All known elements can be arranged in the long form of the periodic table which is purposed by Mendeleev’s. It contains 18 groups and 7 periods with two series of 14 elements at the bottom of the main table. The periodic table is based on the increasing order of atomic number of elements. We know in an atom, the number of protons is equal to the total number of electrons.
In other words, an atom is neutral due to the presence of the same number of electrons and protons. Hence the number of protons or atomic number is equal to the number of electrons. The atomic number can be used to write the electronic configuration of elements. The long form of the periodic table can be divided into four blocks; s, p, d and f-blocks. This classification is based on the position of valence electrons in the elements. For example Na is a s-block element because the valence electronic configuration of Na is 3s1.
In the periodic table, 1st and 2nd group is part of s-block whereas from 13th to 18th group, elements are known p-block elements. Elements from 3rd to 12th group are commonly known as d-block elements or transition elements whereas 2 series of 14 elements at the bottom of main table are called as f-block. Let’s discuss main characteristics of transition elements.
A transition element may be defined as "the element whose atom in the ground state or ion in one of the common oxidation state , has incomplete d-sub shell means having electrons 1 to 9".
- d-block elements are also known as transition elements.
- They are located in between s and p –block elements in the periodic table.
- As s-block elements are metallic in nature and p-block elements are non-metallic, d-block elements show a transition from metallic to non-metallic nature. In other words, they show a transition from the most electropositive s-block elements to the least electropositive or most electronegative p-block elements.
- Because of their metallic nature, transition elements are also named as transition metals.
- Another name for transition metals is d-block due to the presence of incomplete d-sub shell and valence shell filled in (n-1)d orbital and their general electronic configuration is (n-1)d1-10, ns2.
- The other reason for transition element name is variable oxidation state.
In the periodic table, all the elements are arranged in increasing order of their atomic number. The whole periodic table contains 18 vertical columns called groups and 7 horizontal rows called periods.
Periodic table can be studied by using some common properties of elements like atomic radius, ionization potential, electron affinity and electro-negativity, metallic nature. Atomic radius and metallic nature increases in a group from top to bottom and decreases from left to right in a period.
All the other properties decreases in a column from top to bottom and increases from left to right in period. On the basis of chemical and physical properties of elements, the periodic table can be divided in to four blocks. Each block has a characteristic property and shows a regular trend in the group as well as in the period.
Elements which belong to the same group show the same type of properties. The four blocks are -
- s-block element
- p-block element
- d-block element
- f-block element
s-block element: It involves all highly electropositive metals located in the 1st and 2nd group also known as alkali and alkaline earth metals.
p-block element: It consists of metals, non-metals and metalloids from 13th to 18th group.
d-block element: This block formed from 3rd to 12th group contains only metal.
f-block element: It consists of two series of 14th elements. This block contains radioactive and artificial element, commonly known as actinoids and Lanthanoides.
The elements in which the valence electron enters in anti penultimate energy level i.e., (n - 2) f are known as f - block elements.This name is because of the position of the valence electron is in the f - orbital.Since the last electron enters into (n - 2)f - orbital which is inner to the penultimate shell (For d-block elements), they are also called inner transition elements. Hence, the general electronic configuration for these elements is
(n-2)f 1-14, (n-1)d0-1, ns2There are a total of 14 elements in the inner transition elements which are arranged in two series of fourteen elements. The first series is known as lanthanoides and another one as actinoids.
In these elements the valence electron filled in 4f orbital, hence they are also known as 4f-block elements or first inner transition element. This series started from Cerium (Ce : Z = 58) ended at lutetium (Lu: Z = 71). Lanthanoides are also known as "rare earth metals".
Properties of Lanthanide
- Lanthanoides are silvery-white color metals which can get tarnished due to exposure to air and form their oxides.
- They are relatively soft metals compared to d-block elements but the hardness increases with higher atomic number.
- All Lanthanoides show +3 oxidation states. Beside they can show +2 and +4 oxidation state also.
- From left to right across the period the radius of each lanthanide (Ln 3+)ion decreases due to less screening effect of f-electrons. This is referred to as 'lanthanide contraction'.
