JEE Main - Lanthanoids and actinoids electronic configuration oxidation states and contraction E-Book Study Material

Lanthanoids and Actinoids

Electronic Configuration, Oxidation States, and Contraction

The f-block elements, also known as inner transition elements, are characterized by the filling of the f-orbitals. This block is divided into two series: the lanthanoids and the actinoids. These elements exhibit unique chemical properties due to the involvement of electrons in the (n-2)f orbitals in their chemical bonding and reactions.

Electronic Configuration

The electronic configuration of elements is crucial in determining their chemical behavior. For lanthanoids and actinoids, the filling of the f-orbitals follows specific rules, although there are some exceptions.

Lanthanoids

The lanthanoid series comprises elements from atomic number 57 (Lanthanum) to 71 (Lutetium). These elements involve the filling of the 4f orbitals. The general electronic configuration for lanthanoids is [Xe] 4f^1-14 5d^0-1 6s².

Let's look at the electronic configurations of a few key lanthanoids:

Lanthanum (La, Z=57): [Xe] 5d¹ 6s² (Note: 4f is empty)
Cerium (Ce, Z=58): [Xe] 4f¹ 5d¹ 6s²
Praseodymium (Pr, Z=59): [Xe] 4f³ 6s²
Neodymium (Nd, Z=60): [Xe] 4f⁴ 6s²
Promethium (Pm, Z=61): [Xe] 4f⁵ 6s²
Samarium (Sm, Z=62): [Xe] 4f⁶ 6s²
Europium (Eu, Z=63): [Xe] 4f⁷ 6s² (Half-filled f-orbitals, extra stable)
Gadolinium (Gd, Z=64): [Xe] 4f⁷ 5d¹ 6s²
Terbium (Tb, Z=65): [Xe] 4f⁹ 6s²
Dysprosium (Dy, Z=66): [Xe] 4f¹⁰ 6s²
Holmium (Ho, Z=67): [Xe] 4f¹¹ 6s²
Erbium (Er, Z=68): [Xe] 4f¹² 6s²
Thulium (Tm, Z=69): [Xe] 4f¹³ 6s²
Ytterbium (Yb, Z=70): [Xe] 4f¹⁴ 6s² (Fully filled f-orbitals, extra stable)
Lutetium (Lu, Z=71): [Xe] 4f¹⁴ 5d¹ 6s²

There are exceptions to the expected filling of orbitals. For instance, Gadolinium (Gd) has a 5d¹ electron, and Lutetium (Lu) has a 5d¹ electron. This is because a half-filled (4f⁷) or a fully-filled (4f¹⁴) f-subshell is particularly stable. Elements like Cerium (Ce) can have both 4f and 5d electrons.

Actinoids

The actinoid series includes elements from atomic number 89 (Actinium) to 103 (Lawrencium). These elements involve the filling of the 5f orbitals. The general electronic configuration for actinoids is [Rn] 5f^1-14 6d^0-1 7s².

The electronic configurations of actinoids are more complex and less well-established than those of lanthanoids, especially for the later elements. This is partly due to the similar energies of the 5f, 6d, and 7s orbitals, leading to greater variability.

Some examples of actinoid electronic configurations:

Thorium (Th, Z=90): [Rn] 6d² 7s² (Note: 5f is empty)
Protactinium (Pa, Z=91): [Rn] 5f² 6d¹ 7s²
Uranium (U, Z=92): [Rn] 5f³ 6d¹ 7s²
Neptunium (Np, Z=93): [Rn] 5f⁴ 6d¹ 7s²
Plutonium (Pu, Z=94): [Rn] 5f⁶ 7s² (Often written as 5f⁵6d¹7s², but 5f⁶7s² is more stable)
Americium (Am, Z=95): [Rn] 5f⁷ 7s² (Half-filled f-orbitals, extra stable)
Curium (Cm, Z=96): [Rn] 5f⁷ 6d¹ 7s²
Californium (Cf, Z=98): [Rn] 5f¹⁰ 7s²
Einsteinium (Es, Z=99): [Rn] 5f¹¹ 7s²
Fermium (Fm, Z=100): [Rn] 5f¹² 7s²
Mendelevium (Md, Z=101): [Rn] 5f¹³ 7s²
Nobelium (No, Z=102): [Rn] 5f¹⁴ 7s² (Fully filled f-orbitals, extra stable)
Lawrencium (Lr, Z=103): [Rn] 5f¹⁴ 7s² 7p¹ (Or [Rn] 5f¹⁴ 6d¹ 7s², but 7p¹ is more common)

Similar to lanthanoids, actinoids also show exceptions due to the stability associated with half-filled (5f⁷) and fully-filled (5f¹⁴) subshells.

