Transition and Inner Transition Elements PDF

# 8. Transition and Innertransition Elements ## Transition and Innertransition Elements The transition elements belong to the d-block in the periodic table. - Transition metals have incomplete d-orbitals (they give color with incomplete d-orbital). - They exhibit properties between those of s an...

# 8. Transition and Innertransition Elements ## Transition and Innertransition Elements The transition elements belong to the d-block in the periodic table. - Transition metals have incomplete d-orbitals (they give color with incomplete d-orbital). - They exhibit properties between those of s and p-block elements. Transition elements are placed in the periodic table in groups 3 to 12 and 4 periods (n = 4 to 7). (n-1)d orbitals are filled. The general electronic configuration is (n-1)d^1-10 ns^1-2 ### 3d to 6d elements | Group | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | |---|---|---|---|---|---|---|---|---|---|---| | d-series | | | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | | 3d | | | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn | | | | | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | | 4d | | | Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd | | 5d | f-block | 57 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | | | | | La | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg | | 6d | | 89 | 104 | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 | | | | | Ac | Rf | Db | Sg | Bh | Hs | Mt | Ds | Rg | Cn | ### Electronic configuration of d-block elements - 3d = [Ar] 3d^1-10 4s^1-2 - 4d = [Kr] 4d^1-10 5s^1-2 - 5d = [Xe] 5d^1-10 6s^1-2 - 6d = [Rn] 6d^1-10 7s^1-2 ## Electronic configuration of 3d series | Element | Excepted configuration | Observed configuration | |---|---|---| | 21Sc | [Ar] 3d¹ 4s² | [Ar] 3d¹ 4s² | | 22Ti | [Ar] 3d² 4s² | [Ar] 3d² 4s² | | 23V | [Ar] 3d³ 4s² | [Ar] 3d³ 4s² | | 24Cr | [Ar] 3d⁴ 4s² | *[Ar] 3d⁵ 4s¹ | | 25Mn | [Ar] 3d⁵ 4s² | [Ar] 3d⁵ 4s² | | 26Fe | [Ar] 3d⁶ 4s² | [Ar] 3d⁶ 4s² | | 27Co | [Ar] 3d⁷ 4s² | [Ar] 3d⁷ 4s² | | 28Ni | [Ar] 3d⁸ 4s² | [Ar] 3d⁸ 4s² | | 29Cu | [Ar] 3d⁹ 4s² | *[Ar] 3d¹⁰ 4s¹ | | 30Zn | [Ar] 3d¹⁰ 4s² | [Ar] 3d¹⁰ 4s² | ### Electronic configuration of chromium and copper (2M) #### Chromium (Cr): - Excepted configuration = [Ar] 3d⁴ 4s² - Observed configuration = [Ar] 3d⁵ 4s¹ - According to stability, the half-filled orbital is more stable, therefore, 1 electron of 4s is shifted to the 3d orbital/subshell #### Copper (Cu): - Excepted configuration = [Ar] 3d⁹ 4s² - Observed configuration = [Ar] 3d¹⁰ 4s¹ - According to stability, the fully filled orbital is more stable, therefore, 1 electron of 4s is shifted to the 3d subshell - **Oxidation States (OS) of first transition series** - The loss of 4s and 3d electrons leads to the formation of ions. - The oxidation states for the first transition series are: - M -1e^- -> M⁺¹ - M⁺¹ -1e^- -> M⁺² - M⁺² -1e^- -> M⁺³ - M⁺³ -1e^- -> M⁺⁴ - M⁺⁴ -1e^- -> M⁺⁵ - M⁺⁵ -1e^- -> M⁺⁶ - M⁺⁶ -1e^- -> M⁺⁷ - **IE1 < IE2 < IE3 < IE4** - The oxidation states of 3d series are usually +2 and +3 with exceptions. - The oxidation states +1, +4, +5, +6 and +7 generally occur. - **Common oxidation states** for some elements are: - **Copper (Cu)** = +1, +2 - Common OS is +2 - **Element| Expected configuration | e^- representation|** - **21Sc|[Ar] 3d¹ 4s²|↑** - **22Ti|[Ar] 3d² 4s²|↑↑** - **23V|[Ar] 3d³ 4s²|↑↑↑** - **24Cr|[Ar] 3d⁵ 4s¹|↑↑↑↑ ↑** - **25Mn|[Ar] 3d⁵ 4s²|↑↑↑↑↑↑** - **26Fe|[Ar] 3d⁶ 4s²|↑↑↑↑↑↑↑↑** - **27Co|[Ar] 3d⁷ 4s²|↑↑↑↑↑↑↑↑↑↑** - **28Ni|[Ar] 3d⁸ 4s²|↑↑↑↑↑↑↑↑↑↑↑↑** - **29Cu|[Ar] 3d¹⁰ 4s¹|↑↑↑↑↑↑↑↑↑↑↑↑↑↑ ↑** - **30Zn|[Ar] 3d¹⁰ 4s²|↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑** - Ionic radii of transition elements decrease from left to right. - There are two trends in ionic radii: - Elements having the same OS. - Various OS of the same element. - Elements having the same OS: - The ionic radii decrease from left to right due to the increase in atomic radii. - OS of the same element: - Different OS of the same element show different trends. - With a higher OS, the Zeff increases, so the ionic radii decrease. - **Ionization Enthalpy**: - The ionization enthalpy of transition elements is intermediate between those of s and p-block elements. - Transition elements are less electropositive than group 1 & 2. - Generally, in lower OS, they will form ionic bonds and in higher OS they will form covalent bonds. - The ionization enthalpy increases because the Zeff increases. - **IE1 < IE2 < IE3 < IE4** - 5d > 4d > 3d ## Physical Properties of First Transition Elements: All transition elements are metals and show characteristic properties like metals: - They are hard. - They are malleable and ductile. - Can form alloys with different metals. - Good conductors of heat and electricity (except for Zn, Cd, Hg). - They have high MP and BP. ## Trends in atomic properties for the 1st transition series (3d): - **Atomic/Ionic Radii:** - The atomic radii of the elements of the transition series decreases from left to right, because of the increase in Zeff. - **Ionization Enthalpy:** - The ionization enthalpy of transition elements increase from left to right due to: - Increase in Zeff. - Decrease in atomic size. - There are some exceptions depending on the specific element. - **Metallic Character:** - The metallic character decreases gradually from left to right because of the increase in Zeff, due to the increasing nuclear attraction and the electrons being held more tightly. - Since they have incomplete d-orbitals, the vacant d-orbitals attract electrons. ## Color of Transition Elements: - A substance will appear colored if it absorbs a portion of visible light and it depends on the wavelength of absorption in the visible region in the electromagnetic spectrum. - Ions and covalent compounds formed by transition element are colored because: - Presence of unpaired d-orbitals. - d-d transitions. - The nature of the ligand attached to the metal ion. - Geometry of the complex. | Element | e^- configuration | Ions | Ions e^- configuration | No of unpaired e^- | Color | |---|---|---|---|---|---| | 21Sc | 3d¹ 4s² | Sc³⁺ | 3d⁰ | 0 | Colorless | | 22Ti | 3d² 4s² | Ti³⁺ | 3d¹ | 1 | Purple | | | | Ti⁴⁺ | 3d⁰ | 0 | Colorless | | 23V | 3d³ 4s² | V³⁺ | 3d² | 2 | Green | | 24Cr | 3d⁵ 4s¹ | Cr³⁺ | 3d³ | 3 | Violet | | | | Cr⁶⁺ | 3d⁰ | 0 | | | 25Mn | 3d⁵ 4s² | Mn²⁺ | 3d⁵ | 5 | light pink | | | | Mn³⁺ | 3d⁴ | 4 | Violet | | 26Fe | 3d⁶ 