GROUP TRENDS IN ALKALI AND ALKALINE EARTH METALS

In this article,the group trends in properties such as electron negativity,ionization potential,heat of hydration,sizes of atoms and ions and other physical properties are discussed.

(i) Electronegativity:-

The elements have very small electronegativity values and very generally called electropositive elements.Electronegativity decreases from Li through Cs as one moves down the group.Pauling's electronegativity values IA ans IIA group elements are summarized.

(ii) Ionization Potential:-

s-Block elements in general have low first ionization enthalpies because the outer s-electron is shielded very effectively from the nucleus by the inner electrons.The first ionization enthalpy of each of the alkali metal is lower than corresponding alkaline earth element in the same period, because the latter has an extra nuclear proton which causes an increase in the electrostatic attractive force between the nucleus and the outermost electrons.Secondly there is a progressive decrease in ionization enthalpies in each series as the outer most electron is less firmly held as the group is descended.These generalization can be seen by comparing the ionization enthalpy values of s-Block elements.      

The ionization enthalpies of elements that is the firmness with outer s-elements are held,is also related to the sizes of the atoms.The ionization enthalpies decrease with increasing size of the atom down the group.

(iii) Radii Sizes of Atoms and Cations of s-Block Elements:

The alkali metals have the largest covalent radii of all the elements in their respective periods because they possess the smallest nuclear charge which exerts the weakest electrostatic attractive force on the outer most electrons.The elements of Be family have the next largest atoms because of the same reasons.

(iv) Radii (Sizes) of Cations:

Since the elements of IA and IIA groups have large sizes,Low ionization enthalpy, low electron affinity values theses elements readily lose their valence electrons to form cations with a positive charge of 1+ and  2+ respectively.In the process of forming M1+ and M2+ions,Alkali and Alkaline earth metals acquire the configuration of the respective inert gases in the previous period.

In Na+ ions the outer electronic level now being empty, the number of protons in the cation exceeds the number of electrons.The size of the cations relatively much smaller than corresponding alkali metal counterpart in the same period.

(v) Hydration Energy of ions:

The alkali and alkaline earth metals require considerable amount of energy to form M1+ and M2+ ions respectively.But when these ions are formed in solution.M1+ and M2+,the energy required in the formation of gaseous ions is off set by the high negative values for the enthalpies of hydration of the ions.

The alkaline earth metal ions are more strongly hydrated than alkali metal ions because M2+ ions have a stronger electric field than M1+ ions.Hydration enthalpy increases with decreasing size of the ion and it is true both for M1+ and M2+ ions.Thus Li1+ is the most readily hydrated of the alkali metal ionas shown by its high negative value and small ionic size for the enthalpy of hydration.The same is true for the Be2+ ions in the IIA group ions.The ease of hydration falls down the group with increasing of size of the ion.

(vi) Electrode Potentials: 

The s-block elements are powerful reducing agents and this is evident from their high negative aqueous electrode potentials Li1+/Li couple has exceptionally high negative electrode potential because of its large value for the hydration enthalpy which promotes oxidation of Li to Li1+ ion.The electrode potential values are used to predict the ease of formation of M ions form the respective metal forms.

These values immediately point out that the alkali and alkaline earth metals cannot be used in voltaic cells based on water as the solvent because of their rapid oxidation by that solvent.