Properties of transition metals (d block elements) are transitional between the highly reactive metallic elements of s- block (which form ionic compounds) and the elements of p block (which are largely covalent). The d block is present in the centre of the periodic table and the elements are known as transition elements. There are four periods or transition series included in the block. Each having ten elements. Because d-sub-shell can have a maximum of ten electrons. Most of the elements belonging to fourth transition series have been discovered in recent years.
Properties of Transition Metals
Just like s-block elements which are all typical metals, the properties of transition metal elements are also metallic in nature. They depict typical characteristics of metals such as high melting point, high boiling point, metallic lustre, high thermal and electrical conductivity.
The variation in the metallic properties is linked to the variation in the crystal lattices of these elements. Actually, most of the elements belonging to the fourth transition series have been discovered quite recently. Their characteristics are still to be investigated properly.
The physical properties of transition metals show that elements are quite hard and have high melting points also. Actually, they have very strong metallic bonds due to the involvement of both ns and (n-1)d electrons. Along each transition series, they tend to increase with the increase in the atomic numbers (or a number of unpaired electrons). Midway, they become maximum in the elements having d5 configuration (maximum number of unpaired electrons). Then they show downward trends.
It may be defined as enthalpy change involved in breaking the metallic lattice of the crystalline metal into atoms. The high values of enthalpies of atomization tell the strong metallic bonds that are present.
The enthalpy of atomization decreases along the series. The element zinc has the least value due to the reason that there are no free electrons available for the formation of metallic bonds in the atoms. The trend reveals that the metals present in the 4d and 5d series have the greater magnitude of enthalpy of atomization as compared to the elements in the 3d series. The metallic bonds are stronger in these due to the greater involvement of the electrons in the metallic bond formation.
A careful look at the values of the atomic radii reveals that these initially decrease from Sc to V and become almost constant from Cr to Cu. The last element Zn has the comparatively high atomic radius (125 pm).
|Metallic radii (pm)||164||147||135||129||137||126||125||125||128||137|
Within a transition series, the decrease in atomic radii, in the beginning, is due to the increase in the effective nuclear charge with the increase in atomic number. However, with the increase in the number of electrons in the (n – 1) d-subshell, the screening effect of these d-electrons on the outermost ns-electrons also increases. This increased screening effect neutralises the effect of increased nuclear charge and as such the atomic radii remain almost constant in the middle of the series.
Increase in atomic radii towards the end may be attributed to the inter-electronic repulsions. The pairing of electrons in the d-orbitals of the penultimate shell occurs only after the d-subshell is half filled i.e., it has five electrons. The repulsive interactions between the paired electrons in the d-orbitals of the penultimate shell become dominant towards the end of the series and cause the expansion of the electron cloud, thereby increasing the atomic size.
Being properties of transition metals are metallic in nature, the transition elements form cations and their radii as expected, are less than the atoms to which they belong.The trend exists in the ionic radii because with the increase in the atomic number, the effective nuclear charge increases thereby decreasing the radius of the ion.It may be remembered that the radii of a transition metal ions decrease with the increase in the oxidation state. For example, Fe2+ (76 pm) and Fe3+ (64 pm).
Transition elements in general, exhibit all the characteristics of metals i.e., they are hard, lustrous, malleable and ductile. Apart from that, these have also high melting and boiling points. Moreover, they have typical metallic close packed structures as well (hcp, ccp or bcc). With the exception of mercury, which is liquid at room temperature, all others are hard solids.
The electronic configuration of these elements reveals that they have one or more unpaired electrons present in either ns or (n-1) d-orbitals which are available for bond formation. In general, greater the number of such unpaired electrons available, more will be the chances of their mutual combination and more will be the strength of the metallic bond. That is why these metals are very hard.
In a transition series, the density of the elements increases along a series.We have seen that there is a gradual decrease in metallic radius (or atomic radius) in a transition series. At the same time, there is an increase in atomic mass of the elements. The density (Mass/Volume) is expected to increase along the series. The exceptionally low density of zinc is quite expected. Its atomic size or atomic volume is comparatively large.