- They show high melting points and boiling points and high reactivity.
- Like other metals, Lanthanoides also react with water to form hydrogen (H2)gas in cold/quickly upon heating.
- In the same way they can react with acidic solution to form hydrogen gas at room temperature.
- All Lanthanoides are strong reducing agents with paramagnetic nature and form ionic compounds.
- They can easily react with most non-metals to form binary compounds on heating.
- Lanthanoides also form coordination complexes with ligand but coordination numbers of lanthanides are high generally greater than 6; usually 8 or 9 or as high as 12 compare to transition element.
- These elements are located after actinium, hence are collectively termed as actinoids.
- In these elements, the valence electron filled in 5f –orbitals.
- Most of these elements are radioactive and their study in a laboratory is difficult.
- The earlier members of the series have a long half life, but later elements are very unstable with short half lives.
- The last element of actinoids, that is Lawrencium (103), has only 3 minute of half life.
||3, 4, 5
||3, 4, 5, 6
||3, 4, 5, 6, 7
||3, 4, 5, 6, 7
||2, 3, 4
- All actinoids are radioactive in nature with high electro positivity.
- Like other metals, they also react with air and tarnish. They are very dense metals with certain crystal structures.
- One of the unique properties of actinoids is their numerous allotropes.
- Actinoids also react with boiling water or dilute acid and release hydrogen gas.
- Beside +3 oxidation state, they show higher oxidation states of +4, +5, +6, and +7 also.
- They have a greater tendency to form complexes.
- Actinoids compounds are more basic and they can form oxocations also like; UO22+.
- Their magnetic properties are quite complex and cannot be explained easily.
- They are very dense metals with certain crystal structures.
|Period 4 after [Ar]
||Period 5 after [Kr]
||Period 6 after [Xe]
||Period 7 after [Rn]
||6s2 4f14 5d1
|7s2 4f14 7p1
||6s2 4f14 5d2
||7s2 4f14 6d2
||6s2 4f14 5d3
|6s2 4f14 5d4
||6s2 4f14 5d5
||6s2 4f14 5d6
||6s2 4f14 5d7
|6s1 4f14 5d9
||6s1 4f14 5d10
||6s2 4f14 5d10
General properties of transition elements
- There are a total of four series in d-block elements. The first three series contains ten elements in each one, while the last series is incomplete.
- First transition series or 3d series corresponding to filling of 3d-subshell with ten elements from scandium to zinc.
- Second series or 4d series located in the 5th period contains ten elements from Ytterbium to cadmium.
- Third series or 5d series located in 6th period with ten elements from lanthanum to mercury. After lanthanum (57) there is a gap of 14 elements of lanthanide than next element Hafnium (72) again a part of d-block element.
- Last series of transition started from actinium (89) after that 14 elements will be part of actinoids than next element kurchatovium (104) will place in this series. The rest of the elements in this series are artificial and radioactive in nature.
- Transition elements contain an incomplete d-subshell, so they show a variable oxidation state. For example; Osmium (Os) shows a maximum +8 oxidation number and Manganese (Mn) shows +7 oxidation number in potassium per magnate (KMnO4).
- The atomic radius follows the general trend of the periodic table. When we move in a series from left to right, the atomic radius first decreases than becomes stable and increases at the end of series. This trend is due to the screening effect of inner d-electrons.
Except for mercury, all transition metals are solid in nature with typical metallic crystal structure. As the number of unpaired electrons increases in an element, the strength of the metallic bond increases. But half filled orbitals decrease the metallic bond strength. All transition elements have a high melting point directly related to the metallic bond strength. Tungsten (W) has the highest melting point of all the transition elements. They show high ionization potential compared to s-block elements but lesser than p-block elements.
Because of the presence of unpaired electrons in the inner d-orbital, they can form coordination complexes with appropriate ligand. The unpaired electrons are responsible for the magnetic properties of elements. The second and third series elements show almost the same atomic radius and other properties due to Lanthanoides contraction.