Thursday, January 24, 2008

AIEEE Chemistry UNIT 16 d – and f – BLOCK ELEMENTS

Transition Elements :
General introduction, electronic configuration, occurrence and characteristics,

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.

The definition excludes zinc, cadmium, and mercury from the transition elements, however their properties are an extension of the properties of transition elements, they are generally considered along with transition elements.

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

Transition Metals
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. They readily form alloys and have other properties in common. Crossing the period from Sc to Zn there is a small decrease in atomic radius and increase in electronegativity and ionisation energy. Most of the properties of transition metals are related to their electronic structures.

Electronic Structure
Transition elements are characterised by having a partially filled d sub-shell

Sc [Ar] 3d1 4s2

Ti [Ar] 3d2 4s2

V [Ar] 3d3 4s2

*Cr [Ar] 3d5 4s1

Mn [Ar] 3d5 4s2

Fe [Ar] 3d6 4s2

Co [Ar] 3d7 4s2

Ni [Ar] 3d8 4s2

*Cu [Ar] 3d10 4s1

Zn [Ar] 3d10 4s2

*Note that in Cr the arrangement [Ar] 3d5 4s1 with half-filled 3d and 4s sub-shells is more stable than [Ar] 3d4 4s2.

In Cu [Ar] 3d10 4s1 with a completely filled 3d sub-shell and a half-filled 4s sub-shell is more stable than [Ar] 3d9 4s2.

General characteristics

1. They are hard and brittle metals.
2. They have a high melting and boiling points and have higher heats of vaporization than non transition elements.
3. The transition elements have very high densities as compared to the metals of groups I and II (s-block).
4. The first ionization energies of d-block elements are higher than those of s-block elements but are less than those of p-block elements.
5. They are electropositive in nature.
6. Most of them form coloured compounds.
7. The have good tendency to form complexes.
8. They exhibit several oxidation states.
9. Their compounds are generally paramagnetic n nature.
10. They form alloys with other metals.
11. They form interstitial compound with elements such as hydrogen, boron, carbon, nitrogen etc.
12. Most of the transition metals such Mn, Ni, Co, Cr, V, Pt etc. and their compounds have been used as good catalysts.

Oxidation States
Transition metals form ions which are characterised by having a partially filled d sub-shell. In case of 3d series electrons in 4s as well as electrons in 3d participte in reactions. Lower oxidation states represent the participation of electrons in 4s only. Higher oxidation states result when electrons in 3d also participate.

The common oxidation states of various elements are:

Sc 3
Ti 2 3
V 2 3 4 5
Cr (1) 2 3 (5) 6
Mn 2 3 4 (5) 6 7
Fe 2 3 (4) (5) (6)
Co 2 3 (4)
Ni 2 3
Cu 1 2
Zn 2

Oxidation states in brackets are unstable

Sc and Zn are not typical transition metals as they only have one oxidation state which does not have a partially filled d sub-shell. (Sc3+ [Ar], Zn2+ [Ar] 3d10).

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.

No comments: