Lanthanide and Actinides
Lanthanides
The lanthanides are a series of 15 elements (atomic numbers 57-71) in the periodic table. They are characterized by the filling of their 4f orbitals. Lanthanide does not consider as lanthnoids as there is no electron in 4f orbital.
General electronic configuration of Lanthanides: [Xe] 4fn 5d0-1 6s2 (where n = 1 to 14).
or, [Xe] (n-2)f1-14 (n-1)d0-1ns2 (where n = 6)
There are a few exceptions to the general electronic configuration due to the stability of half-filled or fully filled 4f subshells.
Gadolinium (Gd, Z=64): [Xe] 4f7 5d1 6s2
Lutetium (Lu, Z=71): [Xe] 4f14 5d1 6s2
| Electronic Configuration of Lanthanides | |||
|---|---|---|---|
| Atomic Number | Name | Symbol | Electronic Configuration |
| 58 | Cerium | Ce | [Xe] 4f1 5d1 6s2 |
| 59 | Praseodymium | Pr | [Xe] 4f3 5d0 6s2 |
| 60 | Neodymium | Nd | [Xe] 4f4 5d0 6s2 |
| 61 | Promethium | Pm | [Xe] 4f5 5d0 6s2 |
| 62 | Samarium | Sm | [Xe] 4f6 5d0 6s2 |
| 63 | Europium | Eu | [Xe] 4f7 5d0 6s2 |
| 64 | Gadolinium | Gd | [Xe] 4f7 5d1 6s2 |
| 65 | Terbium | Tb | [Xe] 4f9 5d0 6s2 |
| 66 | Dysprosium | Dy | [Xe] 4f10 5d0 6s2 |
| 67 | Holmium | Ho | [Xe] 4f11 5d0 6s2 |
| 68 | Erbium | Er | [Xe] 4f12 5d0 6s2 |
| 69 | Thulium | Tm | [Xe] 4f13 5d0 6s2 |
| 70 | Ytterbium | Yb | [Xe] 4f14 5d0 6s2 |
| 71 | Lutetium | Lu | [Xe] 4f14 5d1 6s2 |
Oxidation Number of Lanthanides
The +3 oxidation state is the most stable and common for all lanthanoids. Some lanthanoids, such as Sm, Eu, Tm, and Yb, can also exhibit the +2 oxidation state. This occurs due to the stabilization of the half-filled or fully-filled f-orbitals. A few lanthanoids, like Ce, Pr, and Tb, can exhibit the +4 oxidation state, though it is less common.
Cerium can exhibit both +3 and +4 oxidation states. In the +4 state, cerium acts as a strong oxidizing agent. Europium can exist in both +2 and +3 oxidation states. The +2 state is relatively stable due to the half-filled 4f7 configuration. Samarium can exhibit +2 and +3 oxidation states but +3 state is more common.
| Oxidation States of Lanthanides | ||
|---|---|---|
| Elements | Symbol | Oxidation States |
| Lanthanum | La | +3 |
| Cerium | Ce | +3, +4 |
| Praseodymium | Pr | +3, +4 |
| Neodymium | Nd | +3 |
| Promethium | Pm | +3 |
| Samarium | Sm | +2, +3 |
| Europium | Eu | +2, +3 |
| Gadolinium | Gd | +3 |
| Terbium | Tb | +3, +4 |
| Dysprosium | Dy | +3 |
| Holmium | Ho | +3 |
| Erbium | Er | +3 |
| Thulium | Tm | +2, +3 |
| Ytterbium | Yb | +2, +3 |
| Lutetium | Lu | +3 |
Chemical Reactivity of Lacthanides
Lanthanides exhibit unique chemical reactivity due to their specific electronic configurations and the presence of the 4f orbitals. Some important reactions are given below-
Reactivity with Water
Lanthanoids react slowly with water to form hydroxides. The reactivity increases with atomic number that means heavier lanthanoids react more readily.
2Ln + 6H2O → 2Ln(OH)3 + 3H2
Reactivity with Oxygen
Lanthanoids react with oxygen to form oxides. Lanthanum and cerium can also form sesquioxides (Ln2O3).
4Ln + 3O2 → 2Ln2O3
Reactivity with Halogens
Lanthanoids react with halogens (F2, Cl2, Br2, I2) to form halides.
2Ln + 3X2 → 2LnX3
Reactivity with Acids
Lanthanoids react with acids to release hydrogen gas. They react more vigorously with dilute acids.
2Ln + 6HCl → 2LnCl3 + 3H2 ↑
Reactivity with Nonmetals
Lanthanoids can react with nonmetals like sulfur, nitrogen, and carbon to form sulfides, nitrides, and carbides, respectively.
2Ln + 3S → 2Ln2S3
2Ln + 3S → 2Ln2C3
2Ln + N2 → 2LnN
Actinides
The actinides are a series of 15 radioactive elements (atomic numbers 89-103) in the periodic table. They are characterized by the filling of their 5f orbitals.
