IIT JEE , AIEEE Forum - Articles Category , Community shelf

Jyothi

TRANSITION METALS INCLUDING LANTHANIDES

The elements lying between s- and p-block elements of the periodic table are collectively known as transition or transitional elements(T.E?s).These elements either in their atomic state or in any of their common oxidation state have partly filled(n-1) d orbitals of (n-1)th main shell. In these elements the differentiating electron enters(n-1)dorbitals of (n-1)d orbitals of (n-1) th main shell and as such these are called d-block elements. This diefinition covers coinage metals, Cu,Ag and Au as transition metals because in their commonly occuring oxidation states, they have partly filled d-sub shells. However, it may be noted that the above definition does not include elements of group 12 i.e., Zn,cd and Hg.

Electronic configurations: All d- block elements show(n-1_d1-10 ns1-2 electronic, configuration. The half filled and completely filled d-orbitals acquire extra-stability and are favoured whenever possible.

General Characteristics of transition elements

1. Metallic Character.All the transition elements are metals and have high density, hardness, high tensile strength, ductility,malleability, high thermal and electrical conductivities and lustre. The metallic character is due to low ionization energies and a number of vacant orbitals in their atoms. The transition metals have very high melting points and boliling points. The high melting points of the transition metals suggest that the atoms in these elements are held together by strog metallic bonds which are formed due to the interaction of electorns in the outermost orbitals. The strength of bonding is roughly related to the number of unpaired electrons, stronger is the metallic bonding and consequently melting points are high. For example in transition series chromium has high melting point because it has five unpaired electrons.

2. Atomic and ionic radii: The atomic radii of elements of a particular transition series decrease with increase in atomic number but this decrease in atomic radius becomes small after midway. The decrease in size is attributed to increase in nuclear charge. However the effect of increased nuclear charge is partly cancelled by the increased screening effect of the electrons in the d-orbitals of penultimate shell. In the first transition series, the atomic radii become almost constant after chromium because the two factors, i.e., increase in nuclear charge and increase in screening effect just balance each other. The ionic radii also follow the similar trend.

3. Ionization Energies: The ionization energies of these elements are high, and in most cases lie between those of s-and p-block elements. This indicates that the transition elements are less electropositive than s-block elements. The ionization energy in any transition series effect of increased nuclear charge is partly neutralized by the extra-screening of the valence electrons from the nuclear attraction by the electrons being progressively added in the inner orbitals.

4. Oxidation States: Most of the transition elements show several oxidation states i.e.,they show variable valency in their compounds.

The outermost electronic configuration of the transition elements is(n-1)d^1-10ns². Since, the energy levels of (n-1)d and ns-orbitals are quite close to each other, hence both the ns-and (n-1) d-electrons are available for bonding purposes. Hence, the number of oxidation states displayed by these elements depends upon the number of d-electrons it has. For example, Sc having a configuration 3d14s2 may show an oxidation state of +2(only s- electrons are lost) and +3 (when d- electrons is also lost). The highest oxidation state which element of this group might show is given by the total number of ns- and unpaired(n-1)d-electrons.

5. Coloured Ions: Most of the transition metal compounds are coloured both in the solid state and in aqueous solutions. The colour of the compounds of transition metals may be attributed to the presence of incomplete(n-)d-subshell. The transition metal ions containing unpaired electrons can be easily promoted from one energy level to another in the same d-subshell. The amount of energy required to excite some of the electrons to higher energy states within the same d-subshell corresponds to energy of certain colours of visible light. Therefore, when light fails on a transition metal compounds, some of its energy corresponding to a certain colour, is absorbed causing excitation of d-electrons. The excess of other colours constituting white light are transimited and the compound appears coloured.

The transition metal ions which have completely filled d-orbitals are colourless. There are no vacant d-orbitals to permit excitation of electrons. Thus; Zn²⁺(3d¹⁰),Cd²⁺(4d¹⁰) and Hg²⁺(5d¹⁰) ions are colourless. The transition metal ions which have completely empty d-orbitals are also colourless. Thus Sc³⁺ and Ti⁴⁺ ions are colourless.

6. Magnetic Properties: The substances, which contain some species(atoms, ions or molecules) with unpaired electrons in their orbitals, behave as paramagnetic substances. Such substances are attracted by magnetic field. On the other hand, the substances whose constituent particles do not contain any unpaired electrons are repelled by magnetic field and are called diamagnetic. The transition metal ions generally contain one or more unpaired electrons in themand hence their compounds are generally paramagentic. The magnetic character is expressed in terms of magnetic moment. Larger the number of unpaired electrons in a substance greater is the paramagentic character and larger is the magentic moment. The magnetic moment is expressed by Bohr magneton and abbreviated as B.M.

