## General characteristics of transition elements preparation and properties, Group 18 d – and f – Block Elements

General Characteristics of Transition elements :

General characteristics of transition elements are as follow :

## Electronic configuration

→ In the transition elements, the last differentiating or valence electron enters in penultimate d-orbitals i.e.. (n-1) d-orbitals are successively filled. The general electronic configuration of transition elements is

(n – 1)d1-10 ns0, 1 or 2

→ Due to very small energy gap between (n-1) d and ns orbitala, transition elements show various exceptions. the most common elements are Cr and Cu. They show exceptions in these electronic configurations due to extra stability of half filled and completely filled orbitals.

→ Table 8.2(a) : Electronic configurations of 3d-series

→ Table 8.2(b) : Electronic configurations of 4d-series

The exception shown above are attributed to factors like nuclear electron and electron-electron forces.

→ Table 8.2(c) Electronic configurations of 5d-series

→ 8.2 (d): Electronic configurations of 6d-series

→ The elements from atomic number 106 to 112 have recently been reported but these heavy elements are very unstable. They are radoiactive in nature and these elements are known as trans actinoid series.

## Characteristics of Transition elecments

→ The characteristics of transition elements are as follow:

• These all elements are metallic in nature. They are good conductors of heat and electricity.
• They do not show vertical similarity as other elements of periodic table show similarities in properties.
• They show property of catalysis.
• They form complex compounds.
• They form coloured complexes.
• They show paramagnetism.
• They form interstitial compounds
• They form many types of alloys.
• Generally, all the elements are strong, hard, high melting and boiling solids.

## General characteristics of elements of first transition series

The study of general characteristics of elements of first transition series can be done as follow :

→ Atomic Radii : The atomic radii of transition metal are lesser than s-block elements but greater than p block elements. On moving left to right in any series of transition metals the atomic radii decrease up to middle ie, from group 3 to group-7 atomic radii decrease regularly, then from group-7 to group-10. The atomic radii are approximately same then after it, upto group-12 atiomic radii increase slightly.

→ The main factor for this change in atomic radii is shielding effect and nuclear force of attraction. The decrease in atomic radii in each series from group-3 to group-7 is due to an increase in nuclear charge from element to element which tends to pull the ns-electrons towards the nucleus i.e., the size decreases. At the same time, due to addition of extra electrons to (n-1)d-orditals screening effect developes. As the number of d-electrons increases, the screening or shielding effect also increases.

→ This screening effect opposes the nuclear force of attraction. In the beginning nuclear force of attraction is powerful than screening effect hence size decreases. In the mid way or middle of the screening effect and nuclear force of attraction is couter balanced by each other, hence in the middle of the series there is no change in atomic radii inspite of the fact that atomic number increaes gradually.

→ At the end of the series, there is slight increase in the atomic radii due to powerful screening effect which creates electron-electron repulsion, among (n-1)d-electrons as after d5 – configuration pairing of electrons starts The group, on moving upto down the atomic size increases i.e., on moving from 3d-series to 4d-series the atomic radii increase due to increase in number of shells. But the atomic size of the elements of 4d-and 5d-transition series is approximate equal due to lanthanoid contraction.

→ Due to inclusion of fourteen elements of lanthanoid series between lanthanum and hafinium, there is regular decrease in atomic size from cerium to Lutetium due to poor shielding of 4f-electrons. Hence, the size of Haffnium becomes nearly equal to the size of Zirconium. For example: The atomic radii of Ti of 3d, Zr of 4d and Hf of 5d is 132, 160 and 159 pm repectively. The atomic radii of the elements of 3d-series are given in table 8.3.

Table 8.3 : Atomie radii of the atom of transition metals in picometer (pm)

 Series 3d Element Atomic radii 2iSc 144 22Ti 132 23V 122 24Cr 117 25Mn 117 26 Fe 117 27Co 116 28Ni 115 19 Cu 117 30Zn 125

→ Ionic Radii : On moving left to right i.e., on increasing atomic number, the ionic radii of the ion (MP) with oxidation number +2 follow the same trend as by atomic radii for first transition series or 3d-series. The values of ionic radii are given in table 8.4.

