Lanthanide shrinkage isLanthanideIt is a phenomenon that the atomic radius and ionic radius of the atom decrease with the increase of atomic number.The atomic radius shrinks slowly, and the difference between adjacent atomic radii is only about 1 pm, but after 15 elements from La to Lu, the atomic radius shrinks and accumulates as much as 14 pm.The contraction of ionic radius is much more obvious than that of atomic radius.
Chinese name
Lanthanide shrinkage
Foreign name
Lanthanide contraction
Alias
rare earth element
Meaning
Element of the sixth cycle starting from lanthanide
The atomic radius of lanthanide metal decreases gradually from lanthanum to lutetium (in accordance withperiodic table of ele mentsThe radius is regular, that is, the radius of the same cycle decreases from left to right, and the radius of the same family increases from top to bottom.For the trivalent lanthanide metal ions, the total decrease is 21.3 pm from left to right, and the average decrease is about 1.5 pm for every two ions.[1]The radius difference between adjacent elements of lanthanide is abnormal for non transition metals and other transition metals, which is called lanthanide shrinkage.
The electron arrangement in the lanthanide element is filled in the inner 4f energy level one after another. Because the f energy level is too scattered, its extension size in space is relatively large, so that the 4f electrons are not completely shielded from the atomic nucleus, and can not shield the atomic nucleus as effectively as the electrons in the s, p, d energy levels. Therefore, as the atomic number increases, the outer electrons sufferEffective nuclear chargeThe number is also increasing (larger than the effective nuclear charge number of s, p and other energy levels), so the outer radius is reduced.
In addition, the shielding between 4f electrons is similar to the above reason, resulting in the reduction of 4f energy level radius.Entire electronicsShielding effectInfluence causes lanthanide shrinkage[2]。
From lanthanum to lutetium, the atomic radius shrinks by 15 picometers, and the radius shrinks by 1 picometre for every increase in nuclear charge on average.The radii of europium and ytterbium are obviously large, while cerium is slightly smaller than praseodymium.This is due to the fact that europium and ytterbium in the lanthanide ions are semi filled and fully filled configurations respectively, cerium is+4 valence ion, and the rest is+3 valence ion.From La3+To Lu3+The ion radius decreases from 106.1 picometers to 84.8 picometers, which is due to Ln3+The change of ionic structure is caused byzeroTo ffourteen，Number of electronsIt changes evenly.Ln2+、Ln4+OfIonic radiusAlso followAtomic numberIncrease and shrink.
Lanthanide shrinkageLanthanideThe properties of lanthanum and lutetium show regular changes: such as metal standardsElectrode potentialValue E ° increases, Ln3+hydrolysisTendency Enhancement, Ln (OH)threeThe alkalinity and solubility ofLigandshisStability constantWhen K increases, the hydrolysis temperature of salt decreases, etc. (Table 2).All these are related to theIonic potentialφ=Z/r (Z isValence, r is the ion radius).All lanthanides with the same valenceCrystal formIt is often the same.
Change in nature
Two adjacentLanthanideThe properties of are very similar.In nature, lanthanides often coexist in whole or in part. It is much more difficult to separate lanthanides from each other than to separate lanthanides from non lanthanides.
Another embodiment of lanthanide shrinkage is that in scandium groupyttriumIon Y of3+The outermost electronic structure of La3+Same as stwopsix, with a radius of 88.1 picometers, and Ho3+、Er3+、Tm3+close.The properties of yttrium compounds are similar to those of corresponding compounds of holmium, erbium and thulium.
The lanthanide shrinkage affects the properties of elements after lutetium, making theAtomic radiusIt is close to zirconium, niobium, etc. in the fifth cycle.The properties of hafnium, tantalum, etc. are very similar to those of zirconium, niobium, etc.Zirconium coexists with hafnium, niobium with tantalum and platinum in nature, and separation is quite difficult.
characteristic
Ionic contraction
The atomic radius shrinks slowly, and the difference between adjacent atomic radii is only about 1 pm, but after 14 elements from La to Lu, the atomic radius shrinks and accumulates as much as 14 pm.The ion radius shrinks significantly more than the atomic radius[3]。
[Question 1] Why does lanthanide shrink
The electron is filled into the f orbital, and the f orbital is loose, which causes the attraction of the outermost electron in the nucleus to increase, leading to the contraction of the atomic (or ion) radius.
