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A gradual increase in the nucleus attraction for outer electrons creates a decrease in the size. This is called Lanthanoids Contraction. As the atomic number increases each succeeding element contains one more electron in the 4f orbital and one proton in the nucleus. The 4f electrons are ineffective in screening the outer electrons from the nucleus causing imperfect shielding.
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Keyterms: Electron, Lanthanoids, nucleus, proton, imperfect shielding, elements, lanthanum, barium, hafnium, subshells, electronic configuration
What are Lanthanoids?
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The f- block (inner transition) elements containing partially filled 4f-subshells are known as Lanthanoids because of their close similarities with element lanthanum. All Lanthanoids are weighted between barium (z=72) and hafnium (z=72) and therefore must be placed between these two elements.
However, a few Lanthanoids have restricted utilizations, individuals from this gathering are found in everything from cigarette lighters to TV screens, and from hued glass to control poles in atomic reactors. Lanthanoid series is typically joined with lanthanum, which has an atomic number of 57, under the overall heading of Lanthanoids.
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| Related Concepts | ||
|---|---|---|
| Lawrencium | Group 18 Elements | Lanthanoids Contraction |
| Actinoids | Colour transition elements | Magnetic properties transition elements |
Electronic Configuration of Lanthanoids
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Given below is electronic configuration of all elements series. The ground state electron configuration of the Lanthanoid elements is generally of the type (Xe)4f n 6s2.
| Name | Symbol | Atomic number | Electron configuration |
|---|---|---|---|
| lanthanum | La | 57 | (Xe)5d1 6s2 |
| Cerium | Ce | 58 | (Xe)4f1 5d1 6s2 |
| Praseodymium | Pr | 59 | (Xe)4f3 6s2 |
| Neodymium | Nd | 60 | (Xe)4f4 6s2 |
| Promthium | Pm | 61 | (Xe)4f5 6s2 |
| Samarium | Sm | 62 | (Xe)4f6 6s2 |
| Europium | Eu | 63 | (Xe)4f7 6s2 |
| Gadolinium | Gd | 64 | (Xe)4f7 5d1 6s2 |
| Terbium | Tb | 65 | (Xe)4f9 6s2 |
| Dysprosium | Dy | 66 | (Xe)4f10 6s2 |
| Holmium | Ho | 67 | (Xe)4f11 6s2 |
| Erbium | Er | 68 | (Xe)4f12 6s2 |
| Thulium | Tm | 69 | (Xe)4f13 6s2 |
| Ytterbium | Yb | 70 | (Xe)4f14 6s2 |
| Lutetium | Lu | 71 | (Xe)4f14 5d1 6s2 |
Oxidation State of Lanthanoids
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The principal oxidation state of all the Ln elements is +3. Cesium shows +4 oxidation state, which is stable in solutions as well. +2 oxidation state is stable in Eu and Yb due to 4f7 (half filled) and 4f14 (filled) electronic configuration. Sm and Tm also show +2 oxidation state in some cases.
The order of penetration of orbitals into the inner electron core decreases as 4f>5d>6s. As successive ionization increases the net charge on the Lanthanoid cation, cases. The formation and stabilization of any ion in a particular oxidation state may be depicted in a relevant Born–Haber cycle of several enthalpy terms like (76) LanthanoidS AND ACTINIDES sublimation, ionization, hydration of the ion etc. The oxidation states of Ln are thus collective effects of several factors.
Chemical Reactivity
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Lanthanoids will in general respond with oxygen to frame oxides. The response at room temperature can be slow while hotness can make the response happen quickly.
The Lanthanoid Contraction alludes to the lessening in atomic size of the elements wherein electrons fill the f-subshell. Since the f subshell isn't safeguarded, the atomic size will diminish as the atomic charge actually increments.
Ionization Energy
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The basis of the Lanthanoid series separation on an ion exchange column is their ability to form complex ions. All Lanthanoids form +3 ions, M+3 whose ionic radii decrease progressively with increasing atomic number from Ce+3 to Lu+3. As a solution containing +3 Lanthanoids ions is placed at the top of a column of cation exchange resin [e.g. is Dowex-50 made of a sulphonated polystyrene and contains functional groups –SO3H.]
Ln+3 (aq) + 3H +R- (s) à Ln+3 (R -) (s) + 3H + (aq)
Properties of Lanthanoids Contraction
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Physical Properties
Some general physical properties of Lanthanoid elements are:
- They are metals with silvery white color appearance.
- They are generally having high melting and boiling point and very hard in nature
- They are good conductor of heat and electricity many Lanthanoid ions form colored ions
- The Lanthanoids exhibit principal oxidation state of +3
- They exhibit paramagnetic properties because presence of unpaired electrons
Magnetic Properties
The presence of paramagnetic properties is found due to the presence of unpaired electrons in it. Since in la+3 and lu+3 ions have no unpaired electrons, they are not paramagnetic but are diamagnetic. All other ln+3 ions show paramagnetic property.
