D-block element and the study of covalent radii, ionisation potential and variable valency of d-,block element.

D block element.

The elements in which electrons progressively enter in the d-orbital of the penultimate shell,I,e in the (n-1) d orbitals are called d-block element. The d-block element is called transition element. It is because they exhibit transition behaviour between highly reactive ionic compound forming s-blo0ck elements (electropositive element) on one side and mainly covalent forming p-block element (electronegative element ) on the other side.

On the other hand, Transition element are d-block elements heaving (incompletely) filled d-orbitals either in element form or in the most common oxidation state.

$$Sc=3d^1\,4s^2$$

$$Ni=3d^8\,4s^2$$

$$Cu=3d^{10}\,4s^1$$

$$Cu^{++}=3d^9$$

$$Incomplete\,filled\,d-\,orbital$$.

$$Therefore\,,d-\,block \, and\,transition\,elements.$$

$$Zn=3d^{10}\,,4s^2\,;Zn^{++}=3d^{10}$$

$$Cd=4d^{10}\,5s^2\,;Cd^{++}=4d{10}$$

$$Hg=5d^{10}\,6s^2\,;Hg^{++}=5d^{10}$$

$$Completely\,filled \,d\,-orbital$$

$$Therefore\,d\,-block\,but\,notransition element$$

Elements of group 12 (Zn, Cd, and Mg ) have completely filled d-orbital in their elemental form and also in their most stable oxidation state +2. Hence, these elements are d-block but not regarded as typical transition elements.

D-block element differs from S block element is due to the following reason.

The general valence shell electronic configuration of S-block element is ns^1-2.Hence the group IA element ( alkali metal) have one selection and group IIA elements (alkaline earth metal) have two electrons in the new quantum shell after the inert gas core. But the valence shell electronic configuration of d-block element is (n-1)d^1-10ns^1-2.From the configuration of the d-block element, we observe that in atoms of d-block elements two outermost shells I,e nth and (n-1) the shell are filled by valence electrons.

Some transition metal is called class ‘a’ acceptor and other as class ‘b’ acceptor.explain reason with the example.

The left-hand side of the transition metals are called calls ‘a’ accepted and the right hand side of the transition metals are called class ‘b’ acceptor this is because the first half( left-hand side) of transition metals behave the first half (left-hand side) of transition metals behave as ‘hard acids and the right-hand side of the transition metal behave as soft acids.

According to HSAB principle, class, ‘a’ acceptor metal from their stable complexes with ligands in which donor atoms are N, O, Or F (Ammonia, amines, water, alcohol, F-,etc). In contrast class ‘b’ acceptor metals form stable complexes with ligands such as I^- , SCN^-, and CN^-

Periodic property of 3d series in the transition series.

1. Covalent radii.

The atomic (covalent )radii of transition elements are smaller than s-block elements. The atomic radii of 3d-series decrease from Sc to Cr remain almost constant till Cu and then increases towards Zn.

For the element of the 1^st transition series, the atomic radii decrease gradually from Sc to Mn but from Fe to Cu these values remains practically constant. The atomic radius of Zn is higher than that of Cu.

Explanation.

For the element from Sc to Mn, the atomic radii decreased. This decreased is because of a gradual increase in nuclear charge with an increase in atomic number. The increased atomic number makes the atom to contrast in size.

Up tp Mn (3d5 4s2), electron-electron repulsion does not come into the picture because there is no pairing of electrons in 3d orbitals. However, in the case of elements from Fe to Zn, the pairing of the electrons in 3d orbitals take place and hence electron-electron repulsion are produced. I the element from Fe to Cu, the decreased in size expected on account of increasing in nuclear charge is almost cancelled by electron-electron repulsion and hence there is no major change in size ongoing from Fe to Cu. The size increased in Zn is due to better shielding of the completely filled d-orbitals.

Ionisation potential.

The ease with which an electron may be removed from a transition atom, (I,e its ionisation Potential) is intermediate between those of the s- and p-blocks.

The ionisation potential of 3d series elements increased as we move across the series from left to right, although the increased is not quite regular eg, the values for Sc, Ti, V, and Cr differ very slightly. Similarly, the values for Fe, Co, Ni, Cu, are fairly close to one another. The values of Zn is appreciably higher due to the additional stability associated with a completely filled 3d level.The slight differences in I, P valve of transition elNote: an element may be explained in terms of screening effect of the d’ electrons. The increased in I,P expected on accounts of increasing in nuclear charge is almost cancelled by the extra screening of the nuclear provided by the inner’ electrons.

Ionic character.

Transition element exhibits variable oxidation state in their compounds. With increased in the oxidation state of a given transition metal, the covalent character of its compound increased.Thus a compound of a given transition metal in lower oxidation state are ionic while those of the same metal ion in higher oxidation states are covalent.For example.VCL₂ [⁺² is ionic].Reason VCl₃[^+3] is less ionic and VCl₄[^+4] is covalent.

Explanation.

As the oxidation state of transition metal increased, the changing density of the metal increased while the size of metal ion decreased.This result in increased in polarisation power of cation and hence character increase (Fagan's rule).

Periodicity in ionisation energy, electronegativity, nature of bond in their compound on passing from fluorine to iodine.

Ionisation energy.

Ionisation energy decreased from F to I, as the atomic size of this element increased from F to I.

Electronegativity.

Electronegativity values decreased from F to I because the size of atoms increased and hence tendency to attain electron decreased.

Nature of bond.

The ionic character of their compound decreased from F to I because Electronegativity values of this element increased from F to I.

Reference:

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.