The elements in the long form of the periodic table has been divided into four blocks, namely s, p, d & f blocks. The elements of group I & II receive their last electron in s-orbital. So they are called as s block elements. The metals Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr) which have one electron in their outermost shell belongs to group I and are called alkali metals as they react with water to form hydroxides which are strong bases or alkalies. The elements of group II are Beryllium (Be), Magnesium (Mg), Calcium (Ca),Strontium (Sr), barium (Ba) and radium (Ra) which have two electrons in their outermost shell. All these elements are also metallic in nature and are commonly known as alkaline earth metals with the exception of beryllium. Because of their low density, alkali metals and alkaline earth metals are called lighter metals. Both alkali and alkaline earth metals are highly reactive and hence do not occur in free state but found in combined state. Whereas alkali metals mostly occur as halides, oxides, silicates, borates and nitrates, alkaline earth metals mainly occur as silicates, carbonates, sulphates and phosphates. Some alkali & alkaline earth metals occur abundantly in nature. Calcium is the fifth, magnesium is the sixth, sodium is seventh and potassium is eight barium is the fourteenth and strontium is the fifteenth most abundant element by weight in the earths crust. Sodium and magnesium are also present in sea water brine wells and few salt lakes. Anomalous behaviour of first elementThe first element of a group differs considerably from the rest of the elements of the same group. This anomalous behaviour is due to(i) Smaller size of their atoms(ii) Their higher ionization energies(iii) Their higher electronegativities(iv) Absence of vacant d orbitals in their valence shell(v) High polarizing power of its cation.