7. Tendency to form complexes: Transition metals have a great tendency to form complexes because the size of their ions is very samll and for this particular reason they have a very high charged density. Moreover, they have vacant orbitals of right type of energy to accept lone pairs of electrons.

GENERAL PROPERTIES OF FIRST ROW TRANSITION METAL COMPOUNDS

1.Oxides: First row transition metals react with oxygen at elevated temperatures to form oxides of the type M₂O,MO,M₂O₃,MO₂,M₂O₅,MO₃ and M₂O₇. When the metal is in the low oxidation state, the oxides are usually basic. At intermediate oxidation states and high oxidation states, the oxides are normally amphoteric and acidic respectively. Some of these oxides dissolve in acid and alkalies to form oxometallic salts like potassium dichromate and potassium permanganate etc.

Potassium dichromate, K₂Cr₂O₇

It is prepared from chromite ore (FeCr₂O₇)

4FeCr₂O₄ + 16NaOH + 7O₂ 8Na₂CrO₄ + 2Fe₂O₃ + 8H₂O

2Na₂CrO₄ + H₂SO₄ Na₂Cr₂O₇ +Na₂SO₄ + H₂O

(orange)

Na₂Cr₂O₇+2KCl K₂Cr₂O₇ + 2NaCl

Potassium dichromate is an orange-red crystalline substance(m.p. 671 K). An aqueous solution of dichromate on treating with alkali turns yellow due to the formation of CrO₄^2- ions which on acidifying gives again Cr₂O₇^2- ions.

Cr₂O₇^2- + 2OH^- 2CrO₄^2- + H₂O

2CrO₄^2- + 2H⁺ Cr₂O₇^2-+ H₂O

It acts as strong oxidizing agent in acidic mediu

K₂Cr₂O₇ + 4H₂SO₄ K₂SO₄ + Cr₂(SO₄)₃ + 4H₂O + 3[O]

Cr₂O₇^2- + 14 H⁺ + 6e^- 2Cr³⁺ + 7H₂O[E^o = 1.31 V]

When any chloride is heated with K₂Cr₂O₇and Conc.H₂SO₄, orange-red fumes of chromyl chloride(CrO₂O₇) are formed

K₂Cr₂O₇+4NaCl + 6H₂SO₄ 2KHSO₄ + 4NaHSO₄+2CrO₂Cl₂ +3H₂O

Potassium Permanganate, KMnO₄

It is prepared from pyrolusite(MnO₂):

heat

2MnO₂ + 4KOH + O₂ 2K₂MNO₄ + 2H₂O

2K₂MnO₄ + Cl₂ 2KMnO₄ + 2KCl

2K₂MnO₄ + O₃ +H₂O 2KMnO₄ + 2KOH + O₂

Potassium permanganate is a purple coloured crystalline solid having melting point 523 K. It is slighlty soluble in water. It is a powerful oxidising agent in acidic, alkaline and neutral media.

Halides: First ?row transition elements react with halogens at high temperatures to form halides. The reactivity of halogens obeys the order F>Cl>Br>I. Usually fluorides are ionic. Bromides, chlorides and iodides become excessively covalnet with the atomic mass of the halogens.

Sulphides: Sulphides of transition metals are prepared either by direct combination of sulphur with metals or by the reaction of H₂S or Na₂S with a solution of their salts. In the sulphides oxidation state of metal is generally low because sulphur is a weak oxidising agent.

GENRAL PROPERTIES OF SECOND AND THIRD ROW TRANSITION ELEMENTS

Group 3 metals Y and La do not show transitional characterisitcs. In Group 4 Zr and Hf and their alloys are used in nuclear power enegineering. In group 5 alloys of Nb and Ta are used in jet enegines. IN group 6 both Mo and W with carbon, nitrogen, boron and silicon give hard refractory and chemically inert interstital compounds. In manganise group Tc and Re are very rare metals. Group 8,9,10 metals constitute three traids out of which first traids are known as ferrous metals.While elements of second and third traids arer known as platinum metals. In group 11 silver and gold are coinage metals used in making jewellery, coins and in dental filling. Group 12 metal Cd and Hg are used in making alloys and amalgams.

INNER TRANSITION ELEMENTS

The elements in which the last electron eneters in (n-2) f-orbitals are called f-block elements or inner transition elements. There are two series of f- block elements.

(i) Lanthanides: These are the elements in which differentiating electrons enter one of the 4f ?orbitals.

(ii) Actnides: These are the elements in which differentiating electrons enter one of the 5f-orbitals.

we observe that there is a general decrease in the ionic radii in the lanthanide series. The regular decrease in the size of lanthanide ions is known as Lanthanide Contraction.

Community Contributions - Articles by goIITians