The elements constituting the f block are those in which the 4f and 5f orbitally progressively filled.These elements are the member of group 3.The f block elements are also termed as inner transition elements.It is because literally speaking they constitute transition series within transition series (d-block elements). In addition to incomplete d-subshell, their f-subshell is also incomplete.
There are seven f-orbitals in a given shell.As much as fourteen electrons can be occupied in a given f block series.The general electronic configuration of the f block elements is (n–2)f1–14(n–1)d0–1ns2 .
f block elements: lanthanides and actinides
The f block elements can be subdivided into two series.It depends upon the nature of the f-orbital of the anti penultimate shell (4f or 5s) in which differentiating electron enters.
4 f Block Elements, First inner transition series, Lanthanides or Lanthanones
In these elements differentiating electron goes to 4f-orbitals. This series consists of lanthanum (Z = 57) and the next fourteen elements (Z = 58 to 71).
The name lanthanide has been derived from lanthanum which is the prototype of lanthanides. These elements are placed in the sixth period of the periodic table. Originally these elements were term as a rare-earths.It is because, for many years, pure compounds of these elements were difficult to obtain. Nowadays, the term rare-earth is avoided because many of these elements are far from rare.
5 f Block elements, Second inner transition series, Actinides or Actinones
In these elements differentiating electron goes to 5 f orbitals. This series includes fifteen elements from actinium (Z = 89) to lawrencium (Z = 103). The name actinide is derived from actinium, the very first member of the series.
Why f block elements placed outside separately
It is interesting to note that the f block elements (lanthanides and actinides) are placed outside the body of the periodic table. The reason for this is the remarkable similarities among the chemical properties of the lanthanides and also among the various members of actinides. The similarities in properties, in turn, is due to the similar electronic configuration of the outermost shell. These elements differ only in the number of f-electrons which do not take part in chemical bonding (difference from d-block elements in which differentiating d-electrons are involved in chemical interaction).
In case, these elements were placed inside the body of the periodic table, the very purpose of the periodic law (i.e. the elements arranged in the periodic table must show series of successive changes in their physical and chemical properties) would have been defeated.
This video helps you in writing electron configuration of f block elements.
Characteristics of f block Elements: Lanthanides
Although La57 and the following elements up to Lu71 resemble each other in their properties, lanthanum itself is studied in d-block elements and not f block.It is because here f-orbital has no electron. The f-orbital actually starts filling at cerium Ce (Z = 58) and is completely filled at lutetium, Lu (Z = 71) and hence elements from cerium (Z = 58) to Lu (Z = 71) are actually grouped as 4 f block elements. Further, since these fourteen elements follow lanthanum, these are also termed as lanthanides or lanthanones.
The electronic configuration represented as:
[Xe] 4f n+1 5d° 6s2 or [Xe] 4f n 5d1 6s2
The valence shell electronic configuration is 4f 1−146s2.
The lanthanides too display variable oxidation states but much less than those displayed by the transition elements. The characteristic and the most stable oxidation state of the lanthanides is +3(Ln3+ ).
Ionic radii (f block elements: Lanthanide contraction)
It consists of a decrease in the size of atoms/ions with the increase in atomic number as we move across from La to Lu. Thus among lanthanides, lanthanum has the largest and lutetium has the smallest radius. This slow decrease in size is the lanthanide contraction.
Consequences of lanthanide contraction
Lanthanide contraction plays a significant role. The three important consequences of lanthanide contraction are discussed below.
An occurrence of yttrium (a transition element) with heavier lanthanides
Since the size of y3+ ion (~ 0.90 Angstrom) is comparable to the heavier lanthanide ions, viz. Tb3+, Dy3+, Ho3+ and Er3+, the former occurs with the latter in natural minerals. Actually, the crystal structure, solubility and chemical properties of the yttrium compounds are so close to those of the corresponding compounds of the heavier lanthanides that the heavier lanthanides are commonly referred to as yttrium earths.
The basic strength of the oxides and hydroxides of lanthanides decreases with increase in atomic number.Broadly speaking, basicity is a measure with which a species loses electrons. Further greater the size of the atom or ion, greater will be the ease with which the species will lose electrons. Thus among lanthanides, La3+, being the largest Ln3+, is the most basic while Lu3+ being the smallest Ln3+, is the least basic.
Similarity 2nd and 3rd transition series
Similarities in properties among the corresponding members of the second and third transition series, such as Zr/Hf, Nb/Ta, Mo/W etc. Transition elements of the third series (i.e. the transition elements following the lanthanides) have virtually the same atomic and ionic sizes as the corresponding elements just above them in their respective sub-groups.
Due to equality in the size of Zr and Hf; Nb and Ta; Mo and W, etc the two elements of a pair have almost the same properties and hence they occur together in nature and are difficult to separate.
f Block Elements: Colour in lanthanide
The lanthanide metals are silvery white.The trivalent lanthanide ions are coloured both in the solid state and in aqueous solution. The colours are unchanged even on alteration of the anions indicating that they are characteristic of the cations.
