The elements belonging to the group 13 to 18 of the periodic table, in which p-orbitals are progressively filled are collectively known as p-block elements.
In all these elements while s-orbitals are completely filled, their p-orbitals are incomplete. These are progressively filled by the addition of one electron as we move from group 13 (ns2np1) to group 17 (ns2np5). In group 18 (ns2np6) both s and p-orbitals are completely filled.
p-block elements show a variety of oxidation state both positive and negative. As we go down the group, two electrons present in the valence `s' orbital become inert and the electrons in the `p' orbital are involved in chemical combination. This is known as `inert pair effect'.
The inert pair effect is really a name, not an explanation. A full explanation involves the decreasing strength of the M-X bond going down the group (for covalent compounds) or the decreasing lattice energies of compounds containing the M4+ ion (for ionic compounds). In this way the energy input needed to form compounds of the formula MX4 are less likely to be balanced by the energy released when the four M-X bonds are formed, so the equilibrium favours the left hand side.
MX2 + X2 -- > MX4
The existence of a positive oxidation state corresponding to the group number and of another state two units lower is an illustration of the inert pair effect, the term referring to the valence `s' electrons, used in bonding in the higher oxidation state but not in the lower.
With the increase in atomic mass, the ionic character of bonds of the compounds of the group 13 (IIIA) elements increases, and some of the heavier metal ions do exist in the +3 oxidation state in aqueous solution. The stability of such compounds with the +3 oxidation state is, however, lower than those with the +1 oxidation state in the case of heavier members of this group. Thus thallium in +1 oxidation state is more stable than in +3 state. This is because, the s electrons in the ns sub-shell do not prefer to form bonds.
This inertness is found only, i) when the `s' electrons are in the fifth or higher principal quantum number ii) when their loss does not afford a species with a noble gas configuration. This property of stabilising the lower oxidation state keeping the paired electron in the ns orbital is referred to as the `inert pair effect'. This effect is also observed in the elements of groups 12 (IIB), 14(IVA) and 15(VA) where the heavier elements exhibit 0, +2 and +3 oxidation states respectively.
Nature of oxides
Oxides of p-block elements may be basic (in case of metallic elements), amphoteric (in case of metalloids) or acidic (in case of non-metals). Non-metals also form a number of oxyacids. In all the groups, the acidic character of the oxide decreases as we move down the group while it increases in the same period from left to right.
Basic oxide - Bi2O3
Amphoteric oxide - SnO, SnO2, PbO, Pb2O3
Acidic oxides - SO3, Cl2O7
Oxyacids - HNO3, H2SO4.
Basic character increases down the
CO2 SiO2 GeO2 SnO PbO
Acidic less acidic amphoteric basic most basic
Acidic character increases across a
Al2O3 SiO2 P4O10 SO2 Cl2O7
amphoteric acidic most acidic
Nature of hydrides
Many of the p-block elements form hydrides. The hydrides of non-metals are more stable. Thus in any group the stability of the hydride decreases from top to bottom; its strength as an acid also increases in this order. Thus among all the hydrides, hydrogen iodide forms the strongest acid solution in water. In group 15, nitrogen forms the stablest hydride of all. Thus the order of stability of these hydrides is
NH3 > PH3 > AsH3 > SbH3 > BiH3
Nature of halides
Out of the p-block elements, the non-metals form covalent halides. Metallic halides show a gradation from an ionic character to covalent character. As we move from left to right across the period, ionic character of the halides decreases and covalent character increases. For example, SbCl2 is partially ionic whereas TeCl4 is covalent.
In case metals forms halides in more than one oxidation states, halides in lower oxidation state are largely ionic and those in higher oxidation state are largely covalent.
Polarizability of a halide ion depends on its size. Iodides and bromides are more covalent while fluorides are more ionic.
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