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Transition elements are those elements that have partially filled d-subshell. They are also known as transition metals and due to the presence of an incompletely filled d-orbital, they can form stable cations. All the elements belonging to the d-block are generally present in the 3rd to 12th groups in the modern periodic table. Lanthanides and actinides which are the f-block elements are also categorised as transition metals. However, due to the presence of incompletely filled f-orbital, they are known as inner transition metals. The 3d subshell fills after the filling of the 4s subshell because the d sublevel is at a lower principle energy level than the s sublevel. Transition elements are metals but they are less reactive as compared to the metals in Group 1 and Group 2. Some transition elements are in their free or uncombined form because of their unreactivity. Compounds formed by transition elements are distinguished on the basis of their wide colours.
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Key Terms: Transition Elements, Periodic Table, Electronic Configuration, Ionic Compounds, Ions, Atomic Radius, Oxidation, Ionization Enthalpy, Complex Compounds, Alloys, Interstitial Compounds
What are Transition Elements?
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Transition elements belong to Group 3 to 12 in the periodic table and are metallic in nature. Due to their presence in between the most reactive metal on the left and non-metals on right, they are named as transition elements. They exhibit variable valency and generally forms coloured compounds. They have partially filled d or f subshell in their common oxidation states. Examples of transition elements are iron, copper, silver, titanium, etc.
Transition Elements
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Electronic Configuration of Transition Elements
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The general electronic configuration of transition elements are (n-1) d 1-10, ns 1-2. In some of the elements, the configuration is (n-1) d 5 ns1 or (n-1) d 10 ns1 because these elements have half-filled or completely filled orbitals. The electronic configuration of first 20 transition elements are as follows-
| Transition Elements | Atomic Number | Electronic Configuration |
|---|---|---|
| Sc | 21 | [Ar]3d1 4s2 |
| Ti | 22 | [Ar]3d2 4s2 |
| V | 23 | [Ar]3d3 4s2 |
| Cr | 24 | [Ar]3d5 4s1 |
| Mn | 25 | [Ar]3d5 4s2 |
| Fe | 26 | [Ar]3d6 4s2 |
| Co | 27 | [Ar]3d7 4s2 |
| Ni | 28 | [Ar]3d8 4s2 |
| Cu | 29 | [Ar]3d10 4s1 |
| Zn | 30 | [Ar]3d10 4s2 |
| Y | 31 | [Kr]4d1 5s2 |
| Zr | 32 | [Kr]4d2 5s2 |
| Nb | 33 | [Kr]4d4 5s1 |
| Mo | 34 | [Kr]4d5 5s1 |
| Tc | 35 | [Kr]4d5 5s2 |
| Ru | 36 | [Kr]4d7 5s1 |
| Rh | 37 | [Kr]4d8 5s1 |
| Pd | 38 | [Kr]4d10 5s0 |
| Ag | 39 | [Kr]4d10 5s1 |
| Cd | 40 | [Kr]4d10 5s2 |
Properties of Transition Elements
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Zinc, Cadmium and mercury are not considered as transition elements due to their stable electronic configuration. The rest of the elements have almost similar properties which are as follows-
- The compounds formed by the transition elements have some characteristics of colour. They also form coloured ions.
- Due to the presence of unpaired electrons, they generally exhibit paramagnetism.
- They exhibit variable oxidation states because of the low gap in energy between the oxidation states of these elements.
- As compared to other elements, transition elements have relatively high densities and are also hard.
- Delocalized d electrons participate in the formation of metallic bonding. This results in high melting and boiling point of transition elements.
- Transition elements are good conductors of electricity due to the presence of metallic bonding of the delocalised d electrons.
- The ratio of charge to the radius is large in transition elements.
- Transition elements can form a wide variety of stable complexes. It happens because a large variety of ligands can bind to the transition elements.
- They create catalytically active compounds which play an important role in the industrial production of some chemicals.
Magnetic Properties of Transition Elements
Majority of transition elements shows paramagnetism because of the presence of unpaired electrons. When the number of unpaired electrons increases from 1 to 5, the paramagnetic character increases. The intermediate elements show the highest paramagnetism. The transition elements which have paired electrons generally exhibits diamagnetic characteristics.
Catalytic Properties of Transition Elements
Due to their variable oxidation States and their ability to form complexes, transition elements exhibit good catalytic properties. They are widely used at an industrial level due to their catalytic properties.