Table 8.4 : Ionic radii of transition metals of 3d-series

 Element Ionic radii (pm) — m2+ m3+ Sc — 81 Ti 90 76 V 88 74 Cr 84 69 Mn 80 66 Fe 76 64 Co 74 63 Ni 72 — Cu 72 — Zn 74 —

→ Generally the ionic radii of transition elements are different for different oxidation state. As oxidation number increases ionic radius decreases because effective nuclear charge increases.

→ Ionisation enthalpy : Transition elements are present in between the elements of s-block and pblock hence the value of the ionisation enthalpies are also lie in between the elements of s-block and p block i.e., the value of first ionisation enthalpy of the elements of d-block is higher than the s-block elements but lower than the p block elements because d-block elements are less electropositive than pblock elements.

→ On moving left to right in each series the ionisation enthalpies increase regularly. However this increase is not regular. It is observed that for a perticular tarnsition series, the difference in the ionisation enthalpies between any two successive d-block elements is much less than the difference in case of any two successive s-block or p block elements.

→ It can be explained on the basis of screening effect created by (n-1)d electrons on ns electrons and increased nuclear charge. As the number of electrons in (n-1)d subshell increases, the shielding of ns electrons also increases. Thus the effect of increased nuclear charge is somewhat neutralised by the increased screening effect. As a result the value of ionisation enthalpy increases but quite slowly among a period of d-block elements.

→ The value of first ionisation tnthalpy of Zinc (Zn). Cadmium (Cd) and Mercury (Hg) of group-12 is appreciably higher due of extra stability associated with completely filled d-orbitals. Their general electronic configuration is (n-1)d10ns2. Slight variations in the value of ionisation enthalpy are mainly due to the slight change in atomic radii. The first ionisation enthalpy values are given in table 8.5

Table 8.5: Ionisation enthalpies (in kJ/mol) of the elements of 3d-series

→ Metallic Nature : All the transition elements are metals because their atoms have one or two electrons in their outermost s-sub shell while their outer p-subshell is empty. They all exhibit most of the properties of metals. They possess malleability, ductility, metallic lusture, high tensile strength, high density, brittleness, hardness etc. Except mercury (Hg) other elements have high melting and boiling points.

→ They are good conductors of heat the electricity. With the exceptions of Zn, Cd, Hg and Mn, they have one or more typical metallic structures at normal temperature. They exhibit all the three types of structure : face centred cubic (fcc), hexagonal closed packed (hcp) and body centred cubic (bcc). These properties confirm that both metallic and covalent bonding are present in the atoms of d-block.

The metallic or lattice structure of these elements are given in table 8.5

Table 8.5: Lattice structures of transition Metals

• (bcc – boby centred cubic)
• (hep – hexagonal close packing)
• (ocp – cubic close packing)
• (X- a typical metal structure)

→ Oxidation states : All transition metals with some exception show a large number of oxidation states. The oxidation states of transition elements are given in table 8.6.

Table 8.6 : Various oxidation states of transition elements

Note: Very rare oxidation states are given in parentheses and most common ones are enclosed in boxes for first transition series (3d series)]

→ The various oxidation states of these elements are related to their electronic congifurations. The less common and unstable oxidation states are given in parentheses. The stability of the oxidation state depends upon the nature of the elements with which the transition elements forms the compounds. It is observed that the highest oxidation states are found in fluorides and oxides because fluorine and oxygen are the most electronegative in nature and have small size.

→ The variable oxidation states of a transition metal are due to the participation of (n-1)d-and ns-electrons in bonding because the energy difference of ns and (n-1)d-subshells are very less. The higher oxidation states are generally shown when only ns-electrons participate in bonding.

→ It should be noted that the oxidation states of transition element differ from each other by unity whereas oxidation states of non-transition elements differ from each other by two because of inert pair effect. Some important points related to oxidation states of transition elements are as follow:

→ The highest oxidation states are shown by the elements lying in middle of the series i.e. the highest oxidation state increases with increase in atomic number, reaches a maximum value in the middle and then starts decreasing. For example, In 3d series maximum oxidation state is shown by manganese (Mn). Manganese shows oxidation state from +2 to +7.

