Study of transition elements and the striking feature of transition elements.

D-block element.

The group of the element in which the last electrons enter into the penultimate energy level,(n-1) d orbital of an atom are called are called d-block element.

These elements are also called transition metal element since their position is in between the s-block and p-block element.They show the transitional behaviour between the highly electropositive ionic compound forming S-block element and the highly electronegative covalent compound forming p-block element.

Their general valence shell electronic configuration is n-1d^1-10 or ns^{1or 2} here n is the periodic number called as ultimate energy level and (n-1) is penultimate energy level (inner to valence).

The elements belong in a d-block element are highly metallic in nature, hard, brittle, and ductile and can show the different typical characteristics property like variable oxidation state, colour, property, magnetic property, behaviour, catalytic property, complex formation ability etc.

The entire d-block element is classified into the three different series I,e 3d series, 4d series and 5d series.It comprises ten group belonging from group IIIB to VIIB, Group VIII consisting of 3 column) and Group IB and Group IIB. The three transition series are listed below.

1. First transition series (3d series) belong to 4^th period consists the element from 21Sc to 30 Zn.
2. Second transition series (4d series); belong to 5^th period consisting the elements from 39Y to 48Cd.
3. Third transition series (5d series); belong to 6^th period, consisting the elements lanthanum57La ,72Hf Hafnium to 80 Hg.
4. Fourth transition series (6d series); belong to 7th-period consists of the elements Actinum 89Ac, elements after 104.

Electronic configuration.

General valence shell electronic configuration=3d^1-104s^{1or 2.}

Atomic radius.

In d-block elements, electrons enters in the d-orbital of a penultimate shell and valence shell remains unaffected. Since d-orbital have a low shielding effect.The nuclear attraction on the valence shell increased.Consequently, the valence shell contract and the atomic radius decreased on moving from Sc to Zn. Decreased in atomic radius along a period due to less shielding effect of the d-block element is called the d-block contraction.

Atomic radii of d-block element decreased very slowly from left to right.Therefore, the atomic radii of the d-block element of a series are approximately said to be constant.

Atomic radius increased on descending from 1^st transition series to 2^nd transition series. This is due to the addition of extra cell. However, the atomic radius remains almost same on descending from 2^nd transition series to 3^rd transition series.This is due to f-block contraction.

Ionisation energy.

The ionisation energy of transition element increase on moving from left to right in the period. The ionisation energy of transition metals is intermediate between s and p block element.

Since the atomic radius of element decreased very slowly along the period, their atomic radii are approximately said to be constant. That’why , the ionisation energy of these elements are reasonably constant.

Metallic property.

Since the atomic radius of d-block elements decreased along a period. Their metallic property also decreased from left to right.

All transition elements exhibit metallic property I,e these elements can form a cation. This is due to the fact that those elements have relatively larger atomic size and lower ionisation energy which favour this element to remove valence electron and to form the cation. Metallic property of these elements is intermediate between s and the p-block element I,e these elements are less electropositive than S-block element and are more electropositive than p-block elements.

Variable valence (oxidation state).

One of the most important properties of the transition element is that they usually exhibit variable oxidation state i,e variable valency in their compounds. The general valence shell configuration is (n-1) d^1-10ns^1or2.

The tendency to show the variable oxidation state by transition element is due to the fact that, the energy level of 3d,4d, and 5d, orbitals are nearly closer or almost same to those of 4s, 5s and 6s orbitals.i,e (n-1) and ns orbitals have the almost same energy level and hence both the ns and n-1d electrons are available for bonding purpose.Therefore, the number of oxidation state exhibited by the transition element depends on the number of unpaired electron present on d-orbitals.

The oxidation state of first transition series is illustrated in the table below.

For example, the outer electronic configuration of Sc is 3d1, 4s2 and exhibits the oxidation state of +II if it used both of 4s electron and also exhibit +III if it used two us electrons and one 3d electron during the chemical bonding.

In general, the II and III row transition element exhibit higher coordination number and are stable in their higher oxidation states whereas the first-row element is more stable in lower oxidation states.

Atomic size

.Generally the atomic radius in any particular period decreased from left to right .So the atomic radius from Sc to Zn is gradually decreased.This is due to increase in number charge and the simultaneous addition of the electrons in the same energy level which causes the greatest force of attraction by the nucleus to the valence shell electrons and hence the size gradually decreased.

However, in a case of d-block element the trend of an atomic radius is not continuous it means the size of an atomic radius is not continued it means the size of an element at a middle of the period and quite remains constant at a middle of the period and quite increase in the last of a period. This is because the screening effect of d-electrons is comparatively less than s and p electrons and hence the effective nuclear charge partly cancelled with each other and hence the size remains almost constant.So, in the 3d series, the size decreased from Sc to Cr, remains almost constant from Mn to Cu and quite increased to Zn.

The ionic size of the 3d series element also follows the same trend as the atomic radius from Sc to Zn.

The atomic radius of transition metal elements along the group increased from top to bottom due to the addition of extra cell in each step.However, the size remains almost constant in 6d and 7d series due to the introduction of Lanthanide and Actinide and the effect of lanthanide and Actinides contraction.

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.