Oxidation States
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Oxidation States determine the ability of an atom to oxidize (lose electron) or reduce (gain electron) the atoms of other species. During the formation of a compound, atoms lose, gain or appear to use the electron. They are all related to oxidation States. Transition elements have variable oxidation States that determines the ability of atoms to oxidize or reduce other atoms or species. After adding or subtracting the negative charge from an atom, ions are formed.
An atom loses its electron if it has an oxidation number of +1 but when it has an oxidation number of -1, it accepts an electron in order to achieve a stable configuration. Transition elements usually lose the electron from s orbital first which is followed by the removal of an electron from d orbital.
Since transition metals lose electrons more easily as compared to alkali metals, they have variable oxidation States. Alkali metals show the oxidation state of +1 because they lose 1 electron while alkali Earth metals have the oxidation state of +2 as they have only two electrons in the outermost shell. In transition elements, because of the removal of d orbital electrons, they show multiple oxidation States.
Atomic Ionic Radii
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From group 3 to group 6, the atomic and ionic radii of the transition elements decrease. It is due to the poor shielding effect of d-electrons. In group 7 to group 10, atoms have almost similar atomic radii. In group 11 and group 12, electron- repulsion balances the nuclear charge and therefore the atoms have larger radii.
Ionic Sizes of Transition Elements
Ionisation Enthalpy
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The amount of energy required to remove the electron from the outermost shell of an atom is known as Ionization Enthalpy. If the effective nuclear charge acting on the electron is more, the greater will be the Ionisation Potential. Due to the greater force of attraction between the electrons and nucleus, the Ionisation energy of transition elements are greater than s-block elements. If the atoms have small atomic radii, more Ionization Energy will be required but if atoms have larger radii, Ionisation Energy will be less. Therefore, the Ionisation Enthalpy of an element is related to its atomic radius. Ionization energy increases on moving from left to right in a periodic table.
Ionisation Enthalpies of Transition Elements
Formations
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Formation of Coloured Ions
Transition elements forms coloured complex as they absorb the radiation from the visible light region for exciting the electrons from one position to another position. d- orbitals split into two sets of different orbital energies in the presence of ligands. Hence, electron transition takes place and radiation is emitted which falls on the visible light region.
Formation of Complex Compounds
A large number of complex compounds are formed by transition elements. This occurs due to the presence of the small size of the metal ions, the availability of d- orbitals for bond formation, and the high ionic charges. In complex compounds, the metal ions bind a number of anions or neutral molecules which give characteristic properties to the complex species.
Formation of Interstitial Compounds
Interstitial compounds are non-stoichiometric and are neither Ionic nor covalent. They are formed when small atoms are trapped inside the crystal lattice of metals. Interstitial compounds formed by the transition elements are- TiC, Mn4N, Fe3H, etc.
Formation of Alloys
Alloys are a homogeneous mixture of two or more metals obtained by melting the components and then cooling the melt. Alloy formation takes place when the atomic radii of the metals do not differ by 15% so that atoms of one metal take up the position in the other’s crystal lattice. Since transition metals are similar in atomic radii, they form good alloys.
Things to Remember
- Transition elements form a bridge between the alkali metals and non-metals in the periodic table. Therefore, they are named as transition elements.
- They are present in the 3rd to 12th group of the periodic table.
- They have partially filled d or f subshell and shows variable oxidation States.
- Their atomic radii decrease from group 3 to 6. In groups 7 to 10, atomic radii are somewhat similar. In groups 11 and 12, atomic radii increase.
- Due to the greater force of attraction between the nucleus and d-electrons, their Ionisation Energy is higher.
- The majority of transition elements are paramagnetic in nature due to unpaired electrons.
- The small size of metal ions, availability of d orbitals and high ionic charges, form a number of complex compounds.
- They also lead to the formation of Coloured compounds.
- Due to their effective catalytic properties, they are used at the industrial level.
- Due to their similarity in atomic radii, they form a good homogeneous mixture.
- They also form interstitial compounds as small atoms of some elements gets trapped inside the crystal lattice.
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Sample Questions
Ques. Why do the transition elements have high melting and boiling point? (5 Marks)
Ans. Transition Elements have high melting and boiling point due to the following reasons-
- Metal-metal covalent bonds are formed due to the presence of unpaired electrons. These bonds are very strong and attribute high melting and boiling points to the elements.
- Due to the presence of partially filled d-orbital, they have a greater number of unpaired electrons which increases the ability of transition elements to form covalent bonds as well as metallic bonds.