→ The elements lying in the beginning of the series show fewer oxidation states because they have small number of electrons which they can lose or share. The elements lying at the last of the series also show fewer oxidation states because they have fewer vacant d-orbitals which can be participated in bonding.

→ The elements in lower oxidation state (+2 and +3) generally from ionic bonds while in higher oxidation state form ionic bond for example In CrO42- ion there is covalent bond between Cr and oxygen.

→ Some transition metals can also show zero oxidation state in their compounds. For example in (Ni(CO)4) and Fe(CO)5) the oxidation state of Ni and Fe is zero. Different oxidation states of transition metals are given in table 8.6

→ The oxides formed by the transition elements with lower oxidation state have basic nature while the oxides formed by the transition elements with higher oxidation state have acidic nature. For example:

etc in which the oxidation number and group number of the elements are same.

→ In a group of d-block elements, the higher oxidation states are more stable for heavier element. For example in group 6, Mo (VI) and W (VI) are more stable than Cr(VI). For example dichromate having Cr(VI) is a strong oxidation agent in acidic medium, while MoO3 and WO3 with VI oxidation state are stable oxides.

→ Melting and boiling points : Transition metals generally have very high melting and boiling points. It is due to strong metallic bonds and the presence of half-filed d-orbitals in them. Due to these half filled orbitals, some covalent bonds also exist between atoms of transition metals.

→ On moving left to right in a period, the melting points of transition metals first increase to a maximum value and then decrease with increase in atomic number regularly towards the end of the period.

→ However, manganese and technetium have abnormally low melting points. It is observed that except the elements of group-12 i.e. Zn, Cd and Hg the melting points of most of the transition metals are above 1173 K.

[Note : Tungsten (W) has the highest melting point (3683 K) amongest transition metals]

→ The high melting and boiling points of transition metals are due to the strong interatomic forces which are roughly related to the number of unpaired electron i.e. half filled orbitals in the begining. On moving left to right the number of half-filled d-orbitals increases up to the middle of the period which causes the increase in strength of interatomic forces that bring their atoms together.

→ But therefore, the pairing of electrons in d-orbitals occurs and the number of half filled orbitals decreases which also decrease the melting points. When no unpaired electrons are present, the melting points also become very low as in the case of Zn, Cd and Hg.

→ However this concept does not explains why manganese (Mn) having five unpaired d-electrons possesses lower melting point than that of vanadium (V) or cobalt (Co) which have only three unpaired electrons. Actually its complex structure is responsible for its low melting point.

The value of enthalpy of atomisation in kJ mol-1 and melting point (K) of 3d-series are given in table 8.7

Table 8.7 : Heat of atomisation (kJ mol-1) and melting points (K) of 3d-series

→ From above given table 8.7 it is clear that manganese have abnormally low value of melting point. It is probably due to its stable electronic configuration (3d-half filled, 4s-fully filled). In Mn, 3d electrons are held very tightly by the nucleus. Due to this strong nuclear force of attraction electrons are not avialable for metallic bonding which result very weak metallic force of attraction and hence very low melting point.

→ Catalytic Property : Catalytic property is an important property of transition elements. The catalytic activity of transition metals and their compounds is associated with their variable oxidation states. Generally, the metals used as catalyst, alloys and many other compounds are transition metals only. In transition elements, partially filled (n – 1)d orbitals are responsible for catalytic activity of transition elements.

→ Transition elements form unstable compounds with active substances by using their empty orbitals. These unstable compounds decompose to give products and catalyst is obtained as such In the presence of catalyst, the reaction proceeds via the way in which value of activation energy is minimum. Thus, reaction takes place smoothly and readily.

→ Some examples are there in which catalyst provide suitable surface for the reaction. The reactants are absorbed on the surface of catalyst. Some of the examples in which transition elements and compounds acts as catalyst are :

→ In short we can say that they act as catalyst because of the following reasons:

• They have variable oxidation state.
• They easily absorb and re-emitt the wide range of energies to provide the necessary activation energy.
• Because of variable oxidation states, they easily combine with one of the reactant to form an intermediate compound which reacts with the second reactant to form final product.

→ There are some examples in which transition elements act as catalyst.

