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Properties of Transition Metals

A transition element may be defined as one which possess partially filed d orbitals in its penultimate shell. The conceptual definition is useful as it helps us in recognizing a transition element merely by looking at its electronic configuration. The definition excludes zinc, cadmium and mercury as these elements do not have partially filled d orbitals but still considered as one the properties are extension of transition elements.

The properties of transition elements are completely dependent upon the valence partially filled electronic configuration and gives them a unique characteristics. In this section we will discuss about the various aspects of transition element properties.

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Physical Properties of Transition Metals

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Metals are rarely in pure state and are most often put to use in form of alloys that exploit or strengthen desirable properties and overcome characteristics that limit the performance.

Alloys based on the carbides and nitrides of transition metals of group 4 through 6 on the periodic table exhibit special physical and mechanical properties that basically result from manipulation of composition and manufacturing process.

• Transition elements are good conductor of heat and electricity
• They exhibit properties which are different from the metals of groups I and II.
• The transition metals are hard with high density and higher melting point and boiling points
• When we move across the first row of transition elements the effective nuclear charge experienced by the valence electrons to increase and consequently lead to decrease in atomic radii from Ti to Cu
• The ionisation energy increase from Ti to Cu
• Transition elements show similar properties without much variance across the d block
• The nitrides have lower melting points, lower hardness, and lower elasticity as compared to the carbides.

The carbides and nitrides have stable chemical characteristics at room temperature. The carbon or nitrogen atoms occupy interstitial sites in metal lattice and are believed to promote strong metal and non-metal bonding. The hardness of metallic carbide increases with the carbon content. The combination properties makes the metal carbides and nitrides and their solids solution ideal materials for providing rigidity of transition materials.

Chemical properties of Transition Metals

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The typical chemical properties which are unique to transition metals are as follows.
• Transition metals display variable oxidation states
• Transition metals exhibit the catalytic properties and hence the presence of several transition metals as catalysts
• These transition metals form stable complexes
• These transition metals are responsible for all the coloured compounds  and ions we see around us
• The weak bonds that are formed between reactant molecules and transition metals at active sites weaken the intra molecular bonds
• There is high concentration of reactants on the catalysts surface of transition metals and thus reactants are brought in close proximity to one another and with the correct orientation for effective reactions to take place

Series of d block elements which give out specific chemical characteristics are as follows
:
• First series  (3 d series):
$\Rightarrow$ Scandium 21, Titanium 22, vanadium 23, chromium 24 manganese 25, ferrum 26, cobalt 27, copper 29, zinc 30
• Second series (4 d series):
$\Rightarrow$ Yittrium 39, Zirconium 40, Niobium 41, Moybdenum 42, Technetium 43, Ruthenium 44, Rhodium 45, Palladium 46, Silver (Ag) 47, Cadmium 48

General Properties of Transition Metals

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The presence of incomplete inner orbitals results in certain characteristics amongst the transition elements.
• The transition elements show well defined horizontal similarities in physical and chemical properties in contrast to s and p block elements as well as the vertical group relationship
• The hardness of transition elements with high melting points and boiling points show strong bonding and these elements possess high density and closed packed structure
• These elements show malleability and ductility as well as conduct heat and electricity
• There is little difference between (n-1) d and ns electrons and hence the transition elements show variable oxidation state in their compounds.
• They are all metals and vary from moderate electropositive to noble and can form alloys with each other
• The transition elements ions generally possess unpaired electrons and are coloured as well as paramagnetic in nature
• The complex ion formation is a typical property of transition elements and all the conditions for the formation of complex ions are fulfilled by the transition elements
• The transition elements form compounds with Lewis acids ligands like CO and NO but in these compounds the elements exists in low zero or even formally negative oxidation states
• The transition elements exhibit catalytic activity and are very good catalysts for all kinds of hydrogenation, oxidation, pyrolysis and dehydration
• They form interstitial compounds with non-metals like carbon, boron, nitrogen and hydrogen
• Properties of metals are considerably altered as these interstitial compounds are often hard with metallic appearance
• Due to comparable stability of the metal ions in different oxidations states the transition elements form non stoichiometric compounds

Typical Properties of Transition Metals

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There are some properties pertaining to some of the metals in transition group which are specific and have typicality about them.
• Presence of cations and anions in stoichiometric ratios and in well-defined spatial relationships.
• There are possibilities of covalent and ionic bonding between cations and anions
• Presence of strong electric field normal to surface due to the columbic nature of the ionic lattice
• Presence of adsorbed species which are charged
• The presence of acidity and basicity on the metal surface
• Presence of vacancies which are both cationic and anionic in nature
• Ability of cations to undergo oxidation and that the possibility of lattice oxygen are reactants in a reaction
• The interactions of the solid with incident photons that leads to light assisted surface chemical process
The high boiling point and high melting point of transition metals is attributed to high valency electron atom ratios. Transition metals quite easily form alloys with each other in definite intermediate compounds like CuZn or $CoZn_{3}$ etc.

The transition metal structures consist of close packed arrays of relatively large atoms and the holes in between these atoms, the small atoms like that of hydrogen, nitrogen and carbon could be inserted without much distortion of original metal structure to provide interstitial compounds.

Adsorption of gases on to transition metal surfaces is important and transition metals or alloys are often used as heterogeneous catalysts. The reactivity of transition metals toward other elements varies widely and the tendency to form other compounds both in solid state should diminish along the series.

Catalytic Properties of Transition Metals

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Transition elements and their compounds are useful catalysts and the two types are:
• Heterogeneous catalysts
• Homogeneous catalysts
Heterogeneous catalysts exists in different phase from the reactants where the reactants might be in gaseous or liquid states while the catalyst in solid state and thus provides an active site where the reaction can take place.

The reactant molecules are adsorbed on to the catalyst surface by forming weak bonds with the atoms on the active sites and this can happen only in case the metal catalyst possess partially filled d orbitals.
• The d electron can be used to form bonds with reactant molecules.
• Low lying vacant orbitals can be used to accommodate lone pairs of electrons from reactant molecules which result in bond formation.
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