4s² | Fe²⁺ | 3d⁶ | 4 | Pale green | | | | Fe³⁺ | 3d⁵ | 5 | Yellow | | 27Co | 3d⁷ 4s² | Co²⁺ | 3d⁷ | 3 | Pink | | 28Ni | 3d⁸ 4s² | Ni²⁺ | 3d⁸ | 2 | Green | | 29Cu | 3d¹⁰ 4s¹ | Cu⁺ | 3d¹⁰ | 0 | Blue | | | | Cu²⁺ | 3d⁹ | 1 | Blue | | 30Zn | 3d¹⁰ 4s² | Zn²⁺ | 3d¹⁰ | 0 | Colorless | - The color of the compound depends on the ligand and also on the geometry of the complex. - For example, when **cobalt chloride (CoCl₂)** is dissolved in water, it gives a pink solution of **[Co(H₂O)₆]²⁺** which has an octahedral geometry. But when this solution is treated with concentrated HCl, it gives a deep blue solution of **[CoCl₄]^2-** which has a tetrahedral geometry. - **[Co(H₂O)₆]**²⁺ + 4Cl^- -> **[CoCl₄]**^2- + 6H₂O - Octahedral -> Tetrahedral - Pink -> Deep blue - The color of the compound depends on the nature of the ligand and the geometry of the complex. - The color depends on the ligand & geometry of the complex. ## Catalytic Properties: - Transition metals and their compounds exhibit good catalytic properties. - They can be homogeneous or heterogeneous catalysts. - **Examples:** Fe, Co, Ni, Pd, Pt are used as catalysts. ### Two types of catalysis: - **Homogeneous catalysis:** - The metal ions precipitate by forming unstable intermediates in the reaction. - **Heterogeneous catalysis:** - The metal provides a surface of action for the reaction. ### Examples of catalytic action: - **MnO₂** acts as a catalyst for the decomposition of KClO³ - **Fe** is used as a catalyst in the Haber process of ammonia production. - **Co/Th alloy** is used as a catalyst in the Fischer Tropsch process, which is used for the synthesis of gasoline. - **Ni** is used in the reduction of alkenes. - **Pt** is used in the contact process of H₂SO₄ where SO₂ is converted to SO³. - 2SO₂ + O₂ -> 2SO₃ - **Fe-Cr catalyst** is used in the reaction of CO and steam (H₂O) to produce CO₂ and H₂ at 500^oC. - CO + H₂O -> CO₂ + H₂ ## Formation of **Interstitial Compounds** - Small atoms like H, C, or N are trapped in the interstitial spaces within the crystal lattice. - These interstitial spaces are usually found in transition metals. - Interstitial compounds can be formed when small atoms like carbon, hydrogen, boron, or nitrogen occupy the interstitial sites in the crystal lattice of a metal. - These compounds are different from alloys because they do not involve the formation of new chemical bonds. - **Interstitial compounds** are important in the metallurgical industry for their properties like hardness, high melting points, high strength, and resistance to corrosion. - **Examples:** - The carbides, hydrides, and nitrides of transition metals. - **Iron** forms interstitial compounds with carbon. - **Steel** is an example of an interstitial compound of carbon in iron. - **Cast iron and steel** are examples of interstitial compounds of carbon in iron. - **Tungsten Carbide** is very hard. ## Properties of Interstitial Compounds: - Hard - Good conductors of heat and electricity. - Chemical properties are similar to the parent metal. - Have higher melting points than the pure metal. - Have lower densities than the pure metal. - Metallic carbides are chemically inert and extremely hard like diamond. - Hydrides of transition metals can be