You are welcome in this website. We shall discuss about f block elements of chemistry 12th. As we know that this topic of d and f block elements is very important for all who are preparing either for board exams 12th or entrance exams for any technical institute. Lanthanoids and actinoids are important series of elements. The elements of these series show various unique features and properties than other elements of the periodic table.
We have included all the important topics of these elements in this notes. We hope that this notes will be very beneficial for you. We shall also study the comparative study of the elements of this block.
What are called f block elements?
The elements in which the last electron enters the antepenultimate energy level (n-2) orbitals are called f-block elements. These elements are also known as inner transition elements. Their general electronic Configuration is (n-2)f1-14 (n-1)d0-1 ns2. Thus, they have three incomplete shells as (n-2), (n-1) and nth shells.
Classification of f block elements:
Depending upon whether the last electron enters a 4f-orbital or 5f- orbital, the f- block elements have been divided into two series as below.
(1) Lanthanoids: the elements in which last electron enters one of the 4f- orbitals are called first inner transition series or lanthanoids. There are 14 elements in this series which are known as rare earths.
(2) Actinoids: the elements in which last electron enters one of the 5f- orbitals are called second inner transition series or actinoids because they come immediately after actinium of d-block elements.
Important atomic and physical properties of Lanthanoids:
(1) Electronic Configuration of Lanthanoids: The general outer- most electronic Configuration of lanthanoids is [Xe]4f1-145d0-16s2.
The electronic configurations are followings .
Cerium (ce) – 58- [Xe] 4f25d06s2
Praseodynium (pr) – 59- [Xe] 4f35d06s2
Neodymium (Nd) – 60- [Xe] 4f45d06s2
Promethium (PM) – 61- [Xe] 4f55d06s2
Samarium (Sm) – 62- [Xe] 4f65d06s2
Europium (Eu) – 63- [Xe] 4f75d06s2
Gadolinium (Gd) – 64- [Xe] 4f75d16s2
Terbium (Tb) – 65- [Xe] 4f95d06s2
Dysprosium (Dy) – 66- [Xe] 4f105d06s2
Holmium (Ho) – 67- [Xe] 4f115d06s2
Erbium (Er) – 68- [Xe] 4f125d06s2
Thulium (Tm) – 69- [Xe] 4f135d06s2
Ytterbium (Yb) – 70- [Xe] 4f145d06s2
Lutetium (Lu) – 71- [Xe] 4f145d16s2
(2) Oxidation states of Lanthanoids: The typical oxidation state of the lanthanoids is +3. The oxidation state of +2 and +4 are exhibited by some of the elements. These are shown by those elements which by losing 2 or 4 electrons acquire a stable Configuration of f0, f7 or f14. Each case tends to revert to the more stable oxidation state of +3 by loss or gain of an electron. That is why Sm2+, Eu2+ and Yb2+ ions in solutions are good reducing agents and aqueous solution of Ce4+ and Tb4+ are good oxidising agents. Lanthanoids show limited number of oxidation states because the energy gap between 4f and 5d subshells is large.
(3) Atomic and Ionic radii of Lanthanoids:
In lanthanoids series, with increase in atomic number, there is a progressive decrease in the atomic as well as Ionic radii of trivalent ions from La3+ to Lu3+. This regular decrease in the atomic and Ionic radii with increasing atomic number is known as lanthanoids contraction.
It may be noted that the decrease in atomic radii is not quite regular whereas for the Ionic radii of Ln3+ ions, it is quite regular.
Consequences of lanthanoid contraction in f block elements:
(a) Difficulty in separation of the elements in lanthanoid series: since the change in Ionic radii in the elements of this series is very small, their chemical properties are similar.
(b) similarities in size of elements belonging to same group of second and third series of transition elements.
(c) As the size of the lanthanoid ion decreases from La3+ to Lu3+, the covalent character of the hydroxide increases and hence the basic strength decreases. Thus, Lu(OH)3 is least basic.
Some important characteristics of Lanthanoids:
(1) All the lanthanoids are silvery white soft metals and tarnish rapidly in the air. Their hardness increases with increasing atomic number.
(2) They have high melting points in the range of 1000 to 1200K except samarium which is very hard and has a very high melting point.
(3) All of them have typical metallic structure and are good conductors of heat and electricity.
(4) The lanthanoids are silvery white metals but most of the trivalent ions are coloured, both in the solid state and in aqueous solution. This is due to the partially filled f-orbitals which permit f-f transition.
