d – and f – Block Elements
→ The elements of the groups 3-12 in periodic table are known as d-block elements. In these elements valence electron enters to the penultimate d-subshell (.e.(n-1) d-subshell). These lie in between s-block and p block elements. The outer two orbitals of these elements are incompletely filled. These elements are known as transition elements.
Table 8.1: The position of transition elements in periodic table
→ In other words we can say that transition element is the one which has incompletely filled d-orbitals in its ground state or in any its oxidation states. The general electronic configuration of these elements is (n – 1)d1-10 ns0, 1 or 2
There are four transition series in the d-block. These are as follow :
→ 3d-series : In this series the last electron enters in 3d-subshell and its electronic configuration is 3d1-10 4s1-2. It includes the elements from atomic number 21 to 30. There are 10 elements in the series, which starts from Scandium (21Sc) and ends at Zinc (30Zn). It is also known a first transition series.
→ 4d-series : In this series the last electron enters in 4d-subshell and its electronic congfiguration is 4d1-105s1-2. It includes the elements having atomic number 39 to 48. There are 10 elements in this series, which starts from Yattrium (39Y) and ends at Cadmium (48Cd). It is also know as second transition series.
→ 5d-series : In this series the last electron enters in 5d1-106s1-2. It includes the elements having atomic number 57 and 72 to 80. Thus there are 10 elements in this series, which has Lanthanum (57La), Hafnium (72Hf) to Mercury (80Hg). It is also known as third transition series.
→ 6d-series : In this series the last electron enters in 6d-subshell and its electronic congfiguration is 6d1-10 7s1-2. It includes the elements having atomic number 89 and 104 to 112.
→ Thus there are 10 elements in this series which has Actinium (89Ac), Rutherfordium (104Rf) to Ununbium (112Uub). It is also known as fourth transition series. The elements with atomic number 104 to 112 are synthesised recently. These elements are knwon as trans actinoid sereis.
Summary and glossory
- Cerium is the most occuring lanthanides. It exists about 3 × 10-4 % on earth crust. It is twentysixth most abundant of all the elements.
- Monazite sand is the most common mineral of lanthanides.
- The E° value of Ce4+/Ce is +1.74 V so it can oxidise water but the reaction is very slow hence Ce (IV) is a good analytical reagent.
- Lathanides show limited number of oxidation atates because the energy gap between of and 5d subshells is large.
- It is very interesting to note that lanthanide ions with xf electrons have a similar colour to those with (14- x)f electrons e.g., La3+ and Lu3+ are colourless. Sm3+ and Dy3+ are yellow and Eu3+ and Th3+ are pink.
- Actinides show a large number of oxidation states. This is because of very small energy gap between 58, 6d and 78 subshells. Hence all their electrons can take part in bond formation.
- Th is used in atomic reactor and treatment of cancer.
- U and Pu are used as fuel in nuclear reactor.
- Misch metal (n = 95%, Fe = 5%, S, C, Ca, Al = traces). It is used in making ug based alloy. It is pysophoric alloy (emits spark when struck) used in making bullets, shells, ligher flints etc.
- Actinides are silvery white metals. Their cations are coloured due to f-f transition.
- The element, whose atom in ground state or ion in one of its common oxidation state has incomplete d-subshell, is known as transition element.
- The general electronic configuration of transition elements is (n-1)d1-10ns1-2.
- Tungstun (w) has the highest melting point among the d-block elements.
- Technetium (TC) is a synthetic transition element.
- All transition elements are d-block elements but all d-block element are not transition element.
- For example group-12 elements i.e. Zn, Cd, Hg are not considered as transition elements yet they belong to d-block because they have completely filled (n-1)d orbitals and they do not show typical transition elements properties.
- Group-3 elements i.e. Sc, Y, La, Ac are also differ from other transition elements as they do not show variable oxidation state. They show only +3 oxidation states. So their compounds are colourless and dimagnetic.
- From the above given reasons group-3 and group-12 elements are called ‘non-typical transition elements’ while the rest are called ‘typical transition elements’.
- Ruthenium (Ru) and Osmium (Os) show highest oxidation state i.e. +8 in their compounds.
- Transition metal compounds show isomorphism due to similarity in the size of their metal ions.
eg.FeSO4 7H2O and ZnSO4 . 7H2O.
- Transition metal are generally coloured both in solid as well as in aqueous state. The colour of compounds is due to presence of unpaired electron and d-d transition. The colour of some common ions are as follow
- The paramagnetism of transition metal ions can be calculated with the help of spin-only formula – µ = √n(n + 2) B.M. (where n = no. of unpaired electrons)