What are Transition Elements?
Transition elements are those elements which partially fill d and F subshells. According to IUPAC definition, a transition metal is an element with a partially-filled d sub-shell atom, or which can give rise to cations that have an incomplete d sub-shell.”
Primarily, transition element refers to the d block elements. However, a few 2B elements such as cadmium, zinc, and mercury are often regarded as the defined elements due to their similar properties. Moreover, the elements of f block are also known as “inner transition elements”.
These elements occupy the middle part of the periodic table and situate between the left-hand side groups and right-hand side groups.
The transition metals remain between s and p block elements. Primarily they are classified into three segments.
First transition series (Sc to Cu)
Second transition series (Y to Ag)
Third transition series (La and the elements from Hf to Au)
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Fig: Periodic table
Sixth and seventh series contain f-block elements that are named as lanthanoids and actinoids. The fourteen elements starting from Cerium to Lutetium are known as lanthanoids.
On the other hand, the fourteen elements starting from nuclear number 90 (Thorium) to 103 (Lawrencium) are actinoids. The elements of actinides are radioactive, and those that are above Z=92, are generally humanmade in accelerators or nuclear reactors.
Some important transition elements are copper, iron and silver. Also, titanium and iron are the most abundant among all.
The first member of the fourth series of transition is Actinium (Ac), which contains elements from Rf to Rg as well. II- B has Zn, Cd, and Hg and III-A possesses Sc, Y, La, and Ac that are non-typical transition elements and the remaining ones are typical transition elements.
However, note that mercury, zinc, cadmium are not listed as transition metals because of their full d orbital.
The second and third-row elements of periodic table gradually show changes in properties if we move from left to right across the table. The outer shells of these elements have negligible shielding effects that increase effective nuclear charge as more protons add to the nucleus.
Therefore, it makes an effect on the atomic properties such as increased first ionisation energy, smaller atomic radius, enhanced electronegativity and other non-metallic characters.
These characteristic persist up to Calcium (Z=20), and then there is a shift. The succeeding ten elements are known as first transition elements, which contain almost same chemical and physical properties. Since additional electrons are added to the penultimate 3d shell, giving a shield between outer 4s shell and the nucleus.
The properties of transition elements are different than the main block elements (s-block). Following are a few characteristics.
Possess high charge/radius ratio.
Contain high density and hard.
Boiling and melting points are high.
Form paramagnetic compounds.
Display variable oxidation states.
Compounds and ions are usually coloured.
Form compounds having catalytic activity.
Produce stable complexes.
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Fig: Electronic configuration of transition elements
The first row of transition elements shows same electronic configuration. If you observe the table, you will see that 3d orbitals are filling gradually, beginning from scandium.
However, this trend is not regular as at chromium and copper, an electron from 4s shell enters 3d shell. This indicates generalised characteristics of orbital energy in the elements of first row series.
Also, in case of chromium, both the orbitals are occupied. This suggests that the 3d and 4s orbitals energy is comparatively close in this row’s atoms.
Similarly, if we study copper, we will see that its 4s orbital contains only one electron while the 3d layer is full. Hence, it is assumed that the orbital energy of 3d level moves from higher to lower as we pass from potassium to zinc.
However, the electronic configuration of transition elements not solely depends on orbital energy.
Following the trend, the 4s orbital energy of chromium is below the 3d, and hence the configuration should have been [Ar] 3d44s2. But in reality, the configuration is [Ar] 3d54s1 where the electrons of outer orbital are unpaired. It happens because of the electronic repulsion effects in outer electrons.
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Fig: Metallic radii of metals
The transition metals have more density than the metals of s-block, and the density increases from scandium to copper. This density factor fluctuates due to an irregular decrease of metallic radii as well as the increase of atomic mass. Consequently, the ionic radius decreases atomic number increases.
The ionisation energies of transition metals vary between s and p block elements. The electro positivity of these elements is lesser than the elements of s-block. Because of these, they form covalent bonds and not iconic bonds.
Moreover, due to the small size, they have high ionisation energy. The ionisation potential of elements of d-block increases from left to right, whereas this energy increases when the nuclear number increases. For example, Cu and Cr possess higher ionisation energies than their surrounded atoms.
These elements have high boiling and melting points because of the overlapping of (n-1)d orbital and d orbitals unpaired electrons in covalent bonding. Metals such as Hg, Cd and Zn possess completely full (n-1)d orbitals. Since they cannot form covalent bonds, their boiling points are lower than other elements of d-block.
Since the transition elements have lesser number of electrons in the outermost shells, all of them are metals. Subsequently, they exhibit all the traits of a metal such as malleability, ductility. They also are good conductors of heat and electricity. All of these elements are hard and fragile except mercury which is liquid and more like alkali metals.
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Except for the first and last member of transition elements, all of them show varying oxidation states. Initially, there is an increase in the oxidation states from the beginning of the table and is the maximum when we reach in the middle of it and then decreases.
The elements Sc through Mn show display the highest oxidation states because they lose all the s and d orbitals electrons from valence shell. For example, the oxidation state of Iron is between 2+ to 6+. Also, these first transition series elements create ions with a charge of 2+ or 3+.
Similarly, the elements from second and third transition series gain more stability in higher oxidation state than the ones from the first series. Also, atomic radius increases as we go down a block. As a result, the ions of the first series are smaller than the second and third ones.
For a matter of fact, the eliminating electrons from the orbits furthest from the nucleus are easier than the ones closest to the nucleus.
For instance, in aqueous solution, molybdenum and tungsten belonging to group 6, has the oxidation state of 6+.
Pop Quiz 1
1. Which one of the following is the oxidation number of a central atom in [Ni(CO)4]?
Transition elements show different chemical characteristics. Based on the reduction potentials, some metals are strong reducing agents, and some have low reductivity. For instance, all lanthanoids create 3+ aqueous cation.
Contrarily, metals such as gold and platinum have high reductivity, and therefore they can resist oxidation and are useful for making jewellery and circuits.
Pop Quiz 2
2. Which ones of the following elements are from group VIB?
Cr Mo W
Zn Cd Hg
Mn Te Re
Fe Ru Os
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1. What Are Inner Transition Elements?
In the periodic table, there are two groups of elements named lanthanides and actinides. These groups contain a total of 30 elements which are called inner transition elements. They are generally placed below periodic table’s core section. These elements are also known as “core metals of transition”.
2. Why Transition Elements Are Named That?
Transition elements are named so because Charles Bury, an English chemist, stated that these elements fill inner shells, i.e. n-1 shell is filled by d block elements. Also, these elements eliminate the transition between stable states.
3. Why All Transition Elements Are Metals?
All transition elements are metals because all of their outermost shells contain only two electrons. Also, they are malleable, hard and ductile because of strong metallic bonds.
4. What Do You Mean By The Charge Of Transition Elements?
The charge of a transition element and its oxidation state are equal. However, their charges generally range between +1 and +7.