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Transition elements are d-block elements. The elements in which the last electron enters the 'd' orbital of the penultimate shell. d- block elements lie in the middle of the periodic table. Their properties are in between those of reactive s-block and less reactive p-block elements.
The elements having partly filled d-orbital in the penultimate shell in their atoms or in their stable oxidation states are called transition elements or transition metals.
The general outer electronic configuration of these elements is (n - 1)10ns1-2.
where 'n' is valence shell and (n - 1) is the penultimate shell.
Example: Scandium, Titanium, Iron, Cobalt, Chromium, Copper, Silver, etc.
1) Electronic configuration
The general outer electronic configuration of transition elements is (n - 1)10ns1-2 where 'n' is valence shell and (n - 1) is the penultimate shell.
2) Variable oxidation state
Transition metals exhibit variable oxidation state.
Example: Iron : + 2 and + 3
Chromium = + 3 and + 6
Manganese: +2, +4, +6 and +7
Copper: +1 and + 2
It is because of the availability of d- electrons in bonding. This is due to very little energy difference between (n- 1)d electrons and ns electrons
3) Colored compounds
The compounds of transition metals are usually colored.
Example: Cr3+ = Green
Fe2+ = Light green
Fe3+ = Light yellow2+
Mn2+ = Pink
Co2+ = Re
Cu2+ = Blue
Ni2+ = Dark green
It is due to the presence of partly filled d-orbital in the metal ions of the compounds which allow d-d electronic transition by the absorption of a certain wavelength of light and it will exhibit the color of reflected or transmitted light complementary to the color of the absorbed light. The transition metal ions with d0 and d10 configuration do not exhibit color.
For example: Zn2+ = [Ar]3d10
and Sc3+ = [Ar]3d0
Transition metal compounds are paramagnetic (attracted towards magnetic field). It is due to the presence of unpaired d - electrons in metal ions.
But zinc compounds are diamagnetic (repelled by the magnetic field) because there is the absence of unpaired electron ( d10 configuration) in zinc ion (Zn2+).
Greater the number of unpaired electrons, greater is the paramagnetism.
5) Complex compounds
Transition metals form complex compounds or coordination compounds.
Complex compounds are those that give complex ions.
-[Cu(NH3)4]SO4 →[Cu(NH3)4]SO4]++ + SO4- -
Tetraamine Copper (II) sulphate ion
- [Ag(NH3)2]Cl (Diamine silver (I) chloride)
- Fe4[Fe(CN)6}3 (ferric ferrosocyanide = prussian blue coloration)
Formation of complex compounds by transition metals is due to the availability of empty d-orbitals in transition metal ion that can accommodate electrons given by the donating species.
6) Used as catalyst
Transition metals and their compounds show catalytic activity.
For example, Iron is used as a catalyst in Haber's process.
Finely divided Nickel is used in hydrogenation.
Vanadium pentoxide is used during contact process.
pulse, Tracy. Introduction to chemistry. u.s.a: flex book, 2010.
Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.