General Characteristics of p-Block elements

Elements in which the last electron enters any one of the three p orbitals of their respective outermost shells are called p block elements.

A p-subshell has three degenerate p-orbitals, each of which can accommodate 2 electrons, therefore, in all, there are six groups of p-block elements i.e. group 13,14,15, 16, 17 and 18 each containing 6 elements.

The atoms of elements of these groups receive their last electron in 2p, 3p, 4p,5p, 6p and 7p orbitals.

Electronic configuration

1) Boron, carbon, nitrogen, oxygen, fluorine and neon head these groups of p block elements. Their valence shell electronic configuration is ns² np^1-6 where n= 2-7.

2) The maximum oxidation state shown by a p block element is equal to the sum of the valence electrons or the group number minus 10. This is called  group oxidation state. Beside group oxidation state, p block elements show a number of other oxidation states.

3) In boron, carbon and nitrogen families, the group oxidation state is the most stable for the lighter elements in the group. A lower oxidation state which is 2 units less than the group oxidation state becomes progressively more stable for the heavier elements in each group.

4) The group oxidation state of group 13 elements is +3 but +1 oxidation state is most stable for thallium. The group oxidation state for group 14 elements is +4 but +2 oxidation state is most stable for lead.

Trend of occurrence of oxidation state two units less than the group oxidation state is called inert pair effect and becomes more prominent as we move down the group.

 

Chemical behaviour

1) p-block is the only one which contains metals, non-metals and metalloids. The common metal among p block elements are : aluminium, gallium, indium and thallium(group 13), tin and lead (group 14) and bismuth (group 15). The common metalloids are silicon, germanium, arsenic, antimony and tellurium  while all the remaining elements are non metals.

2) Non-metals have higher ionisation enthalpies and higher electronegativity than those of metals. Therefore non-metals readily forms anions.

3) The compounds formed by the union of highly reactive metals with non metals are generally ionic because of large differences in their electronegativities.

Compounds formed by the union of non-metals themselves are largely covalent in character due to small differences in their electronegativities. Oxides of non-metals are either acidic or neutral, the oxides of metals are always basic in nature.

More electropositive the metal, the more basic is its oxide and more electronegative the non-metal, more acidic is its oxides. Among p-block elements, the acidic character of the oxides increases or basic character decreases along a period. The basic character of oxide increases or the acidic character decreases down the group.

First member of each group of p block elements differ from its exceeding members of their respective group.

The 2 main reason for the differences are :

1) Size and other properties which depends upon size.

2) Absence of d orbital in their valence shell.

 

Size and other properties which depends upon size : Due to small size, high electronegativity and high ionisation enthalpy, the first element of each group of p-block elements differs from rest of the members of the respective groups.

Absence of d-orbital: The absence of d-orbitals in the elements of 2nd period and the presence of d orbital in the heavier elements.

a) Maximum covalency of four: The first member of each group has 4 orbitals in the valence shell for bonding and hence can accommodate at the maximum 4 pairs or 8 electrons. These elements show a maximum covalency of four. Elements of 3rd period of p-block elements have vacant 3d orbitals lying between 3p and 4s level of energy. Using these d-orbitals, the elements of 3rd period can accommodate more electron and hence can expand their covalency beyond 4.

1) Boron forms only [BF₄]‾ or [BH₄]‾ ion while Al gives [AlF₆]^3- ion.

2)Carbon forms only tetrahalides whereas other members form hexahalides, i.e.[SiF₆]^2–, [GeCl₆]^2- , [SnCl₆]^2- 

3) Nitrogen forms only NF₃ while phosphorus forms both trihalides i.e. PF₃, PCl₃ and pentahalides  i.e. PF₅ and PCl₅.

4) Fluorine does not form FCl₃ having 10 valence electron while chlorine forms ClF₃.

b) Reactivity: Due to presence of d orbital, the elements of 3rd period are more reactive than elements of 2nd period which do not contain d-orbitals.

 

Tetrahalides of carbon are not hydrolysed by water while tetrahalides of other elements of group 14 are readily hydrolysed. This hydrolysis involves the nucleophilic attack by water molecules and the pair of electrons provided by water is accommodated in the vacant d-orbitals.

c)Tendency to form multiple bonds :

1) The presence of d-orbitals also influences the chemistry of heavier elements in a number of other ways.

2) The combined effect of size and availability of d-orbital affects the ability of these elements to form π-bonds.

3) The first member of each group differs from the heavier elements in its ability to form pπ-pπ multiple bonds either with itself or with the other elements of the second period. This type of π-bonding is not strong in case of heavier p-block elements. The heavier elements also form π-bonds but this involves d-orbitals.

For Example: In SO₂ ,one of the two π-bonds between S and O involves dπ-pπ bonding while the other involves pπ-pπ bonding. In SO₃  two of the three π-bonds involved dπ-pπ bonding while the third one involves pπ-pπ bonding. In these dπ-pπ bonds, a half filled 3d orbital of sulphur overlaps with the half filled 2p orbitals of oxygen.

Since d -orbitals are of higher energy then p-orbitals, therefore, they contribute less towards the overall stability of the molecule as compared to pπ-pπ bonding between elements of of 2nd period. But at the same time, such type of bonding may increase the coordination number in species of heavier elements in the same oxidation state.