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The elements in which the last electron enters in (n-2)f orbitals are called f-block elements . In these elements last electron is added to the (third to the outer most shell) ante penultimate shell.They are also called inner transition elements. The valence shell electronic configuration of these elements can be represented as ,
(n-2) f 0,2-14, (n-1) d 0,1,2,ns2
They consist of two series of elements & are placed at the bottom of periodic table. These two series are 4f & 5f.
Lanthanides or Rare Earth elements or 4f – series –
When last electron enters in 4f orbital, then series of elements is 4f or lanthanide series. The Lanthanide series include 14 elements i- e. 58Ce – 71Lu.
Position of Lanthanides in Periodic table-
1) Electronic configuration –
The electronic configuration of lanthanides is [Xe] 4f 0-14 ,5d 0-2, 6 s2.
[54Xe – 1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s2,4d10,5p6 ]. The lanthanides involve the gradual filling of f- orbitals.The energy of 5d & 4f are closely similar . Gadolinium (64Gd) & (71Lu) Lutetium has 5d1 electron to make f -orbital half filled & full filled.
57 La (Lanthanum) – [Xe] 4f 0,5d1,6s2
58 Ce (Cerium) – [Xe] 4f 2,5d0,6s2
59 Pr (Praseodymium) – [Xe] 4f 3,5d0,6s2
60 Nd (Neodymium) – [Xe] 4f 4,5d0,6s2
61 Pm (Promethium) – [Xe] 4f 5,5d0,6s2
62 Sm (Samarium) – [Xe] 4f 6,5d0,6s2
63 Eu (Europium) – [Xe] 4f 7,5d0,6s2
64 Gd (Gadolinium) – [Xe] 4f 7,5d1,6s2
65 Tb (Terbium) – [Xe] 4f 9,5d0,6s2
66 Dy (Dysprosium) – [Xe] 4f 10,5d0,6s2
67 Ho (Holmium) – [Xe] 4f 11,5d0,6s2
68 Er (Erbium) – [Xe] 4f 12,5d0,6s2
69 Tm (Thulium) – [Xe] 4f 13,5d0,6s2
70 Yb (Ytterbium) – [Xe] 4f 14,5d0,6s2
71 Lu (Lutetium) – [Xe] 4f 14,5d 1,6s2
2) Oxidation state –
+3 oxidation state is common to all elements in which 2 electrons of s- subshell & one from d or f- subshell is used . 4f electrons are strongly screened by inner electrons of 5s & 5p electrons. So 4f electrons are not involved in the bonding. In addition some lanthanides show +2 & +4 oxidation state also.They are formed in case of f 0 , f7 , f14 configuration.
For ex. –
Ce4+ (f 0) ; Eu2+ & Tb4+ (f 7) ; Yb2+ (f 14)
3) Colour & Spectra –
Some trivalent ions (M3+) are coloured in solid as well as in solution. The colour of lanthanide ion is due to absorption in visible region of the spectrum resulting f-f transitions because they have partly filled orbitals. The colour of ions having nf electrons is about same with those having (14 – n)f electrons.
Pr3+ 4f 2 green (14 -n)f – electrons =14-2 =12
Tm3+ 4f 12 green nf – electrons =12
Nd3+ 4f 3 pink (14 -n)f – electrons =14-3 =11
Er3+ 4f 11 pink nf – electrons =11
Magnetic property –
Ions which contain all paired electrons are diamagnetic while those containing unpaired electrons are paramagnetic . Among the lanthanides , La3+ [4f 0] & Lu3+ [4f 14] are diamagnetic. All trivalent lanthanide ions are paramagnetic due to unpaired electrons.
Basic character of hydroxides –
All the lanthanides form hydroxides of the formula Ln(OH)3. These are ionic & basic. Since the ionic size decreases from La3+ to Lu3+ , the basicity of hydroxides decreases . La(OH)3 is strongest base while Lu(OH)3 is weakest base.
Atomic & ionic radius (Lanthanide Contraction)-
As we move along the lanthanide series , there is a decrease in atomic & ionic radii. This steady decrease in the atomic & ionic radii is called Lanthanide contraction.
Cause of Lanthanide Contraction –
In the lanthanide series , as we move from one element to another , the nuclear charge increases by one unit & one electron is added. The new electrons are added to the same inner 4f subshell. So 4f electrons shield each other from the nuclear charge quite poorly because of very diffused shapes of the f orbitals. Hence with increasing atomic number & nuclear charge, the effective nuclear charge experienced by each 4f electrons increases. So whole of 4f electron shell contracts at each successive element,so decrease is very small.