Since for most of the ln+3 ions, the energy difference between the two successive j values of a multiplet, there is a strong l-s coupling. In these ions the unpaired electrons in (n-2) f orbitals are quite deeply seated and hence are well shielded by 5s- and 5p- electrons from the effects of other atoms in their compounds. These effective magnetic moments of ln+3 ions, except sm+3and eu+3 is given by following equation
µeff = µj = g√J ( J + 1)B.M
Where g is the lande splitting factor and is given by:
G = 1 +j(j+1) + s(s+1) – l(l+1) 2 j (j+1)
S = resultant spin quantum number
L = resultant orbital quantum number
J = resultant inner quantum number
In most of ln+3 ions, there is almost good agreement between the calculated and experimental values except sm+3 and eu+3 ions.
(ii) we know that for ln+3 ions like la+3(4f 0), gd+3(4f 7) and lu+3(4f 14) which have “s” term symbol, so l=0, i.e., no orbit effect. For these ions when l=0, j=s and hence g=2.
Thus equation (i) reduced to
µeff= µs = 2 √s (s + 1) b.m.
Or µeff = µspin only = 2 √ n 2 (η/2) + 1 = √n (n + 2) b.m
By using this equation, the µs and µj values for la+3, gd+3and lu+3 ions are the same.
Applications of Lanthanoids
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- Use as catalysts: Lanthanoid catalysts have been repeatedly recommended for use in numerous organic reactions, including the hydrogenation of ketones to form secondary alcohols, the hydrogenation of olefins to form alkanes, the dehydrogenation of alcohols and butanes, and the formation of polyesters.
- Other applications: Another significant industrial application of rare earths is in the manufacture of strong permanent magnets. Alloys of cobalt with rare earths, such as cobalt– samarium, produce permanent magnets that are far superior to most of the varieties now on the market
- Use in the television industry: It has been found that if a small amount of europium oxide (Eu2O3) is added to yttrium oxide (Y2O3), it gives a brilliant-red phosphor. Colour television screens utilize red, green, and blue phosphors.
- Use in the metallurgical industry: Small amounts of misch metal and cerium have long been added to other metals or alloys to remove their nonmetallic impurities. Misch metal added to cast iron makes a more malleable nodular iron.
Consequences of Lanthanoid Contraction
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High density of post Lanthanoid elements- because of Lanthanoid contraction the atomic sizes of the post Lanthanoid elements become very small. The arrangement of atoms in metallic grid is a lot of minimal that the densities are exceptionally high. The density of 2nd transition series is slightly higher than 1 st transition series, while the densities of 3rd transition series is almost double than 2 nd transition series.
Basic character of oxides, ln2o3 and hydroxides, ln (oh)3- it is observed that there is decrease in basic strength of oxides and hydroxides of Lanthanoids with increase in atomic number. The basicity decreases as ionic radii decreases. The basicity of ln+3 ions may be expected to decreases in the order, la+3> ce+3> pr+3 ....> lu+3 .
These differences in basicity are reflected in (a) thermal decomposition of oxy-salts. Basically, more basic oxy- salts decompose less easily (b) hydrolysis of ions- more basic ions hydrolyse less readily (c) solubilities of salts (d) formation of complexes and (e) decreasing ease of oxidation of the metals with increasing atomic number – oxidation potential for the couple ln→ ln+3 + 3eregularly goes on decreasing.
Little variety in the properties because of Lanthanoid contraction permits the division of Lanthanoids by the techniques dependent on partial crystallization and basicity differences.
Things to Remember
- The Lanthanoids are located on the top row of the two rows of elements found below the main body of the periodic table.
- All of the Lanthanoid elements are silver-colored, reactive solid metals that tarnish in air.
- always remember Lanthanoids react exothermically with hydrogen gas.
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Sample Questions
Ques. What are characters shown by oxide post contraction? (1 Mark)
Ans. Basic character of oxides, ln2o3 and hydroxides, ln (oh)3- there is decrease in basic strength of oxides and hydroxides of Lanthanoids with increase in atomic number. The basicity decreases as ionic radii decreases. The basicity of ln+3 ions may be expected to decrease in the order, la+3> ce+3> pr+3 ....> lu+3.
Ques. State the oxidation state of Lanthanoid. (2 Marks)
Ans. The principal oxidation state of all the Ln elements is +3. Cesium shows +4 oxidation state, which is stable in solutions as well. +2 oxidation state is stable in Eu and Yb due to 4f7 (half filled) and 4f14 (filled) electronic configuration. Sm and Tm also show +2 oxidation state in some cases.
