The elements of “d” or “f” block which form one or more stable ions with partially filled d or f orbitals are called transition elements. As they have properties between s-block and p-block so they are called transition elements.
They are divided into two categories:
a) Main transition elements or outer transition metals:
d-block elements are called main or outer transition elements.
· They have partially filled (n-1) d orbitals in the ground state.
·Main Transition elements are further divided into following
1st transition series or 3d series consists of 21Sc to 29Cu
2nd transition series or 4d series consists of 39Y to 47Ag
3rd transition series or 5d series consists of 57La to 79Au
4th transition series or 6d series consists of 89Ac to 109Mt (Metnerium)
b) Inner transition elements or metals:
f-block elements are known as inner transition elements.
- F-block elements have partially filled (n-2) f orbitals in the ground state
- They consist of two horizontal rows at the bottom of the periodic table called lanthanides and actinides.
Both lanthanides and actinides belong to group III-B. Lanthanides belong to the 6th period while actinides belong to the 7th period.
Distinguishing Properties of Transition Metals:
1. They are hard metals and have high melting and boiling points.
2. They are good conductors of heat and electricity.
3. They show variable oxidation states and valences.
4. Their compounds are mostly colored in solid-state as well as in the aqueous state.
5. Some of these form paramagnetic compounds.
6. They form complexes or coordination compounds.
7. They form alloys.
8. These elements and some of their compounds act as solid catalysts.
9. They are too hard metals.
10. They are good conductors of heat and electricity due to having free electrons of valence shells.
11. Their general electronic configuration is ns1-2, (n-1) d1-10.
Some important General Characteristics
1. Electronic configuration:
The most occurring transition metals are that of 3d series. Their electronic configuration is:
Their general electronic configuration is ns1-2, (n-1) d1-10
21Sc → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d1 Non-transition metal
22Ti → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d2
23V → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d3
24Cr → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d5
25Mn → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d5 Transition metals
26Fe → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d6
27Co → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d7
28Ni → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d8
29Cu → 1s2, 2s2, 2p6, 3s2, 3p6, 4s1, 3d10
30Zn → 1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10 Non-transition metal
2. Atomic and Ionic Radii:
The atomic radii across the period, decrease rapidly at the start then remains almost the same and then for the last elements it increases. This increase is probably due to the filled 3 d orbitals give more shielding to the outer shell so atomic radii increases. Changes in ionic radii across the period are irregular.
3. Melting and Boiling Points:
Melting and boiling points of transition metals are very high due to strong binding forces among their atoms.
Melting and boiling point increase in a series up to the middle of the series and then decreases up to the end of the series. This is due to the reason that up to the middle of the series number of unpaired electrons increases so binding forces increase thus MP & BP increase, the unpaired d electrons also participate in binding. Then onward pairing of electrons takes place so binding forces decrease thus MP & BP decrease.
4. Binding Energy:
Transition metals are hard to have high mechanical strength due to strong metallic bonding. The strength of metallic bonds increases with the increasing number of unpaired d-electrons. From group III-B to VII-B number of unpaired d-electrons increases and then from VIII-B to I-B to II-B decreases. Thus binding energy increases from left to right in the period up middle and then decreases.
5. Oxidation states:
They show variable oxidation states and this is due to the reason that besides outer s-electrons d-electrons also participate in bonding. In series up to the middle, the number of oxidation states increases and then decreases as can be seen in 3-d series.
Oxidation numbers in parenthesis are uncommon. Highest oxidation state i.e 8, is of Os (Osmium) of 5-d series and also Ru (Ruthenium) of 4-d series has (+8) but it is rare.
The compounds of transition metals are mostly colored. The color is due to the d-d transition. The d-orbitals during bonding split up into two energy levels, one set has high energy than the other. The electrons present in low energy
d-orbitals jumps to higher energy d-orbital ∆E by absorbing some visible radiations and transmit other that gives different colors to the ions e.g. Ti+3 ions absorb yellow light and transmit the other 6 so it appears violet. Cu+2 absorbs red light and transmits others so it appears blue. Co+2 absorbs blue and green light and transmits others so it appears pink.
Substances which are attracted by external magnetic field are called paramagnetic and phenomenon is called paramagnetism.
Substances which are strongly attracted by magnetic field are called ferromagnetic and the phenomenon is called ferromagnetism.
Substances which are repelled by external magnetic field are called diamagnetic and the phenomenon is called diamagnetism.
Paramagnetic behaviour is due to the presence of unpaired electrons in the atom, ion or molecule of the substance. The attraction is due to interaction of magnetic field generated by spinning electrons and external magnetic field. In diamagnetic substances the electrons are paired and their electrons have opposite spin which cancel the effect of each other.
Ni2+ has 2 unpaired electrons so it is paramagnetic
Cu2+ has 1 unpaired electron so it is paramagnetic
Fe2+ has 4 unpaired electrons so it is paramagnetic
Fe3+ has 5 unpaired electrons so it is ferromagnetic
Mn2+ has 5 unpaired electrons so it is ferromagnetic
Cu+ has no unpaired electrons so it is diamagnetic
Most of transition metals have unpaired electrons in their d-orbitals. Due to spin motion of electrons, magnetic moment is generated. The magnetic moments of unpaired electrons are in same direction so they will reinforce each other and thus substance will act as tiny magnet and is called paramagnetic.
On other hand if electrons are paired their spins are in opposite directions and cancel each other’s effect so substance will not be attracted by magnetic field and will be slightly repelled, such substance is called diamagnetic.
The substances which have more number of unpaired electrons will be strongly attracted by magnetic field and are called ferromagnetic. They can be magnetized as they remain permanent magnet even in absence of magnetic field e.g. Iron, cobalt, Nickel.
The magnetic moment is related to number of unpaired electrons and is given as
μ = n(n + 2)
where μ = magnetic moment
n = number of unpaired electrons
It shows that more is number of unpaired electrons more is magnetic moment.
Magnetic moment is practically measured by Gouy’s balance. Its unit is B.M (Bohr magneton).
When n = 1 then μ = 1.73 B.M
When n = 2 then μ = 2.83 B.M
When n = 3 then μ = 3.87 B.M
- Alloy Formation:
Due to similarities in their sizes and structure, some transition metals are able to replace one another in the metallic lattice and thus form alloys e.g. steel (alloys of iron with other metals are called steel). Other examples are brass, bronze etc. They are non-stoichiometric.
- Catalytic properties:
Transition metals show variable oxidation states, therefore, they form unstable intermediate products with various reactants. These intermediate products decompose to give final products, regenerating the catalyst.
In other cases the finally divided metals or their compounds provide a large surface area for adsorption and the adsorbed reactants react faster due to closer contact.
Some of examples of these catalysts are
- Finally divide Fe(iron) is used as catalyst in Haber process (formation of ammonia).
- V2O5 is used as catalyst in Contact process for oxidation of SO2 to SO3.
- Pt is used as catalyst in Oswald process for oxidation of NH3 to NO.
- FeSO4 + H2O2 (Fenton’s reagent) is used as catalyst for oxidation of alcohols to aldehydes.
- Cu is also used as catalyst for oxidation of alcohols to aldehydes.
- Pd is used as catalyst for hydrogenation of phenol to cyclohexanol.
- Pt/PtO (Adam’s catalyst) is used as catalyst for reduction.
- TiCl4 (Ziegler Natta catalyst) is used as catalyst for polymerization of ethylene to polythene (Polyethylene).
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