- The greatest melting point and boiling point are exhibited by those elements which have the greatest number of unpaired electrons. Metals that do not have any unpaired electrons have generally low melting point and boiling point
- Chromium, molybdenum and tungsten have the greatest melting and boiling point while the elements such as zinc and mercury have very low melting and boiling point.
Ques. Describe the general characteristics of the Transition Elements? (5 Marks)
Ans. The characteristic properties of transition elements are given as follows:
- They have high melting and boiling point due to the presence of unpaired electrons in the d orbitals.
- They generally form coloured ions or compounds because they absorb radiation from the visible light region for the excitement of electrons.
- They are paramagnetic in nature as they have unpaired electrons.
- Due to the small size of their atoms and the strong metallic bonding, they have high density and hardness.
- They exhibit variable oxidation States, unlike s and p block elements.
- Their charge/radius ratio is generally high.
- These elements can bind to a large variety of ligands and can form a variety of stable complexes.
- Due to the presence of metallic bonding in these elements, they are good conductors of electricity.
Ques. How transition elements are useful? (5 Marks)
Ans. Transition metals are useful in the following ways-
- Nickel is primarily used in the production of stainless steel.
- Electrical wires are made by copper because copper has high tensile strength, malleability, ductility and electrical conductivity.
- Titanium is used in in aircraft hand piping for nuclear power plants. It is also used in artificial hip replacements.
- Iron is used at the industrial level for the production of construction material. It is also used to make Steel. In Haber's process, it has been used as a catalyst for the production of Ammonia.
- Cobalt is used to make alloys with various metals. It is also used in rechargeable batteries. Paint, ink and pigments are also formed from Cobalt due to its brilliant blue colour.
Ques. Describe the metallic qualities of the Transition Elements? (3 Marks)
Ans. The metallic qualities of transition elements are as follows-
- These elements are basically good conductors of heat and electricity.
- Transition elements show malleability, ductility, high tensile strength and metallic lustre. These qualities are exhibited by a typical metal.
- They also tend to crystallize in BCC ( body-centred cubic), CCP (Cubic Closed pack) or HCP ( Hexagonally closed pack) structures.
- They also show trends in metallic properties. Elements such as Chromium and Molybdenum are hard metals because of the presence of many unpaired electrons.
Ques. Some of the transition metals are referred to as noble metals. Give reasons (3 Marks)
Ans. Some transition elements are referred to as noble metals due to the following reasons-
- Elements such as gold, silver and platinum are considered as noble metals. They are present in the lower right corner of the periodic table.
- They do not react with acids but can be dissolved in aqua regia which is a mixture of hydrochloric acid and nitric acid. However, silver does not dissolve in Aqua regia.
- Due to low enthalpies of hydration and high ionization enthalpies, they are highly unreactive.
Ques. How transition metals are used as catalysts for commercial purposes? (3 Marks)
Ans. The commercial purpose in which transition elements are used as catalysts are as follows-
- Iron is used in Haber’s process. In this process, ammonia is synthesized by the reaction of nitrogen and hydrogen.
- TiCl4 is used for the manufacturing of high-density polythene in industries.
- Nickel is used as a catalyst in the hydrogenation reaction for the manufacturing of vegetable ghee from vegetable oil.
- In the contact process, V2O5 is used in the manufacturing of H2SO4.
Ques. What is the reason that the transition Elements are referred to as coloured compounds? (3 Marks)
Ans. Transition elements are referred to as coloured compounds because of the following reasons-
- Electrons from a lower energy d orbital is excited to a higher energy d orbitals. The energy required for this excitation corresponds to the frequency of light absorbed.
- The absorbed frequency lies in the visible region. The colour which is observed corresponds to the complementary colour of the light absorbed.
- The nature of the ligand determines the frequency of the light absorbed.
Ques. Why do the transition elements exhibit variable Oxidation States? (5 Marks)
Ans. The transition elements show variable oxidation States because of the following reasons-
- The valence electrons of transition elements are present in in two sets of orbitals which are (n-1) d and ns.
- Both are energy levels can be used for bond formation because there is very little difference in the energies of these orbitals.
- +2 oxidation state is formed by ns orbital in simple compounds transition elements while (n-1) d electrons remain unaffected.
- Higher oxidation State such as +3, +4, +5, +6 and +7 is formed when all 4s and 3d electrons are used in the transition elements.
- (n-1) d electrons give variable oxidation State when these electrons become bonding in the excited state.
- Due to the involvement of the electrons of the penultimate d shell, transition elements show variable oxidation States.
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