General electronic configuration of the actinides: [Rn] (n-2)f0-14 (n-1)d0-1ns2 (where n = 7)
There are some exceptions to the general electronic configuration due to the stability of half-filled or fully filled 5f subshells, and the small energy difference between 5f and 6d orbitals.
| Electronic Configuration of Actinides | |||
|---|---|---|---|
| Atomic Number | Name | Symbol | Electronic Configuration |
| 89 | Actinium | Ac | [Rn] 6d1 7s2 |
| 90 | Thorium | Th | [Rn] 5f0 6d0 7s2 |
| 91 | Protactinium | Pa | [Rn] 5f2 6d1 7s2 |
| 92 | Uranium | U | [Rn] 5f3 6d1 7s2 |
| 93 | Neptunium | Np | [Rn] 5f4 6d1 7s2 |
| 94 | Plutonium | Pu | [Rn] 5f6 7s2 |
| 95 | Americium | Am | [Rn] 5f7 7s2 |
| 96 | Curium | Cm | [Rn] 5f7 6d1 7s2 |
| 97 | Berkelium | Bk | [Rn] 5f9 7s2 |
| 98 | Californium | Cf | [Rn] 5f10 7s2 |
| 99 | Einsteinium | Es | [Rn] 5f11 7s2 |
| 100 | Fermium | Fm | [Rn] 5f12 7s2 |
| 101 | Mendelevium | Md | [Rn] 5f13 7s2 |
| 102 | Nobelium | No | [Rn] 5f14 7s2 |
| 103 | Lawrencium | Lr | [Rn] 5f14 6d1 7s2 |
Oxidation Number of Actinides
Actinides exhibit a wide range of oxidation states due to the similar energies of their 5f, 6d, and 7s orbitals. The +3 oxidation state is common for most actinides, similar to lanthanoids. Many actinides, such as Th, Pa, U, Np, and Pu, exhibit the +4 oxidation state. Some actinides, such as U, Np, and Pu, can exhibit higher oxidation states (+5, +6, +7). These higher oxidation states are stabilized by the involvement of 5f and 6d orbitals in bonding. Higher oxidation states (e.g., +5, +6, +7) are relatively stable and more common compared to lanthanoids.
Uranium exhibits oxidation states of +3, +4, +5, and +6. The +6 state is particularly stable and common in uranium hexafluoride (UF6) and uranyl ions (UO22+).
Neptunium exhibits oxidation states of +3, +4, +5, +6, and +7. The +7 state is observed in neptunyl ions (NpO43−).
Plutonium exhibits oxidation states of +3, +4, +5, +6, and +7. The +6 state is common in plutonyl ions (PuO22+).
Mendelevium exhibit +2 and Nobelium exhibit +3 state in aqueous solutions.
| Oxidation States of Actinides | ||
|---|---|---|
| Elements | Symbol | Oxidation States |
| Actinium | Ac | +3 |
| Thorium | Th | +4 |
| Protactinium | Pa | +4, +5 |
| Uranium | U | +3, +4, +5, +6 |
| Neptunium | Np | +3, +4, +5, +6, +7 |
| Plutonium | Pu | +3, +4, +5, +6, +7 |
| Americium | Am | +3, +4, +5, +6 |
| Curium | Cm | +3, +4 |
| Berkelium | Bk | +3, +4 |
| Californium | Cf | +3, +4 |
| Einsteinium | Es | +3 |
| Fermium | Fm | +3 |
| Mendelevium | Md | +2, +3 |
| Nobelium | No | +2, +3 |
| Lawrencium | Lr | +3 |
Lanthanide Contraction and Consequence
As we move along the lanthanoid series, the atomic number increases gradually by one. This means that the number of electrons and protons present in an atom also increases by one. As electrons are being added to the same shell, the effective nuclear charge increases. This happens because the increase in nuclear attraction due to the addition of proton is more pronounced than the increase in the interelectronic repulsions due to the addition of electron. Also, with the increase in atomic number, the number of electrons in the 4f orbital also increases. The 4f electrons have poor shielding effect.
Therefore, the effective nuclear charge experienced by the outer electrons increases. Consequently, the attraction of the nucleus for the outermost electrons increases. This results in a steady decrease in the size of lanthanoids with the increase in the atomic number. This is termed as lanthanoid contraction.
Consequences of Lanthanoid Contraction
1. There is similarity in the properties of second and third transition series.
2. Separation of lanthanoids is possible due to lanthanide contraction.
3. It is due to lanthanide contraction that there is variation in the basic strength of lanthanide hydroxides. (Basic strength decreases from La(OH)3 to Lu(OH)3.)
| Comparision of Lanthanides and Actinides | |
|---|---|
| Lanthanides | Actinides |
| From left to right along 4f series, the decrease of atomic size is nor regular with increase in atomic number. But decrease of ionic radius is regular. | From left to right along 5f series, the decrease of atomic size and ionic radius is regular. |
| Along with +3 oxidation state, +2 and +4 oxidation state are also observed. | Along with +3 oxidation state, +4, +5, +6 and +7 oxidation state are also observed. |
| Lanthanide compounds are less basic. | Actinide compounds are more basic. |
| Ability to form complex compound is low. | Ability to form complex compound is high. |
| They does not form oxocation. | They form oxocation. |
| Paramagnetic (due to unpaired 4f electrons) | Paramagnetic and often more complex magnetic behavior due to 5f electrons |
| Generally light color compounds | Highly colored compounds |
| Except Pm, other lathanides are nonradioactive. | All actinides are radioactive. |