[Question 2] Why are the atomic radii of europium and ytterbium much larger than those of adjacent elements in the overall shrinking trend of atomic radii
This is the influence of the electron layer configuration: Eu and Yb have semi filled 4f and fully filled 4f respectively, and this structure has a greater shielding effect on the atomic nucleus than other states of the 4f electron layer that are not full.
[Question 3] Why does the atomic radius shrink small, while the ionic radius shrinks significantly
In the atom, with the increase of the nuclear charge, the corresponding electrons are filled into the 4f orbital of the third to last layer (the first to last layer is 6s, the second layer is 5s, 5p orbital), which has a greater shielding effect on the nuclear charge than the 6s, 5s, 5p orbital. Therefore, with the increase of the atomic number, the outermost electrons are slowly attracted by the nucleus, which leads to the slow reduction of the atomic radius.However, the ion has one electron layer less than the metal atom. After the lanthanide metal atom loses the outermost 6s electron, the 4f orbital is in the second layer (the first layer to the last is 5s, 5p orbital). The 4f orbital in this state has less shielding effect on the nuclear charge than the 4f orbital in the atom (the third layer to the last), which makes the shrinking effect of the ion radius more obvious than the atomic radius.
[Question 4] In the curve of Ln ion radius reduction, why is there a small discontinuity at Gd ion
Because the electronic layer configuration of Gd is 4fsevenThe shielding effect of this semi filled electronic structure increases slightly,Effective nuclear chargeThe ion radius of Gd ion decreases slightly.This effect is called gadolinium cutoff effect.
Example analysis
For example, theAtomic radiusThe atomic radius of hafnium (Hf, the sixth periodic element) of the same family is 1.56 ∨.Zr4+OfIonic radiusIt is 0.79 ∨, while Hf4+It is 0.78 ∨.althoughAtomic numberFrom 40 to 72, whileRelative atomic massFrom 91.22g/mol to 178.49g/mol, the radii of the two elements are very similar.Due to the significant increase of the relative atomic mass and the almost constant radius, the density of zirconium increases from 6.51 g/cm to 6.51 g/cmthreeSignificantly increased to 13.35g/cm of hafniumthree。
Therefore, zirconium and hafnium are very similarchemical property, they have very similar radii andElectronic layoutBecause of this similarity, hafnium in nature is always associated with zirconium, and the content of zirconium is often much higher than that of hafnium, which makes the discovery of hafnium 134 years later than that of zirconium (zirconium was discovered in 1789, while hafnium was discovered in 1923)[2]。
Consequences
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Lanthanide shrinkage is an important phenomenon in inorganic chemistry. Since 15 kinds of lanthanide elements are in the same position in the periodic table (Group IIIB in the sixth cycle), the atomic radius of the transition from lanthanum (187.94pm) in IIIB to hafnium (156.4pm) in IVB suddenly decreases by 31.51pm, so the influence of lanthanide shrinkage in inorganic chemistry is huge.It is shown in the following aspects[3]：
Separation difficulty
Yttrium (Y), scandium (Sc) and lanthanide are collectively referred to asrare earth element。[5]Among rare earth elements, due to the small difference in atomic radius and the same outer electronic configuration of most rare earth elements, 17 rare earth elements are similar in nature and often coexist in mineralizationFor example, monazite ore rich in Ce, La and Nd is associated with almost all rare earth elements, making separation and purification extremely difficultThe separation of rare earth elements was once a difficult problem in inorganic chemistry[4]。
new blood
The+3 ion radius (89.3pm) of group Ⅲ B yttrium (Y) in the fifth cycle and the lanthanide ion Ho3+(89.4pm)，Er3+(88.1pm), so yttrium and lanthanide often coexist together, becoming a member of rare earth elements.
Inactivity of gold and mercury
Post lanthanideTransitional elementThe metal activity of the metal in the sample decreased obviously;In the same transition system, the metal activity decreases from left to right.The combined effect of these two factors leads to the inactivity of gold (Au) and mercury (Hg).
Inert electron pairs effect
Thallium (Tl), lead (Pb), and bismuth (Bi) in the main group elements in the sixth cycle P region show 6stwoInert electron pair effect.The main reasons are: ① shrinkage of lanthanide; ②6stwoThe penetration ability of electrons is strong, which can effectively avoid the shielding of other electronstwoElectrons can accept largerEffective nuclear chargeThe attraction of is not easy to lose, showing a certain inertia.
In short, due to the lanthanide elementExtranuclear electron arrangementThe particularity of the atomic radius and ionic radius of the lanthanide system shrink significantly due to the particularity of its position in the periodic table, which has a great impact on inorganic chemistry.Similarly, actinide elements also have similar lanthanide shrinkageActinide shrinkageHowever, actinides and elements behind actinides are radioactive elements with very short half-life, so actinide shrinkage is far less important than lanthanide shrinkage.
