Thursday, January 17, 2008

Ch.16.Transition Elements - core Points for Revision


Transition elements (3d series):
Definition, general characteristics, oxidation states and their stabilities, colour (excluding the details of electronic transitions) and calculation of spin-only magnetic moment;

Coordination compounds: nomenclature of mononuclear coordination compounds, cis-trans and ionisation isomerisms, hybridization and geometries of mononuclear coordination compounds (linear, tetrahedral, square planar and octahedral).

A transition element may be defined as an element which in its elementary form or in at least one of its oxidation states, possesses partially filled d orbitals in its penultimate shell.

Three series of elements are formed by filling the 3d, 4d, and 5d shells by electrons.

First series or 3d series: Scandium to Zinc
Second series or 4d series: Yitrium to cadmium
Third series or 5d series: Lanthanum to hafnium to mercury

In JEE syllabus only 3d series is there.

The ten elements from Scandium to Zinc form the first transition metal series. They closely resemble each other and are hard, dense, shiny metals with high melting and boiling points.

oxidation states

Common oxidation states are +2 and +3, with the +2 state more common towards the end. The higher oxidation states are shown in compounds with electronegative elements like O, Cl or F (e.g. Cr2O7^2- [+6], MnO4^- [+7]).

Variable oxidation state is found because of the small difference in energy between the 3d and 4s sub-shells. This allows varying numbers of electrons to be used in bonding. When forming ions transition metals lose electrons from the 4s sub-shell before the 3d.

Catalytic Action
The ability of transition metals to exist in various oxidation states makes them important industrial and biological catalysts.

Coordination compounds are a special class of compounds in which the centgral metal atom is surrounded by ions or molecules beyond their valency.

There are also referred to as coordination complexes or complexes.

Haemoglobin, Chlrophyll, and vitamin B-12 are coordinatio compounds of iron, magnesium and cobalt respectively.

The interesting thing of coordination compound is that these are formed from apparently saturated molecules capable of independent existence.

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