4.4 Important compound of Transition elements#
Oxides and Oxoanions of Metals#
Generally, transition metal oxides are formed by the reaction of transition metals with molecular oxygen at high temperatures. Except the first member of 3d series, Scandium, all other transition elements form ionic metal oxides. The oxidation number of metal in metal oxides ranges from \(+2\) to \(+7\). As the oxidation number of a metal increases, ionic character decreases, for example, \(Mn_2O_7\) is covalent. Mostly higher oxides are acidic in nature, \(Mn_2O_7\) dissolves in water to give permanganic acid \((HMnO_4)\), similarly \(CrO_3\) gives chromic acid \((H_2CrO_4)\) and dichromic acid \((H_2Cr_2O_7)\). Generally lower oxides may be amphoteric or basic, for example, Chromium (III) oxide - \(Cr_2O_3\), is amphoteric and Chromium(II) oxide, \(CrO\) is basic in nature.
Potassium dichromate \(K_2Cr_2O_7\)#
Preparation:#
Potassium dichromate is prepared from chromite ore. The ore is concentrated by gravity separation. It is then mixed with excess sodium carbonate and lime and roasted in a reverberatory furnace.
$$ 4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \xrightarrow{900 - 1000^{\circ}C} 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2 \uparrow $$The roasted mass is treated with water to separate soluble sodium chromate from insoluble iron oxide. The yellow solution of sodium chromate is treated with concentrated sulphuric acid which converts sodium chromate into sodium dichromate.
$$ 2\text{Na}_2\text{CrO}_4 + \text{H}_2\text{SO}_4 \rightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} $$(sodium chromate (yellow)) → (sodium dichromate (orange red))
The above solution is concentrated to remove less soluble sodium sulphate. The resulting solution is filtered and further concentrated. It is cooled to get the crystals of \(Na_2SO_4 \cdot 2H_2O\).
The saturated solution of sodium dichromate in water is mixed with KCl and then concentrated to get crystals of NaCl. It is filtered while hot and the filtrate is cooled to obtain \(K_2Cr_2O_7\) crystals.
$$ Na_2Cr_2O_7 + 2KCl \longrightarrow K_2Cr_2O_7 + 2NaCl $$(sodium dichromate (orange red)) → (potassium dichromate (orange red))
Physical properties:#
Potassium dichromate is an orange red crystalline solid which melts at \(671 \mathrm{K}\) and it is moderately soluble in cold water, but very much soluble in hot water. On heating it decomposes and forms \(Cr_2O_3\) and molecular oxygen. As it emits toxic chromium fumes upon heating, it is mainly replaced by sodium dichromate.
$$ 4K_2Cr_2O_7 \xrightarrow{\Delta} 4K_2CrO_4 + 2Cr_2O_3 + 3O_2 \uparrow $$(potassium dichromate) → (potassium chromate) + (chromium(III) oxide)
Structure of dichromate ion:#
Both chromate and dichromate ion are oxo anions of chromium and they are moderately strong oxidizing agents. In these ions chromium is in \(+6\) oxidation state. In an aqueous solution, chromate and dichromate ions can be interconvertible, and in an alkaline solution chromate ion is predominant, whereas dichromate ion becomes predominant in acidic solutions. Structures of these ions are shown in the figure.
Chemical properties:#
1. Oxidation#
Potassium dichromate is a powerful oxidising agent in acidic medium. Its oxidising action in the presence of \(H^{+}\) ions is shown below. You can note that the change in the oxidation state of chromium from \(Cr^{6+}\) to \(Cr^{3+}\). Its oxidising action is shown below.
$$ Cr_2O_7^{2-} + 14H^{+} + 6e^{-} \longrightarrow 2Cr^{3+} + 7H_2O $$The oxidising nature of potassium dichromate (dichromate ion) is illustrated in the following examples.