They are given below:

→ In conversion of SO2 into SO3, V2O5 act as catalyst.

→ In conversion of unsturated compounds into sturated compounds. Ni catalyst is used.

→ During the formation of ammonia, finely divided iron is used.

→ In some cases, transition metals provide a suitable surface where the reactants are adsorbed and thus come closer to one another so the reaction can proceed with fast rate.

Colour :

→ Elements of d-block form coloured salts in solid state and in solution. The colour of state is due to the presence of unpaired electron. The compound containing unpaired electron absorbs some radiation from visible region which promotes the electron from one of the d-orbitals to another. It can be explained as follow: The colour of compounds is due to the presence of incomplete (n-1) d-subshell.

→ When any anion approaches the metal ion then degeneracy of d-orbitals destroy and they split into two sets i.e., t2g set and eg set The set which contain two d-orbitals i.e. $$d_{x^{2}-y^{2}}$$ and $$d_{z^{2}}$$, is knwon as eg set, while the set which contain three d-orbitals i.e. dxy, dyz and dxz, is knwon as t2g-set.

→ The energy of eg set is higher than the energy of ta-set. When light falls on the compounds then the electron present in d-orbitals having lower energy absorbs radiation and becomes excited.

→ This excited electron jumps from d-orbitals having lower energy to d-orbitals of higher energy. This is called d-d transition.

→ When white light falls on any compound of transition metal having unpaired electron, then some of energy corresponding to a particular colour is absorbed by this electron and it becomes excited. This excited electron raised to higher energy set of d-orbitals and rest of colours of white light are transmitted. This transmitted light makes the compound coloured. The observed colour of the compounds is always the complementary colour of observed colour.

→ Thus we can say that the transition metal ions and the compounds, which have unpaired electrons, are coloured, while those which do not have unpaired electrons are colourless. The colour and outer electronic configuration of some important transition metal ions are given in table 8.8

Table 8.8 : Colours of ions of the first transition series

 Configuration Example Colour 3d0 Sc3+ Colourless 3d0 Ti4+ Colourless 3d1 Ti3+ Purple 3d1 V4+ Blue 3d2 V3+ Green 3d3 V2+ Violet 3d3 Cr3+ Violet 3d4 Mn3+ Violet 3d4 Cr2+ Blue 3d5 Mn2+ Pink 3d5 Fe3+ Yellow 3d6 Fe2+ Green 3d6 3d7 Co3+, Co2+ Bluepink 3d8 Ni2+ Green 3d9 Cu2+ Blue 3d10 Zn2+ Colourless

Magnetic Properties :

→ The magnetic properties of compounds are due to the presence of unpaired electrons. Magnetic properties are very helpful in determination of structure of transition element compounds. The main properties of magnetism are given here: The reason for magnetic property is two type motion of electrons.

• Orbital motion and
• Spin motion

→ Magnetic moment is sum of two motions :

μ = μL + μS B.M.

→ Hence, μL is orbital magnetic moment and is spin magnetic moment. Thus, electron itself works as a small magnet. Magnetic moment is expressed in Bohr Magneton (B.M.).

1 B. M = $$\frac{\text { eh }}{4 \pi m c}$$

→ Armc In transition elements.(n-1)d-electrons are held on the surface only, hence are affected more by external force. As a result, their orbital motion is restricted and becomes negligible. In this conditions,

µ ≅ µS

→ Magnetic moment of magnetic material can be calculated in B.M., Bohr Magneton) by using the following expression,

µ = √n(n + 2) B.M.

→ Where n is the number of unpaired electrons. This formula is known as ‘spin only formula. B.M. -Bohr magneton (a unit of magnetic moment)

1 B.M. = 9.27 × 10-24 A m2 or JT-1

→ The magnetic moment increases with increase in number of unpaired electron. Large the number of unpaired electrons in a substance, the greater is its magnetic moment in Bohr magneton, thus larger will be paramagnetism. The magnetic moment of some important ions are listed in table 8.9

Table 8.9: Observed and calculated magnetic moment (BM) of some ionic species

where n is the number of umpaired electrons.