The ionization enthalpies of the transition metals are higher than those of s-block elements and less than the elements of p-block. Thus, these are less electropositive than the elements of s-block and at the same time more electropositive than the elements belonging to p-block present in the same period. Considering properties of transition metals, the ionisation enthalpies increase from left to right. However, the gaps in the values of the two successive elements are not regular.
The increase in ionisation enthalpy is primarily due to increase in nuclear charge which would tend to attract the electron cloud with greater force. Thus, ionisation enthalpy is expected to increase. As the transition elements involve the gradual filling of electrons in (n-1)d-subshell, this also increases the screening effect. With the increase in the number of (n – 1)d electrons, the outer ns electrons are shielded more and more. Thus, the effect of increasing nuclear charge is opposed by the increase in the magnitude of screening effect.
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Variable oxidation states
Variable valency is one of the most striking features of the transition elements. All transition elements, except the first and the last members of each series exhibit variable oxidation states. The cause of showing different oxidation states is due to the fact that there is only a small difference between the energies of electrons in the ns orbitals and (n-1) d orbitals with the result both ns as well as (n-1) d electrons can be used for compound formation. Thus the variable oxidation states of transition elements are related to their electronic configurations.
|Oxidation States of 3d Series||Oxidation state of 4d series||Oxidation State of 5d Series|
Let us illustrate the relation between the outer electronic configuration of transition elements and its various possible oxidation states by taking the example of Mn (3d5, 4s2) which shows oxidation states from +2 to +7.
- Formation of Mn2+ involves only two 4s electrons.
- Formation of Mn3+ has two 4s and one 3d electron.
- Mn4+ involves two 4s and two 3d electrons.
- Formation of Mn5+ has two 4s and three 3d electrons.
Oxidation state of iron
It must be noted that in some transition elements all of the (n-1) d electrons are not involved in bond formation, e.g. iron (3d6, 4s2) should have +8 as its highest oxidation state, but actually it is only +6 which is also known only in rare cases (+2 and +3 are the common oxidation states of iron). The +6 (and not +8) as the highest oxidation states of iron is explained on the basis that only the unpaired electrons of the 3d subshell take part in bond formation. In iron, there are 4 unpaired and 2 paired 3d electrons.Hence, the effective electrons in 3d orbitals are only four which may give +6 as the highest oxidation state.
Similarly, nickel has +4 as the highest oxidation state because here again only two of its eight 3d electrons are unpaired. The same is applied to all the transition elements.
Highest oxidation state
The highest oxidation state shown by any transition element is + 8 (Ru is 5 d-series and Os is 5 d series). Mn (member of 3 d series) shows the maximum state of +7 is KMnO4.It is important to note that the Cu
It is important to note that the Cu+ ion with d10 configuration is less stable than the Cu2+ ion with d9 configuration. It is due to increased hydration of the Cu2+ ion.
Formation of Colour Ion
Most of the d block metal compounds are coloured in the solid or in solution states (difference from s- and p- block elements whose compounds are generally white). The colour of transition metal ions is due to the presence of unpaired or incomplete d-orbitals. When visible (white) light ( λ = 4000-7000 Å) falls on a coloured substance, the latter absorbs certain radiations of white light and transmit the remaining ones. The transmitted light has the complementary colour to that of the absorbed light. This complementary colour which is actually the colour of the reflected (transmitted) light is the colour of the substance. For example, hydrated Cu2+ ion absorbs radiations corresponding to red light.Hence, it transmits radiations of the wavelength corresponding to the blue colour which is complementary to red colour.
The absorption of visible light and hence coloured nature of the transition metal cations is due to the promotion of one or more unpaired d-electron from a lower to a higher level within the same d-subshell. This promotion requires the small amount of energy which is available in the visible light.
Note that d block metal cations like Sc3+, Ti4+, Cu+ and Zn2+ have either completely empty or fully filled 3d-orbital, i.e. they do not have any unpaired d-electron, and hence appear colourless.
Magnetic properties of transition metals
On the basis of magnetic properties of transition metals, substances are classified into the following two types.