(i) It oxidises ferrous salts to ferric salts.
$$ Cr_2O_7^{2-} + 6Fe^{2+} + 14H^{+} \longrightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_2O $$(ii) It oxidises iodide ions to iodine
$$ Cr_2O_7^{2-} + 6I^{-} + 14H^{+} \longrightarrow 2Cr^{3+} + 3I_2 + 7H_2O $$(iii) It oxidises sulphide ion to sulphur
$$ Cr_2O_7^{2-} + 3S^{2-} + 14H^{+} \longrightarrow 2Cr^{3+} + 3S + 7H_2O $$(iv) It oxidises sulphur dioxide to sulphate ion
$$ Cr_2O_7^{2-} + 3SO_2 + 2H^{+} \longrightarrow 2Cr^{3+} + 3SO_4^{2-} + H_2O $$(v) It oxidises stannous salts to stannic salt
$$ Cr_2O_7^{2-} + 3Sn^{2+} + 14H^{+} \longrightarrow 2Cr^{3+} + 3Sn^{4+} + 7H_2O $$(vi) It oxidises alcohols to acids.
$$ 2K_2Cr_2O_7 + 8H_2SO_4 + 3CH_3CH_2OH \longrightarrow 2K_2SO_4 + 2Cr_2(SO_4)_3 + 3CH_3COOH + 11H_2O $$2. Chromyl chloride test:#
When potassium dichromate is heated with any chloride salt in the presence of Conc \(H_2SO_4\) orange red vapours of chromyl chloride \((CrO_2Cl_2)\) is evolved. This reaction is used to confirm the presence of chloride ion in inorganic qualitative analysis.
$$ K_2Cr_2O_7 + 4NaCl + 6H_2SO_4 \longrightarrow 2KHSO_4 + 4NaHSO_4 + 2CrO_2Cl_2(chromyl chloride) \uparrow + 3H_2O $$The chromyl chloride vapours are dissolved in sodium hydroxide solution and then acidified with acetic acid and treated with lead acetate. A yellow precipitate of lead chromate is obtained.
$$ CrO_2Cl_2 + 4NaOH \longrightarrow Na_2CrO_4 + 2NaCl + 2H_2O $$$$ Na_2CrO_4 + (CH_3COO)_2Pb \longrightarrow PbCrO_4 (Leadchromate(Yellowprecipitate))\downarrow + 2CH_3COONa $$Uses of potassium dichromate:#
Some important uses of potassium dichromate are listed below.
- It is used as a strong oxidizing agent.
- It is used in dyeing and printing.
- It used in leather tanneries for chrome tanning.
- It is used in quantitative analysis for the estimation of iron compounds and iodides.
Potassium permanganate - \(KMnO_4\)#
Preparation:#
Potassium permanganate is prepared from pyrolusite \((MnO_2)\) ore. The preparation involves the following steps.
(i) Conversion of \(MnO_2\) to potassium manganate: Powdered ore is fused with KOH in the presence of air or oxidising agents like \(KNO_3\) or \(KClO_3\). A green coloured potassium manganate is formed.
$$ 2MnO_2 + 4KOH + O_2 \longrightarrow 2K_2MnO_4 (Potassium manganate()Green)+ 2H_2O $$(ii) Oxidation of potassium manganate to potassium permanganate: Potassium manganate thus obtained can be oxidised in two ways, either by chemical oxidation or electrolytic oxidation.
Chemical oxidation:#
In this method potassium manganate is treated with ozone \(O_3\) or chlorine to get potassium permanganate.
$$ 2MnO_4^{2-} + O_3 + H_2O \longrightarrow 2MnO_4^{-} + 2OH^{-} + O_2 $$$$ 2MnO_4^{2-} + Cl_2 \longrightarrow 2MnO_4^{-} + 2Cl^{-} $$Electrolytic oxidation#
In this method aqueous solution of potassium manganate is electrolyzed in the presence of little alkali.
$$ K_2MnO_4 \rightleftharpoons 2K^{+} + MnO_4^{2-} $$$$ H_2O \rightleftharpoons H^{+} + OH^{-} $$Manganate ions are converted into permanganate ions at anode.
$$ 2MnO_4^{2-}(Green) \rightleftharpoons 2MnO_4^{-}(purple) + 2e^{-} $$\(H_2\) is liberated at the cathode.
$$ 2H^{+} + 2e^{-} \longrightarrow H_2 \uparrow $$The purple coloured solution is concentrated by evaporation and forms crystals of potassium permanganate on cooling.
Physical properties:#
Potassium permanganate exists in the form of dark purple crystals which melts at \(513 \mathrm{K}\). It is sparingly soluble in cold water but, fairly soluble in hot water.