Ques. What is reason behind Lanthanoid contraction? (2 Marks)
Ans. The Lanthanoid contraction is because of the flawed shielding of one 4f electron by one more in the equivalent subshell. With an expansion in the nuclear number, the positive charge on the nucleus increments by one unit, and another electron enters the equivalent 4f subshell. The 4f orbital is too diffused to even think about shielding the nucleus adequately, along these lines there is a continuous expansion in the viable atomic charge experienced by the external electrons.
Ques. How would you account for the following? (TN board 2019)
(i) many of the transition elements and then compounds can act as good catalysts.
(ii) the metallic radii of the third (5d) series of transition elements are literally the same as those of the corresponding members of the second series.
(iii) there is a greater range of oxidation states among actinides than those of Lanthanoids. (3 Marks)
Ans. (i) the catalytic activity of transition elements is attributed to the following reasons:
In view of their variable oxidation states change metals structure shaky intermediates and give another way of lower initiations energy for the response.
At times, the change metal furnishes an appropriate enormous surface region with free opportunities, on which reactants are ingested.
(ii) this is due to filling of 4/orbitals which have poor shielding effect or due to lanthanoid contraction.
(iii) this is due to comparable energies of 5f, 5d and 7s orbitals in actinoids.
Ques. How would you account for the following?
(i) Transition metals exhibit variable oxidation states.
(ii) Zr (Z = 40) and Hf (Z = 72) have almost identical radii.
(iii) Transition metals and their compounds act as catalyst. (3 Marks) [Delhi 2013]
Ans. (i) Because the energy difference between (n-1) d-orbitals and ns-orbitals is very less. Since there is very little energy difference between these orbitals, both energy levels can be used for bond formation. Thus transition elements exhibit variable oxidation states.
(ii) Zr and Hf have almost identical radii due to lanthanoid contraction which is due to weak shielding of d-electrons.
(iii) The catalytic properties of the transition elements are due to the presence of unpaired electrons in their incomplete d-orbitals and variable oxidation states.
Ques. What is lanthanoid contraction? What are the consequences of lanthanoid contraction? (4 Marks) [Comptt. Delhi 2015]
Ans. Lanthanoid contraction: The overall decrease in atomic and ionic radii with increasing atomic number from La to Lu due to imperfect shielding of 4f-orbital is known as lanthanoid contraction. Cause : As we move along the lanthanoid series, the effective nuclear charge increases in addition to electrons and the electrons added in the f-subshell causes imperfect shielding which is unable to counterbalance the effect of the increased nuclear charge. Hence the contraction in size occurs.
Consequences: (i) Due to small changes in atomic radii, the chemical properties of lanthanoids are very similar due to which separation of lanthanoids becomes very difficult.
(ii) There is similarity in size of elements belonging to the same group of second and third transition series.
Example: Zr and Hf are known as chemical twins due to their almost identical radii.
Ques. (a) Why do transition elements show variable oxidation states?
(i) Name the element showing the maximum number of oxidation states among the first series of transition metals from Se (Z = 21) to Zn (Z = 30).
(ii) Name the element which shows only +3 oxidation state.
(b) What is lanthanoid contraction? Name an important alloy which contains some of the lanthanoid metals. (5 Marks) [All India 2013]
Ans. (a) Because the energy difference between (n-1) d-orbitals and ns-orbitals is very less. Since there is very little energy difference between these orbitals, both energy levels can be used for bond formation. Thus transition elements exhibit variable oxidation states.
(i) It is Manganese (Mn) which shows oxidation states from +2 to +7
(ii) Scandium (Sc) shows only +3 oxidation state.
(b) Lanthanoid contraction: The overall decrease in atomic and ionic radii with increasing atomic number is known as lanthanoid contraction. In going from La+3 to Lu+3 in lanthanoid series, the size of ion decreases. This decrease in size in the lanthanoid series is known as lanthanoid contraction. The lanthanoid contraction arises due to imperfect shielding of one 4f electron by another present in the same subshell.
Consequences: (i) Similarity in properties: Due to lanthanoid contraction, the size of elements which follow (Hf – Hg) are almost similar to the size of the elements of the previous row (Zr – Cd) and hence these are difficult to separate. Due to small change in atomic radii, the chemical properties of lanthanoids are very similar due to which separation of lanthanoid becomes very difficult.
(ii) Basicity difference: Due to lanthanoid contraction, the size decreases from La+3 to Lu+3. Thus covalent character increases. Hence the basic character of hydroxides also decreases i.e. why La(OH)3 is most basic while Lu(OH)3 is least basic.
Important alloy: Mischmetal alloy which contains 95% lanthanoids and 5% Fe.
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