Class XII Lanthanoids & Actinoids

Class XII Lanthanoids & Actinoids

Sulekha Rani.R,P.G.T Chemistry

KV NTPC Kayamkulam

1. Actinoid contraction is greater from element to element than lanthanoid contraction. Why?

In actinoids, 5f orbitals are filled. These 5forbitals have a poorer shielding effect than 4f orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more that that experienced by lanthanoids. Hence, the size contaction in actinoids is greater as compared to that in lanthanoids.

2. What is lanthanoid contraction? What are the consequences of lanthanoid

contraction?

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

(i) There is similarity in the properties of second and third transition series.

(ii) Separation of lanthanoids not  possible due to lanthanide contraction.

(iii) 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.)

3. What are the different oxidation states exhibited by the lanthanoids?

In the lanthanide series, +3 oxidation state is most common i.e., Ln(III)

compounds are predominant. However, +2 and +4 oxidation states can

also be found in the solution or in solid compounds.

4. What are alloys? Name an important alloy which contains some of the lanthanoid metals. Mention its uses.

An alloy is a solid solution of two or more elements in a metallic matrix. It can

either be a partial solid solution or a complete solid solution. Alloys are usually

found to possess different physical properties than those of the component

elements.

An important alloy of lanthanoids is Mischmetal. It contains lanthanoids

(94−95%), iron (5%), and traces of S, C, Si, Ca, and Al.

Uses

(1) Mischmetal is used in cigarettes and gas lighters.

(2) It is used in flame throwing tanks.

(3) It is used in tracer bullets and shells.

5. Compare the chemistry of actinoids with that of the lanthanoids with special reference to:

(i) electronic configuration (iii) oxidation state

(ii) atomic and ionic sizes and (iv) chemical reactivity.

(i) Electronic configuration

The general electronic configuration for lanthanoids is [Xe]⁵⁴ 4f^0-14 5d^0-1 6s² and that for

actinoids is [Rn]⁸⁶ 5f^1-14 6d^0-1 7s². Unlike 4f orbitals, 5f orbitals are not deeply buried and

participate in bonding to a greater extent.

(ii) Oxidation states

The principal oxidation state of lanthanoids is (+3). However, sometimes we also

encounter oxidation states of + 2 and + 4. This is because of extra stability of fully-filled

and half-filled orbitals. Actinoids exhibit a greater range of oxidation states. This is

because the 5f, 6d, and 7s levels are of comparable energies. Again, (+3) is the principal

oxidation state for actinoids. Actinoids such as lanthanoids have more compounds in +3

state than in +4 state.

(iii) Atomic and lonic sizes

Similar to lanthanoids, actinoids also exhibit actinoid contraction (overall decrease in

atomic and ionic radii). The contraction is greater due to the poor shielding effect of

5f orbitals.

(iv) Chemical reactivity

In the lanthanide series, the earlier members of the series are more reactive. They have

reactivity that is comparable to Ca. With an increase in the atomic number, the

lanthanides start behaving similar to Al. Actinoids, on the other hand, are highly

reactive metals, especially when they are finely divided. When they are added to boiling

water, they give a mixture of oxide and hydride. Actinoids combine with most of the

non-metals at moderate temperatures. Alkalies have no action on these actinoids. In case

of acids, they are slightly affected by nitric acid (because of the formation of a protective

oxide layer).

6. What are inner transition elements? Decide which of the following atomic numbers are the atomic numbers of the inner transition elements: 29, 59, 74, 95, 102, 104?

Inner transition metals are those elements in which the last electron enters the f-orbital. The elements in which the 4f and the 5f orbitals are progressively filled are called f-block elements. Among the given atomic numbers, the atomic numbers of the inner transition elements are 59, 95, and 102.

7. The chemistry of the actinoid elements is not so smooth as that of the Lanthanoids. Justify this statement by giving some examples from the oxidation state of these elements?

Lanthanoids primarily show three oxidation states (+2, +3, +4). Among these oxidation states, +3 state is the most common. Lanthanoids display a limited number of oxidation states because the energy difference between 4f, 5d, and 6s orbitals is quite large. On the other hand, the energy difference between 5f, 6d, and 7s orbitals is very less. Hence, actinoids display a large number of oxidation states. For example, uranium and plutonium display +3, +4, +5, and +6 oxidation states while neptunium displays +3, +4, +5, and +7. The most common oxidation state in case of actinoids is also +3.

8. Which is the last element in the series of the actinoids? Write the electronic configuration of this element. Comment on the possible oxidation state of this element?

The last element in the actinoid series is lawrencium, Lr. Its atomic number is 103 and its electronic configuration is. The most common oxidation state displayed by it is +3; because after losing 3 electrons it attains stable f¹⁴ configuration.

11. Use Hund’s rule to derive the electronic configuration of Ce³⁺ ion and calculate its magnetic moment on the basis of ‘spin-only’ formula?

Magnetic moment can be calculated as:

Where, n = number of unpaired electrons

In Ce, n = 2

12. Name the members of the lanthanoid series which exhibit +4 oxidation state and those which exhibit +2 oxidation state. Try to correlate this type of behavior with the electronic configurations of these elements?

The lanthanides that exhibit +2 and +4 states are shown in the given table. The atomic numbers of the elements are given in the parenthesis.

+2	+4
Nd (60)	Ce (58)
Sm (62)	Pr (59)
Eu (63)	Nd (60)
Tm (69)	Tb (65)
Yb (70)	Dy (66)

Ce after forming Ce⁴⁺ attains a stable electronic configuration of [Xe].

Tb after forming Tb⁴⁺ attains a stable electronic configuration of [Xe] 4f^7.

Eu after forming Eu²⁺ attains a stable electronic configuration of [Xe] 4f^7.

Yb after forming Yb²⁺ attains a stable electronic configuration of [Xe] 4f^14.

13.