1 The f-Block Elements: The Inner Transition Elements The f-block elements are those which have partly filled f- sub shells of the third to the outermost (antepenultimate) i.e. (n-2) th energy shells in their elementary or ionic state. These elements are also called inner transition elements. The f-bock elements are divided into two series. 1. The Lanthanide series (4f) (Atomic number Z = 58-71) 2. The Actinide series (5f) (Atomic number Z = 90-103) Lanthanum (Z=57) and the next fourteen elements (Z=58-71) which follow it are called lanthanide or lanthanones. These fifteen elements closely resemble one another and forms a distinct group with lanthanum as the prototype, hence the name lanthanides or lanthanones. The reason why they resemble lanthanum so closely lies in their electronic configuration. The configuration of lanthanum is [Xe]5d 1 6s 2 and 14 electrons are successively added to 4f subshell. Occurrence: Lanthanides were originally called rare earth elements. The word ‘earth’ was used because they occurred as oxide (which is early usage meant earth) and word ‘rare’ was used because their occurrence was believed to be very scarce. Most commonly sources of lanthanides are India, Scandinavia, USA and Russia. Most occurring lanthanide is Cerium which constitute about 0.0003 % of the earth crust. Monazite sand is the most important mineral containing lanthanides (Thorium 30%). It is exist in the form of orthophosphate. Even atomic number Lanthanides are more abundant in nature and they having larger number of isotopes. Odd atomic number Lanthanides are less abundant and they do not have more than two isotopes. Table 1: The Lanthanide Elements and their naturally occurring isotopes. Elements Symbol Atomic Number No. of naturally occurring isotopes Lanthanum La 57 1 Cerium Ce 58 4 Praseodymium Pr 59 1 Neodymium Nd 60 7 Promethium Pm 61 0 Samarium Sm 62 7
Transcript
1
The f-Block Elements: The Inner Transition Elements
The f-block elements are those which have partly filled f- sub shells of the third to the
outermost (antepenultimate) i.e. (n-2)th energy shells in their elementary or ionic state. These
elements are also called inner transition elements. The f-bock elements are divided into two
series.
1. The Lanthanide series (4f) (Atomic number Z = 58-71)
2. The Actinide series (5f) (Atomic number Z = 90-103)
Lanthanum (Z=57) and the next fourteen elements (Z=58-71) which follow it are called
lanthanide or lanthanones. These fifteen elements closely resemble one another and forms a
distinct group with lanthanum as the prototype, hence the name lanthanides or lanthanones.
The reason why they resemble lanthanum so closely lies in their electronic configuration. The
configuration of lanthanum is [Xe]5d16s2 and 14 electrons are successively added to 4f
subshell.
Occurrence: Lanthanides were originally called rare earth elements. The word ‘earth’ was
used because they occurred as oxide (which is early usage meant earth) and word ‘rare’ was
used because their occurrence was believed to be very scarce. Most commonly sources of
lanthanides are India, Scandinavia, USA and Russia. Most occurring lanthanide is Cerium
which constitute about 0.0003 % of the earth crust. Monazite sand is the most important
mineral containing lanthanides (Thorium 30%). It is exist in the form of orthophosphate.
Even atomic number Lanthanides are more abundant in nature and they having larger number
of isotopes. Odd atomic number Lanthanides are less abundant and they do not have more
than two isotopes.
Table 1: The Lanthanide Elements and their naturally occurring isotopes.
Elements Symbol Atomic Number No. of naturally occurring isotopes
Lanthanum La 57 1
Cerium Ce 58 4
Praseodymium Pr 59 1
Neodymium Nd 60 7
Promethium Pm 61 0
Samarium Sm 62 7
2
Europium Eu 63 2
Gadolinium Gd 64 7
Terbium Tb 65 1
Dysprosium Dy 66 7
Holmium Ho 67 1
Erbium Er 68 6
Thulium Tm 69 1
Ytterbium Yb 70 7
Lutetium Lu 71 2
Table 2: The Lanthanide elements and their electronic configuration & oxidation states.
+3 oxidation states are the characteristic features in all the lanthanides.
Ce4+: [Xe]4f0 Tb4+: [Xe]4f7
Eu2+: [Xe]4f0 Yb2+: [Xe]4f14
f0, f7 and f14 are stable configuration that’s why Ce, Tb show +4 oxidation state and Eu, Yb
show +2 oxidation states.
Sm2+: [Xe]4f6 Tm2+: [Xe]4f13
Sm and Tm shows +2 oxidation state although they have not attain stable configuration.
These facts explain why these elements exercise oxidation states other than +3 as well. These
arguments, however does not hold good when we find that samarium and thulium show
oxidation state of +2.
Pr and Nd also show +4 oxidation states instead of +5 and +6 (this should attain
f0 configuration). These factors are based on highly complicated thermodynamic and kinetic
consideration which is still not quite certain. Ce4+ ion is as strong oxidising agent as MnO4- ;
Pr4+ and Th4+ are even powerful oxidising agents.
Ionic Radii: Lanthanide Contraction
The expected increase in atomic size, when we go down a group (e.g, from Sc to Y and from
Y to La) disappears after the lanthanides and the pairs of elements. Zr-Hf, Nb-Ta, Mo-W, Ru-
Os, Rh-Ir and Pd-Pt have almost the same size. It is thus a direct consequence of Lanthanide
contraction that elements of the second and third transition series resemble each other much
more closely than do the elements of the first and second transition series.