Structure of permanganate ion#
Permanganate ion has tetrahedral geometry in which the central \(Mn^{7+}\) is \(d^{3}s\) hybridised.
Chemical properties:#
1. Action of heat:#
When heated, potassium permanganate decomposes to form potassium manganate and manganese dioxide.
$$ 2KMnO_4 \longrightarrow K_2MnO_4 + MnO_2 + O_2 $$2. Action of conc \(H_2SO_4\)#
On treating with cold conc \(H_2SO_4\) it decomposes to form manganese heptoxide, which subsequently decomposes explosively.
$$ 2KMnO_4 + 2H_2SO_4 (cold)\longrightarrow Mn_2O_7 + 2KHSO_4 + H_2O $$$$ 2Mn_2O_7 \xrightarrow{\Delta} 4MnO_2 + 3O_2 $$But with hot conc \(H_2SO_4\) potassium permanganate give \(MnSO_4\)
$$ 4KMnO_4 + 6H_2SO_4(hot) \longrightarrow 4MnSO_4 + 2K_2SO_4 + 6H_2O + 5O_2 $$3. Oxidising property:#
Potassium permanganate is a strong oxidising agent, its oxidising action differs in different reaction medium.
a) In neutral medium:
In neutral medium, it is reduced to \(MnO_2\)
$$ MnO_4^{-} + 2H_2O + 3e^{-} \longrightarrow MnO_2 + 4OH^{-} $$(i) It oxidises \(H_2S\) to sulphur
$$ 2MnO_4^{-} + 3H_2S \longrightarrow 2MnO_2 + 3S + 2OH^{-} + 2H_2O $$(ii) It oxidises thiosulphate into sulphate
$$ 8MnO_4^{-} + 3S_2O_3^{2-} + H_2O \longrightarrow 6SO_4^{2-} + 8MnO_2 + 2OH^{-} $$b) In alkaline medium:
In the presence of alkali metal hydroxides, the permanganate ion is converted into manganate.
$$ MnO_4^{-} + e^{-} \longrightarrow MnO_4^{2-} $$This manganate is further reduced to \(MnO_2\) by some reducing agents.
$$ MnO_4^{2-} + 2H_2O + 2e^{-} \longrightarrow MnO_2 + 4OH^{-} $$So the overall reaction can be written as follows.
$$ MnO_4^{-} + 2H_2O + 3e^{-} \longrightarrow MnO_2 + 4OH^{-} $$This reaction is similar as that for neutral medium.
Baeyer’s reagent:#
Cold dilute alkaline \(KMnO_4\) is known as Baeyer’s reagent. It is used to oxidise alkenes into diols. For example, ethylene can be converted into ethylene glycol and this reaction is used as a test for unsaturation.
c) In acid medium:
In the presence of dilute sulphuric acid, potassium permanganate acts as a very strong oxidising agent. Permanganate ion is converted into \(Mn^{2+}\) ion.
$$ MnO_4^{-} + 8H^{+} + 5e^{-} \longrightarrow Mn^{2+} + 4H_2O $$The oxidising nature of potassium permanganate (permanganate ion) in acid medium is illustrated in the following examples.
(i) It oxidises ferrous salts to ferric salts.
$$ 2MnO_4^{-} + 10Fe^{2+} + 16H^{+} \longrightarrow 2Mn^{2+} + 10Fe^{3+} + 8H_2O $$(ii) It oxidises iodide ions to iodine
$$ 2MnO_4^{-} + 10I^{-} + 16H^{+} \longrightarrow 2Mn^{2+} + 5I_2 + 8H_2O $$(iii) It oxidises oxalic acid to \(CO_2\)
$$ 2MnO_4^{-} + 5(COO)_2^{2-} + 16H^{+} \longrightarrow 2Mn^{2+} + 10CO_2 + 8H_2O $$(iv) It oxidises sulphide ion to sulphur
$$ 2MnO_4^{-} + 5S^{2-} + 16H^{+} \longrightarrow 2Mn^{2+} + 5S + 8H_2O $$(v) It oxidises nitrites to nitrates
$$ 2MnO_4^{-} + 5NO_2^{-} + 6H^{+} \longrightarrow 2Mn^{2+} + 5NO_3^{-} + 3H_2O $$(vi) It oxidises alcohols to aldehydes.
$$ 2KMnO_4 + 3H_2SO_4 + 5CH_3CH_2OH \longrightarrow K_2SO_4 + 2MnSO_4 + 5CH_3CHO + 8H_2O $$(vii) It oxidises sulphite to sulphate
$$ 2MnO_4^{-} + 5SO_3^{2-} + 6H^{+} \longrightarrow 2Mn^{2+} + 5SO_4^{2-} + 3H_2O $$Uses of potassium permanganate:#
- It is used as a strong oxidizing agent.