(5) The positive ions of lanthanoids are paramagnetic in nature in nature except La3+ and Lu3+. This property of the lanthanoids is due to presence of unpaired electrons in the incomplete 4f subshell.
(6) The first Ionisation enthalpies of lanthanoids are around 600KJ mol-1 and the second about 1200 KJ mol-1 which are comparable with those of calcium. Further, the loss of third electron is easier. Third Ionisation enthalpy is low if it leads to stable empty, half filled or full filled Configuration as is found in La, Gd and Lu.
(7) Their standard electrode potentials lie in the range of -2.2 to -2.4V. Europium has exception for which E0 value is -2.0V.
(8) They readily lose electrons and are thus good reducing agent.
(9) The lanthanoids do not have much tendency to form complexes due to low charge density because of their large size. The tendency to form complexes and their stability increases with increasing in atomic number.
Chemical behaviour of Lanthanoids in f block elements:
The first few elements of this series are quite reactive like calcium. But increasing with atomic number, their behaviour becomes similar to that of aluminum. Some important chemical properties are followings
(1) They combine with H2 on gentle heating. When heated with carbon, they form carbide. On burning in the presence of halogens, they form halides.
(2) They react with dilute acids to liberate H2 gas.
(3) They form oxides and hydroxide of the type M2O3 and M(OH)3 which are basic like alkaline earth metal oxides and hydroxides
Actinoids of f- block elements:
There are also 14 elements in this period. All the elements of this series lie in period 7th and group 3rd. They have been placed with actinium hence, the elements of this series are known as actinoids.
Electronic Configuration of actinoids:
The members of this series along with their electronic Configuration are given below:—-
Oxidation state of Actinoids:
Unlike lanthanoids, actinoids of f block elements show a large number of oxidation states. This is because of very small energy gap between 5f, 6d and 7s subshells. Hence, all their electrons can take part in bond formation. The dominant oxidation state is +3. Besides +3 state, actinoids also exhibit +4 oxidation state. Some actinoids show still higher oxidation states. The maximum oxidation state first increases up to middle of the series and then decreases.
Ionic radii and actinoid contraction:
The actinoids show contraction more than lanthanoids due to poor screening effects of the 5f electrons. As a result, the atomic radii of the atoms or ions of these metals decrease regularly across the series.
General characteristics of actinoids
(1) Like lanthanoids, actinoids are metals with silvery appearance.
(2) These metals are silvery white. However, actinoid cations are generally coloured. The colour of the cation depends upon the number of 5f electrons. The cations containing 0 or 7 electrons in 5f subshell are colourless.
(3) The actinoids like lanthanoids have high melting and boiling points . However, they do not show any regular trend with rise in atomic number.
(4) All the actinoids of f block elements except thorium and amercium have high densities.
(5) The actinoids have lower Ionisation enthalpies than lanthanoids because 5f is less penetrating than 4f and hence is more effectively shielded from the nuclear charge.
(6) Like lanthanoids, actinoids are strongly paramagnetic.
(7) All the actinoids of f block elements are radioactive. First few members have relatively long half lives.
(8) All the actinoids of f block elements are strong reducing agent.
Chemical behaviour of actinoids:
They are highly reactive metals especially in the finely divided state. A few properties are given below:—-
(1) They react with boiling water to give a mixture of oxide and hydroxide.
(2) They combine with most of the non- metals at moderate temperature.
(3) All these metals are attacked by hydrochloric acid but the effect of nitric acid is very small due to the formation of a protective layer of oxide on their surface.
(4) Alkalies have no action on them.
Comparison of Lanthanoids and Actinoids: similarities:
(1) Both show mainly an oxidation state of +3.
(2) Both are electropositive and very reactive.
(3) Both exhibit magnetic and spectral properties.
(4) Actinoids exhibit actinoid contraction like lanthanoid contraction shown by lanthanoids.
Difference between Lanthanoids and actinoids:
| Lanthanoids | Actinoids |
| (1) Besides +3, they show +2 and +4 oxidation state | (1) Besides +3, they show +4, +5, +6 and +7 oxidation state |
| (2) Most of their ions are colourless. | (2) Most of their ions are coloured. |
| (3) They have less tendency towards complex formation | (3) They have greater tendency towards complex formation. |
| (4) Lanthanoid compounds are less basic. | (4) Actinoid compounds are more basic. |
| (5) They do not form oxacations. | (5) They form oxacations. |
| (6) They are non-radioactive except promethium. | (6) They are radioactive. |