Cause of Lanthanide contraction
The mutual shielding effect of f electrons is very little, being smaller than that of d electrons.
This is due to the shape of one at each step. Hence, the inward pull experienced by the 4f
electrons increases. This causes a reduction in size of the entire 4f subshell and that leads to
the total lanthanide contraction.
4
Table 3: Atoms and their atomic size in angstrom.
Atoms Atomic size (in Angstrom)
La3+ 1.15
Ce3+ 1.11
Pr3+ 1.09
Nd3+ 1.08
Pm3+ 1.06
Sm3+ 1.04
Eu3+ -
Gd3+ 1.02
Tb3+ 1.00
Dy3+ 0.99
Ho3+ 0.97
Er3+ 0.96
Tm3+ 0.95
Yb3+ 0.94
Lu3+ 0.93
Colour: The colour appears to depend upon the number of 4f electrons. The colour of ions
containing x 4f electrons as about the same as those with (14-x) 4f electrons. The absorption
bands in the visible region of electronic spectra of rare earth ions in their compounds arise
because of the absorption of light in the visible range resulting in the transition of the
electrons of the ions from the lower energy 4f orbital to the higher energy 4f orbital (f-f
transition).
Table 4: Colour of Lanthanide Ions
Ion Number of 4f
electrons
Colour Ion Number of 4f
electrons
Colour
La3+ 0 Colourless Tb3+ 8 Pale pink
Ce3+ 1 Colourless Dy3+ 9 Yellow
5
Pr3+ 2 Green Ho3+ 10 Pale Yellow
Nd3+ 3 Lilac Er3+ 11 Pink
Pm3+ 4 Pink Tm3+ 12 Pale green
Sm3+ 5 Yellow Yb3+ 13 Colourless
Eu3+ 6 Pale pink Lu3+ 14 Colourless
Gd3+ 7 Colourless
Characteristic features of the spectra of the tripositive lanthanide ions are the sharpness of the
individual bands. Many of these bands are line like and become even narrower as the
temperature is lowered. This is again, due to the fact that the electrons in the 4f orbitals are
effectively shielded from the surrounding by the overlying electrons in the 5s and 5p orbitals
of the rare earth ions and hence absorption bands arise merely from electronic transitions
within the 4f level (f-f transition). Such transitions are more forbidden than the d-d transitions
of the transition metal ions since 4f electrons of lanthanide ions are much less affected by the
ligand electrons than the electrons in the d orbital of transition metal ions in their complexes.
Therefore, the selection rules are more strictly followed for transitions in the compounds of
lanthanides than in the compounds of transition metals.
Magnetic properties:
The La3+, Lu3+, Ce4+ and Yb2+ ions which have 4f0 or 4f14 electronic configuration are
diamagnetic. The rest of the trivalent lanthanide ions which contain unpaired electrons in the
4f orbitals are paramagnetic. The magnetic properties of the lanthanides are different from
those of the transition elements. The magnetic moments arise from two types of motion. The
spin motion of electron around its own axis produces magnetic moment called spin magnetic
moment while the orbital motion of electron around the nucleus produces magnetic moment
called orbital magnetic moment. The observed magnetic properties of a substance are thus the
result of both the spin magnetic moment and orbital magnetic moment.
In the case of transition elements, the d electrons of the metal ions interact strongly with the electrons of the ligands of the ligands (atoms, ions or molecules) surrounding the metal ion. Due to the electric field of the ligands, the orbital motion of the electrons gets restricted and thereby the orbital magnetic moment of these electrons gets almost quenched. The magnetic moment of d block elements thus arises from the contribution of spin motion of the electrons.
6
µ = √n(n+2)
n is the number of unpaired electron.
The above relationship is not valid in the case of f block elements. In the f block elements 4f
orbital are well shielded from the surroundings by the overlapping 5s and 5p subshells. As a
result the electric field of the ligands surrounding the ion does not restrict the orbital motion
of the electrons. The orbital contribution does not ignore in the case of f block elements. The
magnetic moment in such cases is given by the formula
µ = g√J(J+1); J is coupling constant and g is Lande’s splitting factor.
g = 1+ J(J+1)+S(S+1)-L(L+1)/2J(J+1) and (J is equal to l plus minus s)
Table 5: Magnetic moments (Theoretical and experimental) of tripositive Lanthanide ions
Ions Magnetic moments
Experimental Theoretical
La3+ 0 0
Ce3+ 2.3-2.5 2.54
Pr3+ 3.4-3.6 3.58
Nd3+ 3.5-3.6 3.62
Pm3+ - 2.68
Sm3+ 1.4-1.7 0.84
Eu3+ 3.3-3.5 0
Gd3+ 7.9-8.0 7.94
Tb3+ 9.5-9.8 9.72
Dy3+ 10.4-10.6 10.65
Ho3+ 10.4-10.7 10.60
Er3+ 9.4-9.6 9.56
Tm3+ 7.1-7.5 7.56
Yb3+ 4.3-4.9 4.54
Lu3+ 0 0
7
La3+ ion is diamagnetic because of its f0 configuration. The value increases upto Nd3+ ion and
then drops to 1.47 for Sm3+ ion. It starts rising again, becoming maximum for Dy3+ ion when
it is about 11. It again starts dropping zero for Lu3+ (f14 configuration) which is diamagnetic.
Separation of Lanthanides by Ion Exchange Chromatography:
