Following are the properties of transition elements-
1. Metallic character- All transition elements are metals because they have one or two electrons in outermost shell which they can easily loose. These metals are ductile, malleable and good conductor of heat and electricity. In atoms of these elements both covalent and metallic bonding exist. Covalent bonding is responsible or hardness (except Mercury) and hardness increases with increase in number of unpaired d electrons (exception manganese). Metallic bonding is responsible for their conductivity.
2. Atomic radii- In elements of first transition series the atomic radii decreases slowly from scandium to chromium but from chromium to copper it remains almost same. This is because in arrangement of given series, number of shells remains same but nuclear charge increases. The entering electrons from d subshell produce a screening effect and it increases with increase in number of d electrons. Thus, two effects nuclear charge and screening effect opposes each other and in middle of series both effects become nearly equal and no change in radii seems.
Elements- Sc Ti V Cr Mn Fe Co Ni Cu Zn
At. No.- 1.44 1.32 1.22 1.18 1.17 1.17 1.16 1.15 1.17 1.25
3. Ionization potential- First ionization energy of fourth period transition element vary irregularly but tend to increase from left to right across series. The increase in ionization enthalpy along transition series is due to effectiveness of attraction of nuclear charge on electron. The electronic configuration of first row d block element reveal that addition of electron to (n-1)d subshell results in more and more shielding of 4 s electrons. Due to presence of 4 f orbitals in atoms of elements of third transition series which poorly shields outermost electrons from nuclear attraction, the ionization enthalpy of third transition series are higher than those of first and second transition series.
4. Oxidation state- Since there is very small difference in energy of outermost s and d subshell and electrons from both subshell may be used for bond formation so all transition elements show variable oxidation state (except zinc group). Some of transition metals such as chromium, iron and nickel also show zero oxidation state in metal carbonyl
like chromium carbonyl, iron carbonyl and nickel carbonyl. The relative stability of different oxidation states can be explained on basis of d0, d5 and d10 configurations.
Example- titanium in +4 oxidation state (3 d0, 4 s0) > titanium in +3 oxidation state (3 d1, 4 s0).
5. Magnetic property- Except elements of zinc group, all transition elements are para magnetic i.e, they are attracted by magnetic field and this is due to presence of unpaired electrons in (n-1)d orbitals. The elements of zinc group are dia magnetic and they are repelled by magnetic field.
The electrons determine magnetic properties in two ways-
(a) Each electron behave as a tiny magnet spinning about its own axis. Spinning of charge produces a magnetic moment known as spin magnetic moment of electron.
(b) Moving electron in an orbit also produces a magnetic moment known as orbital magnetic moment. Thus, magnetic properties of an atom, ion or molecule results from combination of spin and orbital magnetic moment.
where s= Spin quantum number
and, L= Sum of orbital angular momentum quantum number
It has been found that in number of compounds including first row of transition elements, orbital contribution is quenched or suppressed by electric fields of surrounding atoms i.e, L = 0 so that magnetic moment of species is practically equal to
6. Complex formation- The ability of transition elements to form complexes is due to
(a) Small size of transition metal cation.
(b) High effective nuclear charge.
(c) Availability of vacant d orbitals of appropriate energy of bonding.
The chemistry of transition metals is dominated in cases where metal ion interacts with two or more donor ligands. These have been called co-ordination compounds.
7. Catalytic properties of transition metals–
(a) Transition metals show catalytic properties due to various oxidation state due to which they form unstable intermediates with various reactants. These intermediate products decomposes to give final product.
A + B + catalyst = AB catalyst = AB + catalyst
(b) In cases finely divided metals provide a large surface area for adsorption of reactants. The adsorbed reactant react faster due to their closer contact on surface of catalyst giving rise to product.
8. Colour formation- The phenomenon of colour is associated with incomplete d electron shells and excitation of electrons from d orbitals of lower energy to d orbital of higher energy. The amount of energy required for d-d electron transition is very small and is absorbed from visible region. It has been observed that transition metal ions having completely filled d orbitals such as Cu(+1), Ag(+1), Au(+1), Zn(+2), Cd(+2), Hg(+2) are all colorless as excitation of electron is not possible within d orbital.