La3+ (4f0)and Lu3+ (4f14), having no unpaired electron; these do not show paramagnetism while all other tri positive ions of lanthanides are paramagnetic.
Although the lanthanide ions have a high charge (+3), their large size (0.85 – 1:03 Angstrom) imparts them low charges density (charge to size ratio) with the result they cannot cause much polarisation and hence do not have much tendency to form complexes. Their complexes with unidentate ligands like β-diketones, oximes and ethylene diamine tetra-acetate (EDTA) are fairly common.
Reactivity of lanthanides
Since all the lanthanides have similar outer electronic configuration and display mainly +3 oxidation state in their compounds, they have nearly similar chemical properties. Their similarity is much more than that among the ordinary transition elements because lanthanides differ mainly in the number of 4f electrons which are very effectively shielded from interaction with other elements by the overlying 5s, 5p and 6s electrons. Moreover, due to lanthanide contraction, there is a very small difference (maximum 0:21 Angstrom) among the size of the trivalent lanthanide cations. Thus, for practical purposes, the size of these ions is almost identical which results in similar chemical properties of these elements.
The lanthanides are highly reactive which is an agreement with the values of their ionisation energy and electronegativity. The ionisation energies of lanthanides are quite comparable with those of alkaline earth metals particularly calcium. Thus like alkaline earth metals, lanthanides are highly electropositive and very reactive metals.
Uses of lanthanoids
- Used for the production of alloy steels for plates and pipes. e.g. mischmetal which consists of lanthanoid metal (~95%) and iron (-5%) and traces of S, C, Ca and AI. Mischmetal is used in Mg based alloy to produce bullets, shell and lighter flint.
- Mixed oxides of lanthanoids are employed as a catalyst in petroleum cracking.
- Some individual Ln oxides are used as phosphors in television screens and similar fluorescing surfaces.
- Because of their paramagnetic and ferromagnetic character, their compounds are used in making magnetic & electronic devices.
- Ceric sulphate is an oxidising agent in volumetric analysis.
Characteristics of f block Elements: Actinoids
Actinoids (5f-series): The actinoids include the fourteen elements from Th to Lr. The actinoids are radioactive elements and the earlier members have relatively long half lives, the latter ones have half life values ranging from a day to 3 minutes for lawrencium (Z=103).
All the actinoids are believed to have the electronic configuration of 7s2 and variable occupancy of the 5f and 6d subshell. The fourteen electrons are formally added to 5f, though not in thorium (Z= 90) from Pa onwards, the 5f orbitals are complete at element 103. The irregularities in the electronic configuration of the actinoid, like those in the lanthanoids, are related to the stabilities of the f0, f7 and f14 occupancies of the 5f orbitals. Thus the configurations of Am and Cm are (Rn) 5f 7s2 and [Rn] 5f76d17s2 .
Ionic Sizes in Actinoids
The general trend in lanthanoids is observable in the actinoids as well. There is a gradual decrease in the size of atoms or M3+ ions across the series. This may be referred to as the actinoids contraction (like lanthanoids contraction). The contraction is, however, greater from elements to an element in this series resulting from poor shielding by 5f electrons.
There is a greater range of oxidation states, which is in part attributed to the fact that the 5f, 6d and 7s levels are of comparable energies.
The actinoids show in general +3 oxidation state. The elements, in the first half to the series frequently exhibit higher oxidation state. e.g. The maximum oxidation state increases from +4 in Th to +5, +6 and +7 respectively in Pa, U and Np but decreases in succeeding elements. The actinoids resemble the lanthanoids in having more compounds in +3 state than in the +4 state. However, +3 and +4 ions tend to hydrolyze.
Uses of actinoids
1. Thorium is used in atomic reactors and in the treatment of cancer. Its salts are used in making incandescent gas mantles.
2. Uranium is used as a nuclear fuel. Its salts are used in glass industry (for imparting green colour), textile industry, ceramic industry and in medicines.
3. Plutonium it is used as a fuel for atomic reactors as well as for making atomic bombs.
Lanthanides and Actinides Comparison
1. The actinoids metals are all silvery white in appearance but display a variety of structures. The structural variability is obtained due to irregularities in metallic radii which are far greater than in lanthanoids.
2. The actinoids are highly reactive metals, especially when divided, the action of boiling water on them, for example, gives a mixture of Oxide and hydride and Combination with most metals takes place at moderate temperatures, hydrochloric acid attacks all metals but most are slightly affected by nitric acid owing to the formation of protective oxide layers, alkalies have no action.
3. It is evident from the chemistry of lanthanoids that the ionisation enthalpies of the early actinoids, though not accurately known but are lower than for the early lanthanoids. This is quite reasonable since it is to be expected that when 5f orbitals are beginning to be occupied, they will penetrate less into the inner core of electrons. The 5f electrons, will, therefore, be more effectively shielded from the nuclear charge than the 4f electrons of the corresponding lanthanoids. Because the outer electrons are less firmly held, they are available for bonding in the actinoids.
This is about the f block elements.
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