The elements which lie in the middle of s and p- block are called transition elements.
•those elements which have partially filled d-Subshells in their elementary form or in their commonly occurring Oxidation states belongs to d-block.
Zinc ,Cadmium, mercury are not transition series elements because they have fully filled d-orbitals in their elementary state or in their commonly occurring oxidation states.
●Cu ,Ag ,Au have also fully filled d-orbitals in their elementary states but in their commonly occurring (Cu2+,Ag2+, Au2+) ions it have partially filled d-orbitals and hence they behaves as “Transition Elements”.
FIRST Transition Series :Scandium(z=21 ) to Zinc (Z=30) [4th period:3d orbital]
SECOND Transition Series: Yttrium(Z=39) to Cadmium(Z=48) [5th period:4d orbital]
THIRD Transition Series: Lanthanum(Z=57),Hafnium(Z=72) to Mercury(Z=80) [6th period:5d orbital]
FORTH Transition Series :Actinium(Z=89),Rutherfordium(Z=104) to Copernicium(Z=112) [7th period: 6d-orbital]
Why d-Block Elements ionize by the loss of 4s electron First.
The Reason for this behaviour is that after the electrons have entered 3d subshell,The energy of 3d Subshell becomes Lower than that of 4s-Subshell.
As a Result 3d orbital becomes more stable than 4s orbital and hence transition elements of 1st series ionize by loss of 4s electrons first.
General Characteristic of Transition Elements
1.All the transition metals are hard ,brittle and have high melting & boiling points.
2.They have higher heat of vaporisation than non-transition elements.
3.They are Electropositive in nature.
4.Except mercury which is liquid at room temperature ,other transition elements are solid and have typical metallic character.
5.Most of the transition elements have high densities.•Osmium and Iridium have very high density.
6.Most of them are good electrical conductor and form coloured compounds.
7.They Exhibit Several Oxidation States and have good tendency to form Complexes.
8.Their Compounds are Generally Paramagnetic in nature.
9.They Form alloys with other metals.
10.Most of the transition metal and their compounds have been used as good Catalysts.
General Trends of Some Important Properties
In a particular Series the atomic Radii Decreases with increase in atomic Number.Due to increase in Nuclear Charge.
•Atomic Radii decreases across a period but this decrease is not uniform and is less in comparison to the decrease in the Same period for s & p block elements.
•Atomic Radii decreases gradually from Sc to Cr but from Cr to Cu it Remain practically Constant. This is due to the better Screening by the ( n-1)d Subshells.
•Atomic Radii of Elements increases with increase in At.number towards the end of the series.This is due to the increased force of repulsion between the added electrons which exceed the attractive force due to Increased Nuclear Charge.
-Ionic Radii decreases with increasing Oxidation States .
For Same Oxidation State, Ionic Radii Decreases with increase in Nuclear Charge.
The Density of Transition Elements Are Quite high due to the decrease in the Atomic Volume.
•Atomic volume Increases after Group 10 and Consequently, density decreases.
Melting and Boiling Point
d-block elements have very high melting and boiling points.this is due to more valence electrons.
•It is observed that melting and boiling points of these elements reach to a max. Value near group 7 or 8 and then decreases with increase in atomic number.
First ionisation energy of d-block elements is higher than s-block but less than p-block elements.
I.E. Increases as we move Across the period but “Quite Slowly”.
The Reducing Character of a metal depends upon its tendency to be Oxidised ,it is measured in terms of reduction potential.
•d-block elements are Strong Reducing Agent ,but their reducing ch. Is lower than s-block elements. This is due to large I.E. and Large Enthalpies of atomization.
They all are metal and their metallic character is due to loe ionisation energy and a number of vacant orbitals in their atoms.
Ionisation Energies of transition metals are fairly high and their atoms are small in size.
•they require a large amount of energy to change them from Solid to vapour State. Also they do not get hydrated easily.
All this leads to small tendency to react.
Transition elements has ability to exhibit a large no. Of oxidation states.
•Variable oxidation state of d-block elements are due to the participation of inner (n-1)d electrons in addition to outer ns electrons.
Relative Stability of Oxidation States of Transition Metals
•Lesser is the ionisation energy more stable is the oxidation state.
Ni —-> Ni2+ + 2e– I.E=2.49×103 kj/mol
Pt —->Pt2+ + 2e– I.E=2.66×103 kj/ mol
Here Ni2+ is more stable than Pt2+
•Lower is the Electrode Reduction potential value,greater is the stability of that Oxidation state.
Cu+(aq) + e– —–> Cu(s) E°red =O.52 V
Cu2+(aq) +2 e– —–> Cu(s) E°red =O.34 V
here Cu2+ is more stable than Cu+
•Nature of Solvent
Stability of oxidation state also depends upon the nature of Solvent.
Eg. Cr3+ is stable in water while Cr2+ is unstable .
Formation of coloured ions
Transition elements mostly forms coloured compounds this is due incomplete (n-1)d Subshells.
Ions containing unpaired electrons can be easily promoted from one energy level to another in the same d-subshell.
•the amt of energy required to excite some of electrons to higher energy levels Corresponding to energy of Certain Colour of Visible Light.
Most of the transition Compounds are paramagnetic in character
and their magnetic Ch. Is Calculated by
μ=√ n(n+2) B.M. where n=number of unpaired electrons.
Tendency to form Complexes :-
is due to
•Small size of the atoms and ions of Transtion metals.
•high Nuclear Charge
•Availability of vacant d-orbitals of appropriate energy in their atoms for bonding with ligands.
The catalytic properties of transition metal is due to the formation of appropriate reaction intermediates with the substrate by using empty d-orbitals.
These intermediates give reaction paths of lower activation energy. Therefore increases the rate of reaction
Transition metals form large no of alloys .this is due to the fact that transition metals are quite similar in size.
Thereby atoms of one metal can substitute the atoms of other metal in its Crystal lattice.
Chemical Reactivity and General Trends in some important Compounds.
They are Chemically less reactive due to their higher ionisation energy ,High enthalpy of Sublimation & Smaller Size.
Oxides of higher oxidation state of metal are acidic .
the oxides having intermediate oxidation state are amphoteric.
•Tendency of the metals to give electrons becomes more as we move in a period.
ie Reducing ch. Increases.
Acidic Ch increases as : +2 < +3 < +4 < +5 < +6 < +7
Transition Elements reacts with halogens at higher temp to form transition metal halides .
These reactions have high activation energies , therefore high temp. Is required to start the reaction.
Order of Reactivity : F> Cl >Br> I
Sulphides are either formed by direct combination of sulphur with metal or by the reaction of compounds Such as H2S or Na2S with a solution of the metallic salts.
•Sulphides are formed in lower Oxidation state and are insoluble in water.