- It is used for the treatment of various skin infections and fungal infections of the foot.
- It used in water treatment industries to remove iron and hydrogen sulphide from well water.
- It is used as Bayer’s reagent for detecting unsaturation in an organic compound.
- It is used in quantitative analysis for the estimation of ferrous salts, oxalates, hydrogen peroxide and iodides.
Note: \(HCl\) cannot be used for making the medium acidic since it reacts with \(KMnO_4\) as follows.
$$ 2MnO_4^{-} + 10Cl^{-} + 16H^{+} \longrightarrow 2Mn^{2+} + 5Cl_2 + 8H_2O $$\(HNO_3\) also cannot be used since it is good oxidising agent and reacts with reducing agents in the reaction.
However, \(H_2SO_4\) is found to be most suitable since it does not react with potassium permanganate.
Note
Equivalent weight of \(KMnO_4\) in acid medium = \(\frac{\text{Molecular weight of } KMnO_4}{\text{no of mols of electrons transferred}} = \frac{158}{5} = 31.6\)
Equivalent weight of \(KMnO_4\) in basic medium = \(\frac{\text{Molecular weight of } KMnO_4}{\text{no of mols of electrons transferred}} = \frac{158}{1} = 158\)
Equivalent weight of \(KMnO_4\) in neutral medium = \(\frac{\text{Molecular weight of } KMnO_4}{\text{no of mols of electrons transferred}} = \frac{158}{3} = 52.67\)
f-block elements - Inner transition elements#
In the inner transition elements there are two series of elements.
- Lanthanoids (previously called lanthanides)
- Actinoids (previously called actinides)
Lanthanoid series consists of fourteen elements from Cerium \((_{58}Ce)\) to Lutetium \((_{71}Lu)\) following Lanthanum \((_{57}La)\). These elements are characterised by the preferential filling of 4f orbitals, Similarly actinoids consists of 14 elements from Thorium \((_{90}Th)\) to Lawrencium \((_{103}Lr)\) following Actinium \((_{89}Ac)\). These elements are characterised by the preferential filling of 5f orbital.
The position of Lanthanoids in the periodic table#
The actual position of Lanthanoids in the periodic table is at group number 3 and period number 6. However, in the sixth period after lanthanum, the electrons are preferentially filled in inner $4f$ sub shell and these fourteen elements following lanthanum show similar chemical properties. Therefore these elements are grouped together and placed at the bottom of the periodic table. This position can be justified as follows..
- Lanthanoids have general electronic configuration [Xe] \(4f^{1-14}5d^{0-1}6s^{2}\)
- The common oxidation state of lanthanoides is \(+3\)
- All these elements have similar physical and chemical properties.
Similarly the fourteen elements following actinium resemble in their physical and chemical properties. If we place these elements after Lanthanum in the periodic table below 4d series, the properties of the elements belongs to a group would be different and it would affect the proper structure of the periodic table. Hence a separate position is provided to the inner transition elements as shown in the figure.
Electronic configuration of Lanthanoids:#
We know that the electrons are filled in different orbitals in the order of their increasing energy in accordance with Aufbau principle. As per this rule after filling 5s,5p and 6s and 4f level begin to fill from lanthanum, and hence the expected electronic configuration of Lanthanum(La) is [Xe] \(4f^{1}5d^{0}6s^{2}\) but the actual electronic configuration of Lanthanum is [Xe] \(4f^{0}\) \(5d^{1}\) \(6s^{2}\) and it belongs to d block. Filling of 4f orbital starts from Cerium (Ce) and its electronic configuration is [Xe] \(4f^{1}\) \(5d^{1}\) \(6s^{2}\). As we move from Cerium to other elements the additional electrons are progressively filled in 4f orbitals as shown in the table.
