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TRANSITION ELEMENTS
1.1 General physical and chemical properties of the first row of transition
elements
Transition elements are d-block elements which forms one or more stable ions with incomplete d orbitals.
Scandium is not a transition element because – it forms only one Sc ion which has no electrons in its 3d sub
3+
shell.
Zinc is not a transition element because – it forms only one Zn ion which has complete 3d sub shell.
2+
For chromium and copper – one electron from 4s subshell jumps to 3d subshell to become stable. This is
5 10
possible as 4s and 3d has similar energy level. (chromium is 3d which is half filled and copper is 3d which is
fully filled)
The 4s and 3d sub shell energy level is similar – similar energy required to remove different number of
electrons - results in variable oxidation state (when electron is removed, it is first removed from 4s orbital)
Properties of transition elements
They have variable oxidation states
They behave as catalysts - more than one stable oxidation state vacant (d) orbitals are energetically
accessible OR empty / vacant (d) orbitals can form dative bonds with ligands
They form complex ions
They form coloured compounds
Physical – high melting and boiling points, high density, paramagnetic or diamagnetic
, 1.2 Ligands
Ligands is a species that has one or more lone pairs of electrons which it can donate to central metal
atom/ion by coordinate covalent bonds.
Monodentate ligands can form only one dative bond to the central metal ion.
Examples - Water (H2O) molecules, Ammonia (NH3) molecules, Chloride (Cl-) ions, Cyanide (CN-)
ions
Bidentate ligands can each form two dative bonds to the central metal ion as they contain two
atoms with lone pairs of electrons.
Examples - 1,2-iaminoethane (H2NCH2CH2NH2) ‘en’ and Ethanedioate ion (C2O42- ) ‘ox’
Polydentate ligands can form more than two dative bonds to the central metal ion.
Example - EDTA4- , which is a hexadentate ligand
1.3 Transition elements complexes
A complex is a molecule or ion formed by a central metal ion (cation) is surrounded by one or
more ligands by donating its lone pair of electrons, which is formed by coordinate covalent bonding
The coordination number of a complex is the number of coordinate bonds that are formed
The overall charge is found by subtracting charges of ligand from the central metal atom/ion.
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