Introduction To Block Elements

The s – Block Elements General Introduction: The elements of group 1 and 2 and helium in-group 18 of the periodic table are called s- block elements. Except hydrogen and helium all the elements of s- block are metals.

Group 1 elements are called alkali metals because they react with water to form hydroxides, which are strong bases or alkalies. Francium (Fr) the last element of the group1 is radioactive and is formed by the radioactive decay of actinium.

The elements of group 2 are called alkaline earth elements. Because of their low density, alkali metals and alkaline earth elements are also known as lighter metals. Before studying the properties of group 1 & 2 elements, the two important aspects of chemistry are to be understood i.e. Diagonal relationship and anomalous behavior of first element of these main groups. DIAGONAL RELATIONSHIP

The elements of s- and p- block elements also show diagonal relationship, in addition to the group and period relationships. When

we move diagonally across the periodic able the elements show certain similarities, which are however far less pronounced than the similarities within the group. The diagonal relationship is particularly noticeable in the elements of second & third periods of the periodic table. The most common diagonal pairs are: Lithium and Magnesium, Beryllium and Aluminium, Boron and Silicon Cause of diagonal Relationship: As we move left to right in a period, the electro positive character goes on decreasing while on moving in a group from top to bottom, the electropositive character goes on increasing. These two effects tend to cancel each other in moving diagonally from top left to bottom right. There fore, the elements diagonally related in this way tend to have similar properties.

The diagonal relationship can also be explained in terms of polarizing power [Ionic charge/ ionic radius2] on moving along a period from left to right, the charge on the ions increases but ionic size decreases, hence polarizing power increases. It is vice versa for the groups. But on moving diagonally ionic charge increases and ionic radius also increases and hence the polarizing power remains constant & the elements exhibit similar properties. Anomalous Behaviour of Head Elements: The elements of group 2 are sometimes also called head elements of their respective groups. The first element in each groups1, 2, 13,14,15,16 and 17 shows certain properties which are different from that of other elements in their respective groups. This anomalous behaviour can be explained on the basis of following properties;

i) ii) iii) iv)

Smaller size of their atoms Their higher ionization enthalpies Their higher electronegativites Absence of vacant d- orbital in their valence shells.

This can be explained by taking the example of lithium and beryllium halides, which show more covalent character than the corresponding halides of other metals in their groups. Lithium hydroxides are thermally unstable whereas hydroxides of other alkali metals are stable to heat. Lithium unlike their alkali metals does not form a solid hydrogen carbonate. GROUP- 1 ELEMENTS OF ALKALI METALS

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