Table: electronic configuration of Lanthanum and Lanthanoids
| Name of the element | Atomic number | Symbol | Electronic configuration |
|---|---|---|---|
| Lanthanum | 57 | La | [Xe] \(4f^{0}\) \(5d^{1}\) \(6s^{2}\) |
| Cerium | 58 | Ce | [Xe] \(4f^{1}\) \(5d^{1}\) \(6s^{2}\) |
| Praseodymium | 59 | Pr | [Xe] \(4f^{3}\) \(5d^{0}\) \(6s^{2}\) |
| Neodymium | 60 | Nd | [Xe] \(4f^{4}\) \(5d^{0}\) \(6s^{2}\) |
| Promethium | 61 | Pm | [Xe] \(4f^{5}\) \(5d^{0}\) \(6s^{2}\) |
| Samarium | 62 | Sm | [Xe] \(4f^{6}\) \(5d^{0}\) \(6s^{2}\) |
| Europium | 63 | Eu | [Xe] \(4f^{7}\) \(5d^{0}\) \(6s^{2}\) |
| Gadolinium | 64 | Gd | [Xe] \(4f^{7}\) \(5d^{1}\) \(6s^{2}\) |
| Terbium | 65 | Tb | [Xe] \(4f^{9}\) \(5d^{0}\) \(6s^{2}\) |
| Dysprosium | 66 | Dy | [Xe] \(4f^{10}\) \(5d^{0}\) \(6s^{2}\) |
| Holmium | 67 | Ho | [Xe] \(4f^{11}\) \(5d^{0}\) \(6s^{2}\) |
| Erbium | 68 | Er | [Xe] \(4f^{12}\) \(5d^{0}\) \(6s^{2}\) |
| Thulium | 69 | Tm | [Xe] \(4f^{13}\) \(5d^{0}\) \(6s^{2}\) |
| Ytterbium | 70 | Yb | [Xe] \(4f^{14}\) \(5d^{0}\) \(6s^{2}\) |
| Lutetium | 71 | Lu | [Xe] \(4f^{14}\) \(5d^{1}\) \(6s^{2}\) |
In Gadolinium (Gd) and Lutetium (Lu) the 4f orbitals, are half- filled and completely filled, and one electron enters 5d orbitals. Hence the general electronic configuration of 4f series of elements can be written as [Xe] \(4f^{1-14}\) \(5d^{0-1}\) \(6s^{2}\)
Oxidation state of lanthanoids:#
The common oxidation state of lanthanoids is \(+3\). In addition to that some of the lanthanoids also show either \(+2\) or \(+4\) oxidation states.
\(Gd^{3+}\) and \(Lu^{3+}\) ions have extra stability, it is due to the fact that they have exactly half filled and completely filled f- orbitals respectively. Their electronic configurations are
$$ Gd^{3+} : [Xe]4f^{7} $$$$ Lu^{3+} : [Xe]4f^{14} $$Similarly Cerium and Terbium attain \(4f^{0}\) and \(4f^{7}\) configurations respectively in the \(+4\) oxidation states. \(Eu^{2+}\) and \(Yb^{2+}\) ions have exactly half filled and completely filled f orbitals respectively.
The stability of different oxidation states has an impact on the properties of these elements. the following table shows the different oxidation states of lanthanoids.
Atomic and ionic radii:#
As we move across 4f series, the atomic and ionic radii of lanthanoids show gradual decrease with increase in atomic number. This decrease in ionic size is called lanthanoid contraction.
Cause of lanthanoid contraction:#
As we move from one element to another in 4f series (Ce to Lu) the nuclear charge increases by one unit and an additional electron is added into the same inner 4f sub shell. We know that 4f sub shell have a diffused shapes and therefore the shielding effect of 4f electrons relatively poor. hence, with increase of nuclear charge, the valence shell is pulled slightly towards nucleus. As a result, the effective nuclear charge experienced by the 4f electrons increases and the size of \(Ln^{3+}\) ions decreases. Lanthanoid contraction of various lanthanoids is shown in the graph
Consequences of lanthanoid contraction:#
1. Basic nature#
As we from \(Ce^{3+}\) to \(Lu^{3+}\), the basic character of \(Ln^{3+}\) ions decrease. Due to the decrease in the size of \(Ln^{3+}\) ions, the ionic character of \(Ln - OH\) bond decreases (covalent character increases) which results in the decrease in the basic nature.
