The elements in the groups from 3 to 12 or The elements in the groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and, VIIIB of the periodic table are called as d-block elements. The 4th, 5th, 6th and 7th period consists of 10 elements each and are called as d-block elements. Thus, In all there are 40 d-block elements. The d-block elements have partially filled d-orbitals. They are placed in between the s-block and p-block elements in the periodic table. They are also called as Transition elements. The name " Transition " originates from their place between s and p-block elements in the periodic table. The d-block elements are all metals with their last two shells incompletely filled.
Characteristic Properties of elements in d-block of Modern Periodic Table :-
The properties of d-block elements are intermediate between s-block and p-block elements.
1) Electronic Configuration :- The general electronic configuration of d-block elements is ns 2 ( n - 1 ) d1 to 10 . But in the elements of the 12th group i.e Zn, Cd and Hg ,the incoming electron occupy the ns shell instead of ( n - 1 ) d subshell which are already saturated and they should therefore, be excluded from d-block elements , But, since they posses properties similar to those of d-block elements, for all practical purposes they are placed with d-block elements.
The d-block elements are classified into 4 series:-
a) 3d series containing 10 elements :-
They have incomplete 3d-orbital. Elements starting from Sc (21) to Zn (30) are present in these series.
b) 4d series containing 10 elements :-
They have incomplete 4d-orbital. Elements starting from Y (39) to Cd(48) are present in these series.
c) 5d series containing 10 elements :-
They have incomplete 5d-orbital. Elements starting from La (57) to Hf (72) to Hg (80) are present in these series.
d) 6d series containing 10 elements :-
They have incomplete 6d-orbital. This is the incomplete series.
2) Metallic Character :- All the d-block elements have 1 or 2 electrons in their outermost shell and thus all are metals. All are hard,ductile and malleable solids with strong bonding due to their greater effective nuclear charge and large number of valence electrons
(except Hg which is liquid) .
3) Atomic and Ionic Radii :- The Atomic and Ionic Radii of transition elements are smaller than their corresponding s-block elements and are greater than their corresponding p-block elements. The Atomic and Ionic Radii of transition elements for a given series show a decreasing trend for first 5 elements and then becomes almost constant for next 5 elements of the series. For eg :- In 3d series atomic radius decreases from Sc(21) to Mn(25) and then becomes constant for next five, i.e. Fe(26) to Zn(30). This is due to the combined effect of increasing effective nuclear charge and increasing screening effect along the period.
4) Atomic volume and density :- The size decreases along the period and therefore atomic volume also decreases along the period. Atomic volumes are smaller than group 1 and 2 members, i.e. s-block elements. The density,however,increases along the period.
5) Boiling and Melting points :- All the transition elements have high M.P. and B.P. , as compared to s-block elements due to their strong metallic bonding and unpaired d-electrons. M.P. of transition elements of a given series increases on moving from left to right in a period and attains a maximum value and after that M.P. goes on decreasing towards the end of period. These higher values are due to small atomic radii of transition elements which provides greater inter atomic forces of attraction. However Zn, Cd, Hg have relatively low values of M.P. and B.P. Due to their completely filled d-orbitals
6) Ionisation Energy :- The Ionisation energy of d-block elements lies in between the of s-block and p-block elements. Thus they are more electropositive than p-block elements and less electropositive than s-block elements. Since smaller being atomic size, thus high Ionisation Energy is noticed for transition elements. Ionization Energy values first increases upto Mn(25) and then becomes irregular or constant due to irregular trend of atomic size after Mn(25) in 3d series,also similar trend is noticed in 5d and 6d series. However Zn, Cd, Hg have abnormally high values of Ionization Energy on account of greater stability of s-subshell.
7) Oxidation state :- The Transition elements show variable oxidation state in their compounds. Reason for this variable oxidation state is that there is a very small energy difference in between (n-1)d and ns orbitals. As a result ,electrons of (n-1)d orbitals as well as ns-orbitals take part in bond formation. Variation in oxidation state is related to their electronic configuration. For eg :-
Ti : +2, +3, +4.
Cr : +1, +2, +3, +4, +5, +6.
Mn : +2, +3, +4, +6, +7.
Fe : +2, +3.
Cu : +1, +2.
8) Standard Electrode Potential and Reducing properties :- Expect Cu and Hg, the standard reduction potential of d-block elements in acid solution is generally negative, but however Cu and Hg are having positive electrode potential. The d-block elements are not good reducing agents in comparision to s-block elements due to their high heats of sublimation , high ionisation potential and low heats of hydration of their ions.
9) General Chemical Reactivity :- The d-block elements have been unreactive in comparision to s-block elements due to their high ionisation energy, high heats of sublimation and low heats of hydration.
But however Transition metals vary widely in their chemical reactivity. Many of them are sufficiently electropositive to dissolve in mineral acids, although a few are 'noble' ,that is, they areunaffected by simple acids. The metals of the first series with the exception of copper are relatively more reactive and can be oxidised.
Transition metals form alloys among themselves. The alloys of transition metals are hard and have high melting point as compared to other metal.
10) Conductivity :- All the Transition elements are good conductors of heat and electricity.
11) Colour :- Except zinc all the transition metals complex ions are colourless due to presence of unpaired electrons.
The colours exhibited by some transition metal ions are :-
1. Cu2+ (Blue).
2. Fe2+ (Green).
3. Ti3+ (Purple).
4. Mn3+ (Violet).
5. Fe3+ (Yellow).
12) Flame colouration :-The d-block elements impart no characteristic colour to the flame. We know that when an electron gains energy, it gets excited to a higher energy level, and when it looses that energy, it again comes back to the same level, by giving out some amount of radiation. This released energy appears in the form of colour. The d-block elements do not impart colour to the flame as their electron don't jump to the higher energy level when brought in the flame.
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