p-block Elements
Those elements in which the last electron enters any one of the three p-orbitals of their respective outermost shells are called p-block elements. Since a p-subshell has three degenerate p-orbitals, each one of which can accommodate two electrons, therefore, in all, there are six groups of p-block elements. These are 13, 14, 15,16, 17 and 18 (helium is excluded from group 18). In all these elements, the s-orbital of their outermost shells is already complete but their respective p-orbitals get progressively filled in by the addition of one electron at a time as one moves from group 13, 14, 15, 16, 17 and 18. Since these elements contain one to six electrons in their respective outermost p-orbitals, therefore, their outer shell electronic configurations vary from ns? np to ns?npº as we move from groups 13 to groups 18. Since each group has five elements, therefore, in all, there are 30p-block elements in the periodic table. General outer shell electronic configuration of p-block elements: ns2 np1- 6. Like s-block elements, even p-block elements have general characteristics. Some of these are discussed below.
1. p-block elements include both metals and non-metals along with certain metalloids. However, the number of non-metals is much higher than that of metals. Further, the metallic character increases from top to bottom within a group and non-metallic character increases from left to right along a period in this block.
2. Their ionization enthalpies are relatively higher as compared to those of s-block elements.
3 Their oxidizing character increases from left to right in a period and reducing character increases from
top to bottom in a group. As such F2 is the strongest known oxidizing agent.
4. Some of them show more than one oxidation states in their compounds. It is a due to inert pair effect.
d-block Elements
Elements in which the last electron enters any one of the five available d-orbitals of their respective penultimate shells (last but one shell) are called d-block elements. Since a d-subshell has five d-orbitals, each one of which can accommodate two electrons, therefore, in all there are ten vertical columns comprising ten groups of d-block elements, namely 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12. The atoms of the elements belonging to these groups usually have 1 or 2 (zero in some cases) electrons in the s-orbital of the outermost shell, i.e., n s-orbital while the electrons are being progressively filled in, one at a time, in the d-orbitals of their respective penultimate shells, i.e., (n - 1) d-orbitals. Thus, General outer shell electronic configuration of d-block elements: (n - 1)d0-10ns1-2
Since the properties of these elements are midway or transitional between those s-block and p-block elements, they are also called transition elements. All these elements are further divided into four series called first,second, third and fourth transition series.
The first ionization series forms a part of the fourth period of the long form of the periodic table. It contains ten elements from Scandium to Zinc (2,Sc - 30Zn) in which 3d-orbitals are being progressively filled in. The second transition series which forms a part of the fifth period also contains ten elements from Yttrium to Cadmium(39Y-49Cd) in which 4d-orbitals are being progressively filled in. The third transition series which forms a partof the sixth period also contains ten elements i e, lanthanum (La) and from Hafnium to Mercury (72H1-goHg) in which 5d-orbitals are being progressively filled in.
The fourth transition series which forms a part of the seventh period also contains ten elements, I.e., Actinium (99 Ac) and elements form, kurchatovium (104 Ku) or rutherfordium (104R), to eka Mercury or Ununblum (Uu. = 112). In all these elements, 6d-orbitals are being successively filled in.
Thus the elements of groups, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12 belonging to fourth, fifth, sixth and the incomplete seventh periods in which the last electron enters the d orbitals of their respective penultimate shells are collectively called d-block elements. Some of the general characteristics of d-block elements are,
1 These elements frequently show variable oxidation state,
2. They are hard, malleable (malleable means these elements can be converted into sheets) and ductile (can be drawn into wires) metals with high melting and boiling points.
3. Their ionization energies are between s- and p-block elements.
4. Most of the transition metals such as V, Cr, Mn, Fe, Co, Ni, Cu etc, and their compounds are used as catalysts.
5. They form both ionic and covalent compounds.
6. Their compounds are generally coloured and paramagnetic. The colour is due to d-d transition or sometimes due to charge transfer mechanism,
7. They form complexes.
8. They are good conductors of heat and electricity.
9 These elements can form a number of interstitial compounds.
10. These elements are good at forming alloys due to their comparable sizes.
f-block Elements
Elements in which the last electrons enters any one of the seven f-orbitals of their respective ante-penultimate shells (last but two) are called f-block elements. In all these elements, the s-orbital of the last shell (n) is completely filled, the d-orbitals of the penultimate (n-1) shell invariably contains zero or one electron but the f-orbitals of the ante-penultimate (n - 2) shell (being lower in energy than d-orbitals of the penultimate shell) gets progressively filled. There are two series of f-block elements each containing 14 elements. Therefore, in all three are 28 f-block Elements in their periodic table. These are placed at the bottom of the periodic table.
The elements of the First series, t.e., Cerium to Lutetium (59Ce-7, Lu) which form apart of the sixth period are collectively called as lanthanoids since all these elements follow lanthanum in the periodic table and also closely resemble Lanthanum (57La) in their properties. These are also called rare earth elements since they occur scarcely in the earth's crust. In lanthanoids, 4f-orbitals are being progressively filled. The elements of the second series, i.e., Thorium to Lawrencium (90Th - 103Lr) which form a part of the incomplete seventh period are collectively called actinoids since all these elements follow actinium in the periodic table and also closely resemble Actinium (Ac) in their properties. In actinides or actinolds, 5f-orbitals are being progressively filled in.
All the actinides are radioactive elements. The first three elements, l.e., Thorium (Th), Protactinium (Pa) and Uranium (U) occur in nature but the remaining 11 elements, i.e., from Neptunium to vawrencium (93Np 103 Ls) have been prepared artificially through nuclear reactions. These eleven elements are called transuranic elements since they follow uranium in the periodic table and also have been derived from it through nuclear reactions. All the f block elements, I.e., lanthanides and actinides are also called inner transition elements since these elements from transition series within the transition elements of d-block. Some of the general
characteristics of f block elements are,
1.Most of the elements of the actinide series are radioactive.
2. They have generally high melting and boiling points,
3 They show variable oxidation states.
4. They are heavy metals.
5. They have a high tendency to form complexes,
6. Their compound are generally coloured. It is due to f-f transition
Thus, the periodicity of chemical and physical properties among elements is a natural result of the periodic recurrence of similar electronic configuration in the outer shell of the respective atoms when they are arranged in the order of their increasing atomic number,
Post a Comment