2. Similarities among lanthanoids:#
In the complete f - series only \(10 \ \mathrm{pm}\) decrease in atomic radii and \(20 \ \mathrm{pm}\) decrease in ionic radii is observed. because of this very small change in radii of lanthanoids, their chemical properties are quite similar.
3. Similarities between 4d and 5d series:#
The elements of the second and third transition series resemble each other more closely than the elements of the first and second transition series. For example
| Series | Element | Atomic radius |
|---|---|---|
| 3d Series | Ti | 132 pm |
| 4d Series | Zr | 145 pm |
| 5d Series | Hf | 144 pm |
Actinoids:#
The fourteen elements following actinium i.e., from thorium (Th) to lawrentium (Lr) are called actinoids. Unlike the lanthanoids, all the actinoids are radioactive and most of them have short half lives. Only thorium and uranium(U) occur in significant amount in nature and a trace amounts of Plutonium(Pu) is also found in Uranium ores. Neptunium(Np) and successive heavier elements are produced synthetically by the artificial transformation of naturally occurring elements by nuclear reactions.
Similar to lanthanoids, they are placed at the bottom of the periodic table.
Electronic configuration:#
The electronic configuration of actinoids is not definite. The general valence shell electronic configuration of 5f elements is represented as \([Rn]5f^{0-14}6d^{0-2}7s^{2}\). The following table show the electronic configuration of actinoids.
Table: electronic configuration of actinoids
| Name of the element | Atomic number | Symbol | Electronic configuration |
|---|---|---|---|
| Actinium | 89 | Ac | [Rn] \(5f^{0}\) \(6d^{1}\) \(7s^{2}\) |
| Thorium | 90 | Th | [Rn] \(5f^{0}\) \(6d^{2}\) \(7s^{2}\) |
| Protactinium | 91 | Pa | [Rn] \(5f^{2}\) \(6d^{1}\) \(7s^{2}\) |
| Uranium | 92 | U | [Rn] \(5f^{3}\) \(6d^{1}\) \(7s^{2}\) |
| Neptunium | 93 | Np | [Rn] \(5f^{4}\) \(6d^{1}\) \(7s^{2}\) |
| Plutonium | 94 | Pu | [Rn] \(5f^{6}\) \(6d^{0}\) \(7s^{2}\) |
| Americium | 95 | Am | [Rn] \(5f^{7}\) \(6d^{0}\) \(7s^{2}\) |
| Curium | 96 | Cm | [Rn] \(5f^{7}\) \(6d^{1}\) \(7s^{2}\) |
| Berkelium | 97 | Bk | [Rn] \(5f^{9}\) \(6d^{0}\) \(7s^{2}\) |
| Californium | 98 | Cf | [Rn] \(5f^{10}\) \(6d^{0}\) \(7s^{2}\) |
| Einsteinium | 99 | Es | [Rn] \(5f^{11}\) \(6d^{0}\) \(7s^{2}\) |
| Fermium | 100 | Fm | [Rn] \(5f^{12}\) \(6d^{0}\) \(7s^{2}\) |
| Mendelevium | 101 | Md | [Rn] \(5f^{13}\) \(6d^{0}\) \(7s^{2}\) |
| Nobelium | 102 | No | [Rn] \(5f^{14}\) \(6d^{0}\) \(7s^{2}\) |
| Lawrentium | 103 | Lr | [Rn] \(5f^{14}\) \(6d^{1}\) \(7s^{2}\) |
Oxidation state of actinoids:#
Like lanthanoids, the most common state of actinoids is \(+3\). In addition to that actinoids show variable oxidation states such as \(+2\), \(+3\), \(+4\), \(+5\), \(+6\) and \(+7\).
The elements Americium(Am) and Thorium (Th) show \(+2\) oxidation state in some compounds, for example thorium iodide \((ThI_2)\). The elements Th, Pa, U, Np, Pu and Am show \(+5\) oxidation states. Np and Pu exhibit \(+7\) oxidation state.