In periodic system Ⅲ B familyAtomic numberIt is the general name of 15 chemical elements from 57 to 71.They include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetiumrare earth elementMembers of.
Lanthanide is usually a silvery white lustrous metal, which is relatively soft and hasDuctilityAnd hasParamagnetism。The chemical properties of lanthanides are relatively active.The newly cut shiny metal darkens rapidly in the air and forms a layer on the surfaceOxide filmIt is not compact and will be further oxidized. The metal will be heated to 200~400 ℃ to generate oxides.Metal reacts slowly with cold water and violently with hot water to produce hydrogen, which is soluble in acid and insoluble in alkali.Metal burns violently in halogen at above 200 ℃ and generates at above 1000 ℃nitrides, slowly absorb hydrogen at room temperature, rapidly generate at 300 ℃hydride。LanthanideIt is a strong reducing agent more active than aluminum and ignites at 150~180 ℃.Lanthanide outermost layer (6s)Number of electronsNo change, all are 2.And lanthanumNucleusThere are 57 charges, from lanthanum to lutetium, the nuclear charge increases to 71Atomic radiusandIonic radiusThe phenomenon of gradual shrinkage is called lanthanide shrinkage.Due to the contraction of lanthanide, the properties of the compounds of these 15 elements are very similarhydroxideIt has low solubility in water and strong alkalinity,chloride、Nitrate、sulfateSoluble in water,oxalate、fluorideCarbonate and phosphate are insoluble in water.
Law theory
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Lanthanide shrinkage
rare earth elementIt shows very rich optical, electrical and magnetic properties, and has been listed as "strategic elements in the 21st century" by developed countriesLanthanideAn important physical and chemical property of is analyzed theoretically in line with the cognitive level of middle school students, and verified by calculation.
Using de Broglie relation v=E/h, λ=h/p to establish the inverse relationship between energy and wavelengthatomapproximateenergy levelThe diagram shows that the difference in energy levels between the 4f sublayer and 5d sublayer of lanthanide+3 valence ion is the reason for their different colors.
useHunt ruleTo explain the orbital occupation of the 4f sublayer of lanthanide+3 valence ion, and found that the 4f sublayer of the corresponding ions above gadolinium and below gadoliniumSublayerAt the same time, to reach a stable semi full state, the former needs to getNumber of electronsIt is consistent with the number of electrons that need to be lost in the latter.The energy of electron transfer is very close.It is inferred that the wavelength of the corresponding ion will be within the wavelength range corresponding to the same color.
Combined with the scientific and technological practice activities in the State Key Laboratory of Rare Earth Material Chemistry and Applications of Peking University“transition stateMethod, using the quantum chemical calculation software purchased from the Netherlands--density functional theory The calculation program ADF is used to calculate the relevant excitation energy, which verifies the correctness of the theoretical analysis.
It is not a simple coincidence that the color of lanthanide contraction+3 valence ions is symmetrically distributed with gadolinium as the center, but with the number of electrons they fill in 4f orbit andEmpty orbitThe three special states, full and half full, are closely related.
periodic table of ele ments
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periodic table of ele ments
The second cycle is more than the first cycleElement of area pThe elements of the third cycle are the same as those of the second cycle, and the fourth cycle is more than the third cycleElements of zone dThe types of elements in the fifth cycle and the fourth cycle are the same, and the sixth cycle is more than the fifth cycleElement of zone fThe types of elements in the seventh cycle and the sixth cycle are the same.
If the types of upper and lower periodic elements are the same, the law of gradual change is very regular from top to bottom,
If the types of upper and lower periodic elements suddenly increase, it often brings anomalies.
In fact, it is all because of the sudden addition of elements in a certain area,Nuclear charge numberThe increase of exceeds the previous rule, resulting inEffective nuclear chargeMore increases bring about anomalies in the law of property changes.
Therefore, the elements of the same family, from top to bottom, will have basic laws, but often there will be anomalies in the elements of the second cycle, the fourth cycle, and the sixth cycle.
For example:
1. The elements N, O, F in the second cycle's p region,Single bondBond energyEven smaller than that of the third cycle, the firstElectron affinityIt is also smaller than the family elements in the third cycle, and is easy to formhydrogen bond。
2. High valence compounds of As, Se and Br in the fourth cycleOxidizabilityIt is stronger than the elements of the same family in the third period.
