which is equal and opposite to first one. So no magnetic moment present in substances having paired electrons.
Diamagnetism increases with increase in atomic number.
(ii) Paramagnetic :
It is found in substances which have unpaired electrons like transition elements. This type of substances having permanent magnetism. When this type of substances placed in external magnetic field, they aligns themselves in the direction of magnetic field. Paramagnetism occurs due to motion and spin of electrons. Paramagnetism decreases with increase in temperature. These types of substances are attracted in magnetic field. If number of unpaired electrons in a substance is n then magnetic moment.
Number of unpaired electrons in substances is calculated by the magnetic.
Substances which have very high paramagnetic character are known as ferromagnetic substances. And found in some alloys or compounds of Fe, Co, Ni, Mn. These types of substances even remain magnetic characters after removing from the external magnetic field. This type of substances contains tiny magnets in them which arrange randomly. On placing this type of substances in magnetic field these tiny magnets arrange them in one direction so these show very high magnetism.
These types of substances do not show magnetism (Paramagnetism) even they have unpaired electrons like MnF2, MnO.
Properties of elements of first transition series:
Elements from Sc (21) to Zn (30) are known as elements of first transition series (i.e. 3d transition series). In the atoms of first transition series last electron goes in 3d sub-sell.
The compounds which are formed by two types of elements and ions are known as binary compounds. Elements of the first transition series react with so many non metallic elements like carbon, oxygen, phosphorus, sulphur and nitrogen etc. to form binary compounds. Oxides, halides, sulphides, carbides are main binary compounds of first transition series.
Oxides: -When element of first transition series heated with oxygen at high temperature metal oxides are formed. Important oxides of first transaction series are as follows.
Acidic oxides : V205, CrO3, MnO3.
Basic Oxides : Sc2O3, TiO, Ti2O3, VO, V2O3, MnO, CrO, FeO, Fe2O3, Fe3O4, CoO, NiO, Cu2O.
Amphoteric Oxides : TiO2, VO2, Cr2O3, CrO2, Mn3O4, Mn2O3, MnO2, CuO, ZnO.
Main properties of oxides :
1. Acidic, Basic or Amphoteric nature : As the oxidation No. of metal increases its acidic nature of oxides also increases.
Oxides of Vanadium VO V2O3 VO2 V2O5
Oxidation No. of vanadium +2 +3 +4 +5
Nature of oxides Basic Basic Amphoteric Acidic
2. Solubility : Amphoteric and basic oxides are soluble in acids which do not act as oxidants. Acidic oxides form oxy acids in water and oxy salts in bases to get dissolved.
3. Reducing nature of oxides : Electron donor substances act as reductant. Substances (atoms, ions and molecules) which donate their electrons easily have higher reducing character.
Halides : Elements of first transition series (3d series) react with halogens at high temperature to form halides. Order of reactivity of halogens with the metals is as given below.
F2 >Cl2> Br2> I2
Generally fluorides formed in higher oxidation states. Formation of halides require high activation energy so this reaction occurs at high temperature.
Properties of halides
1. Transition metal halides are less volatile and more susceptible to hydrolysis. Metal halides in higher oxidation states have high tendency to undergo hydrolysis.
TiCl4 + 2H2O ® TiO2 + 4HCl
2. In lower oxidation states more stable oxides are formed.
Eg. :ZnCl2, VCl2 etc.
3. Fluorides are ionic in nature. Chlorides, Bromides and iodides have both ionic and covalent character.
Fluoride > Chloride > Bromide > Iodide
Sulphides :Sulphides are obtained on heating metal with sulphur. Metal sulphides are also produced on reacting aqueous solution of metal salts with Na2S or H2S.
Properties of sulphides :
1. First transition metal sulphides are mainly dark colored or black.
CuS – Black
NiS – Black
CoS – Black
2. Sulphides are insoluble in water.
3. They get oxidised to metal sulphates on oxidation.
4. Some sulphides such as CoS, NiS and FeS behave as an alloy or exibit the semi-metallic character.
5. FeS2, CoS2 contain discrete S2 units with S-S bonding.
Carbides : carbides are produced on heating transition metals or metal oxides with carbon at very high temperature about 2000-2200°C.
Carbides formed by first transition series elements are of two types-
(a) Salt like carbides : These carbides are also known as electrovalent carbides or ionic carbides. Metals like Sc, Cu, Zn etc. form this type of carbides.
(b) Interstitial Carbides : These carbides are also known as metallic carbides. Metals like Ti, V, Mn, Fe, Co form such type of carbides. These type of carbides are obtained on heating a carbon and metal..
Properties of interstitial carbides
1. interstitial carbides are extremely hard.
2. interstitial carbides have high melting point.
3. interstitial carbides have high electrical conductivity.
4. interstitial carbides show inertness to chemical reactions.
5. interstitial carbides have metallic lustre.