(i) Paramagnetic substances
Substances which are weakly attracted into the magnetic field are called paramagnetic. These substances lose their magnetism on removing the magnetic field. Paramagnetism is caused by the presence of unpaired electrons and since most of the transition metal atoms have unpaired d-electrons, they are paramagnetic in behaviour.
(ii) Diamagnetic substances
Substances which are repelled by the magnetic field are called diamagnetic, It is the property of the completely filled electronic subshells.
Since most of the transition metal ions have unpaired d-electrons, they show paramagnetic behaviour. The exceptions are Sc3+, Ti4+, Cu+ and Zn2+ etc. which do not contain any unpaired d electron and hence these are diamagnetic. The presence of an unpaired electron in an element causes it to behave like a permanent magnet. As a result of the permanent magnet, a paramagnetic substance when placed in an applied magnetic field, takes up a parallel position to the field.
It is important to note that diamagnetic substances show a decrease in weight while paramagnetic substances show an increase in weight in presence of a magnetic field.
Since each unpaired electron is regarded as a micro magnet having a certain value of the magnetic moment, the total magnetic moment of a cation depends upon the number of unpaired electrons and is given by the following expression.
μ = √[n(n + 2)] BM
When n (the number of unpaired electron) = 1, μ =√[1 x 3] = 1.73 B.M.
and so on. Thus metal with maximum unpaired electrons (i.e. 5) should have maximum magnetic activity. Hence Mn with 5 unpaired electrons has maximum magnetic activity.
Bigger the magnetic moment’s value, more the paramagnetic character.
In the case of Fe, Co and Ni, the unpaired electron spins are exceptionally more pronounced. As a result, these elements are much more paramagnetic than the rest of elements. Hence these are said to be ferromagnetic, i.e. they can be magnetised.
Certain transition elements form compounds of indefinite structure and proportion (non-stoichiometric compounds) with group 16 elements (O, S, Se and Te). The formation of non-stoichiometric compounds is partly due to the variable valency of the element and partly due to defects in the solid structures.
Interstitial compound formation
Transition elements have remarkable power to combine with atoms of relatively small size to form interstitial compounds, e.g. hydrides with hydrogen, carbides. with carbon etc. The steel and cast iron are hard because of the interstitial compound formation with carbon.
Catalytic Properties of transition metals
Due to variation in properties of transition metals, elements and their compounds are frequently used as catalysts.The most important being Fe, Pt, Pd, Ni and V2O5. In some cases, the transition elements provide unpaired d-electrons.This forms the unstable intermediate compound with the reactant. While in other, the transition metals provide a large surface area for the reactants to be adsorbed.
Resistant to corrosion
With the exception of iron, other transition metals are resistant to corrosion.Chromium is a very important corrosion resisting metal. The properties of transition metals like high heats of sublimation (due to the existence of covalent bonding), high ionization potentials, and low heats of hydration of their ions make these metals to remain unreactive or noble. Within a transition series, the noble character general creases with the increase in atomic number. This tendency is pronounced in platinum and gold.
All transition elements, except copper and mercury, have oxidation potentials higher than that of standard hydrogen electrode (taken as zero). Hence the oxidation properties of transition metals except copper and mercury are good reducing agents. Copper has a negative oxidation potential, and hence it is not able to displace H* ions from acid solutions. Sometimes, other transition metals also do not displace hydrogen from acids; it is because of their surface being covered with insoluble inert oxides. For example, Cr is so unreactive that it can be used as a protective non-oxidising metal.
Formation of Covalent and Ionic Bond
These metals form ionic as well as covalent compounds. Generally, ionic compounds are formed by the elements in their lower valency states. While covalent compounds are formed by the elements in their higher valency states.
Since d block elements are quite similar in atomic size, they can mutually substitute one another in crystal lattices. This gives solid solution and smooth alloys. An alloy is a solid mixture of two or more different elements, at least one of which is a metal. Alloys are homogeneous in the solid state. Alloys containing mercury (a liquid at ordinary temperature) as one of the constituent elements are term as amalgams. In alloys, chemical properties of the component elements remain same, but certain physical properties are improved. Actually, the purpose of making alloys is to develop some useful characteristics which are absent in the constituent elements.
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