used as powerful reducing agents. ## Formation of Alloys: - Alloys are obtained by combining two or more metals or a metal with a non-metal. - Alloys are generally homogeneous mixtures with metallic properties. - **Two types of alloys:** - **Ferrous alloys:** Contain iron as the primary component. - Examples: Nickel steel, stainless steel, chromium steel. - **Non-ferrous alloys:** Do not contain iron as the primary component. - Examples: Brass (Cu & Zn), bronze (Cu & Sn), nichrome (Ni & Cr). ## Uses of Alloys: - **Bronze**: - It is tough, strong, and corrosion resistant. - It is used for making statues, medals, trophies. - **Cupra-Nickel**: - It is used for making machinery parts, marine ships, boats. - **Stainless Steel**: - It is used for making house articles and ultra-high speed aircraft. - **Nichrome**: - It is used in the ration of 80:20 and has been developed specifically for gas turbine engines. - **Titanium**: - It is used for high-temperature articles and ultra-high-speed flights and fireproof articles. ## Compounds of **Mn** and **Cr**: KMnO₄ & K₂Cr₂O₇ - **Preparation of KMnO₄** - It is prepared by chemical oxidation. - Manganese dioxide (MnO₂) is heated strongly with KOH and oxidizing agent potassium chlorate (KClO₃), to form dark green potassium manganate (K₂MnO₄). - 3MnO₂ + 6KOH + KClO₃ → 3K₂MnO₄ + KCl + 3H₂O - In neutral or acidic medium, potassium manganate is disproportionated to KMnO₄ and MnO₂. - **KMnO₄ & K₂Cr₂O₇** both are strong oxidizing agents. ## Oxidation: - Oxidation is the addition of oxygen (O₂) or the removal of hydrogen (H₂) or the loss of electrons. - **Oxidation of KMnO₄**: - **In acidic medium:** - KMnO₄ acts as an oxidizing agent and oxidizes iodide ions (I^-) to iodine (I₂). - 2KMnO₄ + 10I^- + 16H⁺ → 2Mn²⁺ + 8H₂O + I₂ - KMnO₄ oxidizes Fe²⁺ to Fe³⁺. - 2KMnO₄ + 5Fe²⁺ + 8H⁺ → 5Fe³⁺ + 2Mn²⁺ + 4H₂O - KMnO₄ oxidizes H₂S to S. - 2KMnO₄ + 5H₂S + 6H⁺ → 2Mn²⁺ + 5S + 8H₂O - KMnO₄ oxidizes oxalic acid to CO₂. - 2KMnO₄ + 5C₂H₂O₄ + 6H→ 2Mn²⁺ + 10CO₂ + 8H₂O - **In neutral or weakly basic condition:** - KMnO4 oxidizes iodide ions to iodate ions (IO₃^-). - 2KMnO₄ + H₂O + 2I^- → 2MnO₂ + 2OH^- + IO₃^- - **In alkaline solutions:** - KMnO₄ is electrolyzed, MnO₂ is formed. - 2KMnO₄ + H₂O + 2e^- → 2MnO₂ + 2KOH - KMnO₄ crystals are deep purple black. ## Uses of KMnO₄: - It is a powerful oxidizing agent used in various chemical reactions and applications. - As an antiseptic - In unsaturation test in laboratory. - In volumetric analysis for reducing agents. - In qualitative analysis for detecting halides. - As a powerful oxidizing agent in laboratory and industry. ## K₂Cr₂O₇ (Potassium Dichromate): - **Preparation of K₂Cr₂O₇**: - In industrial production, cromite ore (FeOCr₂O₃) is heated with anhydrous sodium carbonate (Na₂CO₃) and flux of lime in furnaces. - 4(FeOCr₂O₃) + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂ - Sodium chromate (Na₂CrO₄) is extracted with water and treated with concentrated H₂SO₄ to get sodium dichromate. - 2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + 2NaCl + Na₂Cr₂O₇. H₂O - Addition of potassium dichromate and potassium chloride gives an orange-red color. - Na₂Cr₂O₇ + 2KCI → K₂Cr₂O₇ + 2NaCl - **Chemical reactions of K₂Cr₂O₇**: - Oxidation of I^- from an aqueous solution of KI by acidified K₂Cr₂O₇ gives I₂. - K₂Cr₂O₇ + 6KI + 7H₂SO₄→ 2K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3I₂ - H₂S is oxidized to pale yellow ppt of sulfur. - K₂Cr₂O₇ + 4H₂SO₄ + 3H₂S → K₂SO₄ + Cr₂(SO₄)₃ + 7H₂O + 3S ## Chemical Properties of d-block Elements: - They are electropositive metals. - They exhibit variable valencies and form colored salts and complexes. - They are good reducing agents. - They form insoluble oxides and hydroxides. - Fe, Co, Cu, Mo and Zn are biologically important metals. ## Catalysis in Biological Reactions: - The oxidation states of 3d, 4d, and 5d elements are affected due to different properties. - Mo(VI), W(VI) - 4d -> More stable - Cr(VI), Mn(VII) - 3d -> Less stable - The highest OS for the first row is +7 (Mn⁷⁺) - The highest OS for the 2^nd row is +8 (CrO₄^2- & OsO₄) ## Extraction of d-metals: - Most metals are found in the earth’s crust in the form of their salts, such as carbonates, sulfates, sulfides, and oxides. - A few metals are non-reactive and found in the free state in earth’s crust. - **Examples:** Gold, Silver, Platinum. ## Minerals: - A naturally occurring solid found in the earth's crust containing inorganic salts, silicones, metals, etc. is called a mineral. ## Ores: - The mineral which contains a high percentage of a metal and from which the metal can be economically extracted is called an ore. - Ores are usually found in the earth's crust and are mined for their valuable mineral content. - ** List of some ores of some transition elements: ** - **Iron:** Hematite/Hematite - **Copper:** Chalcopyrite, Chalcocite - **Zinc:** Zinc blende - **Others:** Many transition metals have their own ores. ## Metallurgy: - The commercial extraction of metals from their ores is known as metallurgy. - There are different types of metallurgy: - **Pyrometallurgy:** - The ore is reduced to metal at a high temperature using a reducing agent like C, H₂, or Al. - **Examples:** Extraction of iron from its oxides. - **Hydrometallurgy:** - Metal is extracted from a suitable aqueous solution of its salts using a suitable reducing agent. - **Electrometallurgy:** - Metal is extracted by electrolytic reduction of molten(fused) metallic compounds. ## Extraction of Iron from Hematite using Blast Furnaces: - **Hematite:** Fe₂O₃ + SiO₂ + Al₂O₃ + Phosphate + sand, mud and other impurities - The impurities are called gangue. - **Raw Material:** - Hematite (Fe₂O₃) oxide. - Coke - Carbon - Limestone - CaCO₃ - **Processes used in Blast Furnaces:** - **Concentration:** The ore is crushed and washed to remove impurities. - **Roasting:** The concentrated ore is heated in air to remove moisture, volatile impurities, and convert sulfides to oxides. - **Reduction:** The roasted ore is reduced to iron metal by using coke as a reducing agent in the blast furnace. - **Melting:** The reduced iron is melted and collected at the bottom of the blast furnace. - **Blast Furnaces::** - Height = 25 m, Diameter(d) = 5 to 10 m. ## Reactions in Blast Furnaces: - **Zone of Combustion:** - Coke burns with O₂ in the air. - C + O₂ -> CO₂ + Heat - **Zone of Reduction:** - CO₂ is reduced to CO. - CO₂ + C -> 2CO - Iron oxides are reduced to iron