→ There are many types of magnetic properties are found in substance. Main three types are given as:

• Diamagnetism
• Paramagnetism
• Ferromagnetism

→ Diamagnetism : The substance in which paired electrons are present have zero magnetic moment because both the electrons neutralise effects of each other. Such substances are known as diamagnetic substances.

→ They do not show any type of magnetic behaviour in absence of magnetic field. When these substances are placed in magnetic filed, their resultant magnetic moment becomes zero but orbital magnetic moment is induced in the opposite direction of the field.

→ Hence these substance are repelled in magnetic field. Diamagnetism does not depend on temperature and almost all the substance show this property. It is weaker than para magnetism. Due to this, it is difficult to observe this property in substances having unpaired electrons.

→ Paramagnetism : It is found in the substances in which unpaired electrons are present. The electrons do not cancel effect of each other. Such substances are show paramagnetic properties in substances. An atom, ion or molecule of substance can be considered as a magnet. Their spin and orbital motion generates small electric and magnetic field.

→ When electron are paired, their electronic field is equal and opposite. Hence, they neutralise each other and resultant magnetic filed is zero. The magnetic field produced by unpaired electron is small but effective. Therefore, paramagnetism depends on number of unpaired electrons present in that substance. When paramagnetic substance are placed in magnetic field, than they are strongly attracted towards magnetic field.

→ If the field is heterogeneous, the substance will move in that directions in which the speed of field is maximum so that maimum lines can be passed through the substance. Paramagnetism is inversaly proportional to the temperature.

→ The paramagnetic and dimagnetic behaviour of a substance can be calculated by weighing the substance first in air and then suspending it between the poles of magnetic field. If the weight of the substance is more in magnetic field then it is paramagnetic because this substance is attractcted by the magnetic field. If the weight of substance is less in the magnetic field then it is dimagnetic because it is repelled by the magnetic field.

→ Ferromagnetic substances: The substances which are very strongly attracted by the magnetic field, then these are known as ferromagnetic substances. Ferromagnetism is the extreme condition of paramagnetism. These substances show permanent magnetism even in the absence of magnetic field. Hence ferromagnetic substances are used to form magnet. Example- iron, cobalt, nickel, godolinium, CrO2 Fe3O4, Mn ete.

→ On increasing or decreasing the temperature or beating etc, the arrangement of ionic magnets is disturbed and As a result, their magnetic property vanishes.

→ Formation of Interstitial Compounds The size of transition element are quite large. When they are arranged in lattice then larger interstitial voids are formed in these voids smali non metallic atoms like H, B, C, N, He etc are trapped. The compounds formed by the trapping of smaller atoms in the interstitial voids of these transition elements are known as interstitial elements are known as interstitial compounds.

→ When H, He, B, C, N etc are trapped then interstitial compounds are known as Hydrides, Helidea, Borides, Carbides, Nitrides etc. respectively. These interstitial compounds are generally non-stoichiometric compounds having formula like Tic, Mn4N, Fe3H, VH0.50, VH0.56 TiH1.7 etc. The bonds formed between these element are neither ionic nor covalent in naute. These formulas are also not correspond to any normal oxidation state to the metal.

Some of the important chemical and physical properties of these interstitial compounds are as follow :

• The melting points of these compounds are generally high.
• They are very hard and rigid. For example-steel is very hard in nature.
• It is an interstitial compounds of Fe and C.
• Simililarly some of the borides are as hard as diamond.
• They retain metallic conductivity.
• They are chemically inert.
• In interstitial compound, the malleability and ductility of the metals decreases but tensile strength increases, due to the presence of these atoms.

Formation of Alloys :

→ A large number of alloys are formed by transtition metals. The size of transition metals are quite similar hence the atom of one metal can substitute the atoms of other metal in its crystal lattice. In this way when more than one metal are melted mixed well and then on cooling this solution of mixture of more than one transition metals, a smooth solid alloy is obtained.

→ Alloys are generally hard, strong and have high melting point. they are resistant to corrosion Stainless steel, normal steel, vanadium steel, c-steel etc. are alloys of iron with chromium, vanadium, molybdenum, tungsten, nickel, manganese etc with some amount of carbon. Some other examples of alloys are brass, (copper-Zine), Bronze (Copper-tin) etc.