Mnemonic for Lanthanoid Electronic Configurations: There is no simple mnemonic for all lanthanoid configurations due to exceptions. However, remembering the general pattern [Xe] 4f^x 5d^y 6s² and focusing on the stable configurations for Eu (4f⁷6s²), Gd (4f⁷5d¹6s²), Yb (4f¹⁴6s²), and Lu (4f¹⁴5d¹6s²) is key.

Mnemonic for Actinoid Electronic Configurations: Actinoid configurations are more variable. Focus on the general trend [Rn] 5f^x 6d^y 7s² and the stable configurations for Am (5f⁷7s²), Cm (5f⁷6d¹7s²), No (5f¹⁴7s²), and Lr (5f¹⁴7s²7p¹ or 5f¹⁴6d¹7s²).

Oxidation States

The oxidation states exhibited by f-block elements are a direct consequence of their electronic configurations. The outermost electrons are most readily involved in bonding, but the (n-2)f electrons can also participate, especially in actinoids.

Lanthanoids

The most common oxidation state for lanthanoids is +3. This is because after losing three electrons (two from 6s and one from 5d or 4f), they achieve a stable noble gas configuration or a configuration with a half-filled or fully-filled 4f subshell.

For example, after losing two 6s electrons and one 4f electron, Ce³⁺ ([Xe] 4f¹ 5d⁰ 6s⁰) is formed. Similarly, Yb³⁺ ([Xe] 4f¹³) and Lu³⁺ ([Xe] 4f¹⁴) are stable.

However, lanthanoids can exhibit other oxidation states, typically +2 and +4, though these are less common. The occurrence of these states depends on the stability of the resulting electronic configuration.

+2 oxidation state: Achieved when the resulting ion has a stable 4f⁷ or 4f¹⁴ configuration. Examples:
- Europium (Eu, [Xe] 4f⁷ 6s²) readily forms Eu²⁺ ([Xe] 4f⁷), which is stable.
- Ytterbium (Yb, [Xe] 4f¹⁴ 6s²) readily forms Yb²⁺ ([Xe] 4f¹⁴), which is stable.
+4 oxidation state: Achieved when the resulting ion has a stable 4f⁰, 4f⁷, or 4f¹⁴ configuration. Examples:
- Cerium (Ce, [Xe] 4f¹ 5d¹ 6s²) forms Ce⁴⁺ ([Xe] 4f⁰), which is stable.
- Praseodymium (Pr) and Neodymium (Nd) also show a +4 oxidation state, but it is less stable than Ce⁴⁺.
- Terbium (Tb) can also exhibit a +4 oxidation state.

The stability of oxidation states in lanthanoids generally follows the order: +3 >> +2, +4. The +3 oxidation state is predominant due to the balanced energy required to remove two 6s electrons and one 4f or 5d electron.

Actinoids

Actinoids exhibit a much wider range of oxidation states compared to lanthanoids. This is due to the comparable energies of the 5f, 6d, and 7s electrons, making it easier for them to participate in chemical bonding.

The most common oxidation state is +3, similar to lanthanoids. However, oxidation states from +3 up to +7 are observed.

+3 oxidation state: This is the most stable and common oxidation state for most actinoids, especially in their compounds.
+4 oxidation state: Observed in elements like Thorium (Th), Uranium (U), and Plutonium (Pu). For example, Th⁴⁺ ([Rn] 5f⁰ 6d⁰ 7s⁰) is very stable.
+5 oxidation state: Seen in elements like Protactinium (Pa), Uranium (U), and Neptunium (Np). For example, UO₂⁺ ion contains U in the +5 state.
+6 oxidation state: Exhibited by Uranium (U) and Plutonium (Pu). For example, the uranyl ion (UO₂²⁺) contains U in the +6 state.
+7 oxidation state: This is a very high oxidation state, observed only for Neptunium (Np) and Plutonium (Pu) under strongly oxidizing conditions, for example, in the NpO₅^3- and PuO₅^3- ions.

The variation in oxidation states for actinoids is more pronounced than for lanthanoids. The stability of oxidation states in actinoids generally increases from +3 to +4 and then decreases.