Differences between lanthanoids and actinoids:
| s.no | Lanthanoids | Actinoids |
|---|---|---|
| 1 | Differentiating electron enters in 4f orbital | Differentiating electron enters in 5f orbital |
| 2 | Binding energy of 4f orbitals are higher | Binding energy of 5f orbitals are lower |
| 3 | They show less tendency to form complexes | They show greater tendency to form complexes |
| 4 | Most of the lanthanoids are colourless | Most of the actinoids are coloured. For example. \(U^{3+}\) (red), \(U^{4+}\) (green), \(UO_2^{+}\) (yellow) |
| 5 | They do not form oxo cations | They do form oxo cations such as \(UO_2^{2+}, NpO_2^{2+}\) etc |
| 6 | Besides \(+3\) oxidation states lanthanoids show \(+2\) and \(+4\) oxidation states in few cases. | Besides \(+3\) oxidation states actinoids show higher oxidation states such as \(+4, +5, +6\) and \(+7\) |
Summary#
- IUPAC defines transition metal as an element whose atom has an incomplete d sub shell or which can give rise to cations with an incomplete d sub shell. They occupy the central position of the periodic table, between s and p block elements,
- d- Block elements composed of 3d series (4th period) Scandium to Zinc (10 elements), 4d series (\(5^{\mathrm{th}}\) period) Yttrium to Cadmium (10 elements) and 5d series (\(6^{\mathrm{th}}\) period) Lanthanum, Hafnium to mercury.
- The general electronic configuration of d- block elements can be written as [Noble gas] \((n - 1)d^{1 - 10}ns^{1 - 2}\) Here, \(n = 4\) to 7. In periods 6 and 7, the configuration includes \(((n - 2)f\) orbital; [Noble gas] \((n - 2)f^{14}(n - 1)d^{1 - 10}ns^{1 - 2}\)
- All the transition elements are metals. Similar to all metals the transition metals are good conductors of heat and electricity. Unlike the metals of Group- 1 and group- 2, all the transition metals except group 11 elements are hard.
- As we move from left to right along the transition metal series, melting point first increases as the number of unpaired d electrons available for metallic bonding increases, reach a maximum value and then decreases, as the d electrons pair up and become less available for bonding.
- Ionization energy of transition element is intermediate between those of s and p block elements. As we move from left to right in a transition metal series, the ionization enthalpy increases as expected.
- The first transition metal Scandium exhibits only \(+3\) oxidation state, but all other transition elements exhibit variable oxidation states by loosing electrons from (n- 1)d orbital and ns orbital as the energy difference between them is very small.
- In 3d series as we move from Ti to Zn, the standard reduction potential \((E_{M^{2+}/M}^{0})\) value is approaching towards less negative value and copper has a positive reduction potential. i.e., elemental copper is more stable than \(Cu^{2+}\)
- Most of the compounds of transition elements are paramagnetic. Magnetic properties are related to the electronic configuration of atoms.
- Many industrial processes use transition metals or their compounds as catalysts. Transition metal has energetically available d orbitals that can accept electrons from reactant molecule or metal can form bond with reactant molecule using its d electrons.
- Transition metals form a number of interstitial compounds such as TiC, \(ZrH_{1.92}\), \(Mn_4N\) etc.
- Transition elements have a tendency to form coordination compounds with a species that has an ability to donate an electron pair to form a coordinate covalent bond.
- In the inner transition elements there are two series of elements. 1) Lanthanoids (previously called lanthanides) 2) Actinoids (previously called actinides)
- Lanthanoids have general electronic configuration [Xe] \(4f^{1-4}5d^{0-1}6s^{2}\)
- The common oxidation state of lanthanoides is \(+3\)
- As we move across 4f series, the atomic and ionic radii of lanthanoids show gradual decrease with increase in atomic number. This decrease in ionic size is called lanthanoid contraction.
- The electronic configuration of actinoids is not definite. The general valence shell electronic configuration of 5f elements is represented as [Rn]5f \(^{0-14}6d^{0-2}7s^{2}\)
- Like lanthanoids, the most common state of actinoids is \(+3\). In addition to that actinoids show variable oxidation states such as \(+2\), \(+3\), \(+4\), \(+5\), \(+6\) and \(+7\).