Name: Electronic Arrangement of s-block Elements
Uploaded: 2019-07-19
Duration: 24 min 8 s
Description: Do you know how to write the electronic configuration of s-block elements? Is there any similarity in the electronic configuration of all the members of the...

Question

Explain why the alkali metals have low densities, are electrically conducting, good reducing agents, form monovalent ions, soluble salts and soluble hydroxides, which become stronger alkalis on descending the group.

Accepted Answer

The densities of alkali metals are quite low as compared to other metals. $Li, Na$ and $K$ are even lighter than water. The densities increase on moving down the group. However, $K$ is lighter than $Na$ .

The densities of metallic elements depend upon the type of packing of atoms in metallic state and also on their size. The alkali metals have close packing of metal atoms in their lattice because of the large size of their atoms, they have low densities. As, we move down the group from $Li$ to $Cs$ , there is increase in atomic size is well as atomic mass. But the increase in atomic mass is more to compensate the increase in atomic size. As a result, the densities (mass volume) of alkali metals gradually increase from $Li$ to $Cs$ . However, potassium is lighter than sodium probably due to increase in atomic size of potassium.

As smaller the size of the ion, larger the charge of the ion hence greater the hydration and smaller ionic conductance and mobility of the ion. So the increasing order of mobility the alkali metal ions in aqueous solution is ${Li}^{+} < {Na}^{+} < K^{+} < {Rb}^{+} < {Cs}^{+}$ .

Alkali metals are strong reducing agents. This is due to their greater ease to lose electrons. This is also indicated by the larger negative values of their reduction potentials. All of them are better reducing agents than hydrogen ( $E ° = 0$ ). Therefore, these metals react with compounds containing acidic hydrogen atoms such as alcohols and acetylene liberating hydrogen.

All the alkali metals exhibit an oxidation state of $+ 1$ in their compounds. The alkali metals have only one electron in their valence shell and therefore they can lose the single valence electron readily to acquire the stable configuration of a noble gas. Thus, they form monovalent ions, $M^{+}$ (e.g., $Li, Na, K, Rb, Cs$ ). Since the second ionization energies are very high, they cannot form divalent ions. Thus, alkali metals are univalent and form ionic compounds.

The compounds of the alkali metals are ionic in nature. Due to the low ionization energies and large atomic sizes the atoms of alkali metals form cations readily by losing the valence electrons. Consequently, they form ionic bonds with the non-metals of the $p -$ block.

The hydroxides of alkali metals are strongly basic and the strength of the base increases down the group. The $M - OH$ bond in the hydroxides of alkali metals is very weak and it can easily ionize to $M^{+}$ and ${OH}^{-}$ ions. This accounts for their basic character. Since the ionization energy decreases down the group, the bond between metal and oxygen becomes weak. Therefore, the base strength of the hydroxides increases accordingly. Thus, $NaOH$ is a stronger base than $LiOH$ and so on.

Explain why the alkali metals have low densities, are electrically conducting, good reducing agents, form monovalent ions, soluble salts and soluble hydroxides, which become stronger alkalis on descending the group.

Important Questions on s-Block Elements:Alkali and Alkaline Earth Metals. Alkali Metals:(Group 1)

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