Key Difference in Oxidation States: Lanthanoids primarily show +3 oxidation state with occasional +2 and +4. Actinoids show a wider range of oxidation states, from +3 up to +7, with +3 being the most common. This is because the 5f electrons are less tightly held than the 4f electrons.

Lanthanoid Contraction

Lanthanoid contraction refers to the gradual decrease in the atomic and ionic radii of the lanthanoid elements as the atomic number increases across the series.

As we move from left to right across the lanthanoid series (from La to Lu), the atomic number increases by one at each step. This means one proton is added to the nucleus, and one electron is added to the outermost shell (4f orbitals).

While the nuclear charge increases, the added electron enters the 4f subshell. The 4f orbitals are located in the inner shells and have poor shielding or screening effects on the outer electrons. The poorly shielding 4f electrons do not effectively counteract the increasing attraction of the nucleus for the outer electrons.

Consequently, the effective nuclear charge experienced by the outer electrons increases gradually, leading to a contraction in the size of the atom and its ions.

Reasons for Lanthanoid Contraction:

Increasing Nuclear Charge: Each subsequent element has one more proton in the nucleus, increasing the attractive force.
Poor Shielding by 4f electrons: The 4f electrons are not very effective at shielding the outer electrons from the nucleus's pull. They are diffused and penetrate poorly.
Filling of 4f orbitals: Electrons are added to the (n-2)f orbitals, which are deep within the atom.

Consequences of Lanthanoid Contraction:

Lanthanoid contraction has several significant consequences:

Similarities in properties of elements in the next transition series: The contraction in radii across the lanthanoid series is so significant that the radii of elements in the second transition series (4d) are very similar to those in the third transition series (5d). For example, Zr (Z=40) and Hf (Z=72) have very similar atomic radii (Zr = 160 pm, Hf = 159 pm). This is because Hf follows the lanthanoids, and its radius is contracted due to the lanthanoid contraction effect.
Difficulties in separation: Due to their very similar chemical properties and ionic radii, it is very difficult to separate lanthanoids from each other.
Physical properties: Certain physical properties, like density and ionization enthalpy, show a gradual increase across the series.
Basicity of hydroxides: The basicity of the lanthanoid hydroxides decreases across the series (e.g., La(OH)₃ is more basic than Lu(OH)₃). This is because as the radius decreases, the covalent character of the M-OH bond increases, making the hydroxide weaker base.

Ionic Radii Trend (in pm):

Element	La³⁺	Ce³⁺	Pr³⁺	Nd³⁺	Pm³⁺	Sm³⁺	Eu³⁺	Gd³⁺	Tb³⁺	Dy³⁺	Ho³⁺	Er³⁺	Tm³⁺	Yb³⁺	Lu³⁺
Ionic Radius	106.1	101.4	99.0	98.3	97.5	95.8	95.0	93.9	92.3	91.2	90.1	88.1	86.9	85.8	84.8

Observe the steady decrease in ionic radii from La³⁺ to Lu³⁺.

Actinoid Contraction

Similar to lanthanoids, actinoids also exhibit contraction across their series, known as actinoid contraction. This is due to the filling of the 5f orbitals.

The 5f electrons have an even poorer shielding effect than the 4f electrons. As the nuclear charge increases across the actinoid series, the attraction of the nucleus on the outer electrons becomes stronger, leading to a significant decrease in atomic and ionic radii.

Reasons for Actinoid Contraction:

Increasing Nuclear Charge: Each element gains a proton.
Very Poor Shielding by 5f electrons: The 5f orbitals are even more diffused and penetrate less effectively than 4f orbitals, resulting in a weaker shielding effect.
Filling of 5f orbitals: Electrons are added to the (n-2)f orbitals.

Comparison with Lanthanoid Contraction:

The actinoid contraction is generally more pronounced than lanthanoid contraction. This is because the 5f orbitals are less effectively shielded than the 4f orbitals.

Consequences of Actinoid Contraction:

The consequences of actinoid contraction are similar to those of lanthanoid contraction:

Similarities in properties: Elements within the actinoid series show more similar properties than lanthanoids.
Difficulties in separation: Separation of actinoids is extremely challenging due to their similar chemical properties.
Physical properties: Physical properties show a gradual change across the series.

Key Takeaway: Lanthanoid contraction is due to the filling of 4f orbitals, leading to a decrease in size. Actinoid contraction is due to the filling of 5f orbitals, and it is even more pronounced due to the poorer shielding effect of 5f electrons. Both phenomena result in similarities in properties among elements within their respective series and make their separation difficult.