by CO and C - Fe₂O₃ + 3CO -> 2Fe + 3CO₂ - FeO + C -> Fe + CO - **Zone of Slag Formation:** - The gangue reacts with limestone to form slag. - CaCO3 + SiO₂ -> CaSiO₃ + CO₂ - The slag is molten and floats on the molten iron. - It is collected separately and used as building material. ## Inner Transition Elements: - Lanthanide and Actinide series (f-block elements). ### Lanthanide series: | Element | Symbol | Excepted configuration | Observed configuration | |---|---|---|---| | Lanthanum | La | [Xe] 5d¹ 6s² | [Xe] 5d¹ 6s² | | Cerium | Ce | [Xe] 4f¹ 5d¹ 6s² | [Xe] 4f¹ 5d¹ 6s² | | Praseodymium | Pr | [Xe] 4f³ 5d⁰ 6s² | [Xe] 4f³ 5d⁰ 6s² | | Neodymium | Nd | [Xe] 4f⁴ 5d⁰ 6s² | [Xe] 4f⁴ 5d⁰ 6s² | | Promethium | Pm | [Xe] 4f⁵ 5d⁰ 6s² | [Xe] 4f⁵ 5d⁰ 6s² | | Samarium | Sm | [Xe] 4f⁶ 5d⁰ 6s² | [Xe] 4f⁶ 5d⁰ 6s² | | Europium | Eu | [Xe] 4f⁷ 5d⁰ 6s² | [Xe] 4f⁷ 5d⁰ 6s² | | Gadolinium | Gd | [Xe] 4f⁷ 5d¹ 6s² | [Xe] 4f⁷ 5d¹ 6s² | | Terbium | Tb | [Xe] 4f⁹ 5d⁰ 6s² | [Xe] 4f⁹ 5d⁰ 6s² | | Dysprosium | Dy | [Xe] 4f¹⁰ 5d⁰ 6s² | [Xe] 4f¹⁰ 5d⁰ 6s² | | Holmium | Ho | [Xe] 4f¹¹ 5d⁰ 6s² | [Xe] 4f¹¹ 5d⁰ 6s² | | Erbium | Er | [Xe] 4f¹² 5d⁰ 6s² | [Xe] 4f¹² 5d⁰ 6s² | | Thulium | Tm | [Xe] 4f¹³ 5d⁰ 6s² | [Xe] 4f¹³ 5d⁰ 6s² | | -Ytterbium | Yb | [Xe] 4f¹⁴ 5d⁰ 6s² | [Xe] 4f¹⁴ 5d⁰ 6s² | | Lutetium | Lu | [Xe] 4f¹⁴ 5d¹ 6s² | [Xe] 4f¹⁴ 5d¹ 6s² | ### Actinide series: | Element | Symbol | Excepted configuration | Observed configuration | |---|---|---|---| | Actinium | Ac | [Rn] 5f⁰ 6d¹ 7s² | [Rn] 5f⁰ 6d¹ 7s² | | Thorium | Th | [Rn] 5f⁰ 6d² 7s² | [Rn] 5f⁰ 6d² 7s² | | Protactinium | Pa | [Rn] 5f² 6d¹ 7s² | [Rn] 5f² 6d¹ 7s² | | Uranium | U | [Rn] 5f³ 6d¹ 7s² | [Rn] 5f³ 6d¹ 7s² | | Neptunium | Np | [Rn] 5f⁴ 6d¹ 7s² | [Rn] 5f⁴ 6d¹ 7s² | | Plutonium | Pu | [Rn] 5f⁶ 6d⁰ 7s² | [Rn] 5f⁶ 6d⁰ 7s² | | Americium | Am | [Rn] 5f⁷ 6d⁰ 7s² | [Rn] 5f⁷ 6d⁰ 7s² | | Curium | Cm | [Rn] 5f⁷ 6d¹ 7s² | [Rn] 5f⁷ 6d¹ 7s² | | Berkelium | Bk | [Rn] 5f⁹ 6d⁰ 7s² | [Rn] 5f⁹ 6d⁰ 7s² | | Californium | Cf | [Rn] 5f¹⁰ 6d⁰ 7s² | [Rn] 5f¹⁰ 6d⁰ 7s² | | Einsteinium | Es | [Rn] 5f¹¹ 6d⁰ 7s² | [Rn] 5f¹¹ 6d⁰ 7s² | | Fermium | Fm | [Rn] 5f¹² 6d⁰ 7s² | [Rn] 5f¹² 6d⁰ 7s² | | Mendelevium | Md | [Rn] 5f¹³ 6d⁰ 7s² | [Rn] 5f¹³ 6d⁰ 7s² | | Nobelium | No | [Rn] 5f¹⁴ 6d⁰ 7s² | [Rn] 5f¹⁴ 6d⁰ 7s² | | Lawrencium | Lr | [Rn] 5f¹⁴ 6d¹ 7s² | [Rn] 5f¹⁴ 6d¹ 7s² | ## F block Elements: * F-block elements are those where the f-block gets filled by electrons. * They are placed separately in the periodic table at the bottom * This orbital subset of f lies inside the d-orbital. * These elements occur in period 6 and 7. * They are called **inner transition elements.** ## Aufbau Principle: * This principle explains the filling of orbitals in the periodic table. * The orbitals are filled in the increasing order of the sum of the principal quantum number (n) and the azimuthal quantum number (l). * For example, 4s (n+l = 4+0 = 4), 3d (n+l = 3 + 2 = 5). ## Lanthanide Contraction: - The overall atom's atomic and ionic radii decrease from La to Lu. - This is known as lanthanide contraction. - The decrease in atomic size across the lanthanide series from La to Lu is known as lanthanide contraction. - It is caused by the poor shielding effect of the 4f electrons. ## Ionization Enthalpy of Lanthanides