[Explained] Why do Alkali Metals have different Oxides?

 

Alkali metals burn vigorously when heated in oxygen and form different types of oxides depending upon the nature of the metal. Lithium forms monoxide (LI₂O), sodium forms a peroxide, while the other alkali metals (Potassium, Rubidium, Cesium) form superoxides having the general formula MO₂. Thus,

                                                        4Li + O₂  ⟶  2Li₂O

                                                        2Na + O₂ ⟶  Na₂O₂

                                                         K + O₂  ⟶  KO₂

Why does alkali metal form different kinds of oxides is explained below.

The reason why alkali metals have different oxides is because of the difference in size of the atom which is defined by the atomic and ionic radii of the alkali metal.

Why do Alkali Metals have different oxides?

The formation of different types of oxides by different alkali metals can be explained on the basis of their ionic sizes. Lithium ion is the smallest of all alkali metal ions. Due to small size, it has a strong positive field around it. The strong positive field around lithium ion attracts the negative charge so strong that it does not permit the monoxide ion, O^2- to combine with another oxygen atom to form peroxide ion, O₂^2-. 

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On the other hand, the weaker positive field around larger sodium ion permits the O^2- ion to combine with another oxygen to form O₂^2- ion. This is why lithium forms only monoxide, whereas sodium forms mainly peroxide. The larger K⁺,Rb⁺ and Cs⁺ possess still weaker positive fields around them and allow peroxide ions to further combine with oxygen to form superoxide ions, O^2-. This is why potassium, rubidium and cesium form mainly superoxides.

Thus it is generally said that a small cation can stabilize a small anion, whereas a large cation can stabilize a large anion.

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Atomic and Ionic Radii

The atomic and ionic radii of alkali metals show the following characteristics:

The ionic radii of alkali metals ions are smaller than the atomic radii of the corresponding atoms.  

For example the ionic radius of Na⁺ ion is 102 pm whereas the atomic radius of Na atom is 186 pm.

Alkali metals possess only one electron in their valence shell. During the formation of cation, the valence s electron is lost. The cation thus formed has one electrons shell less than the parent atom. The removal of an electron shell decreases the size. 

      Na           ⟶     Na⁺ + e^-

     1s²2s²2p⁶3s¹              1s²2s²2p⁶         

Moreover, the removal of an electron from the valence shell increases the effective nuclear charge experienced by the remaining electrons. Thus, the remaining electrons are pulled closer to the nucleus resulting in a further decrease in the size of the ion. 

The combined effect of the decrease in the number of the electron shells and an increase in the effective nuclear charge is responsible for the smaller size of alkali metal cations as compared to those of the corresponding alkali metal atoms. 

The atomic and ionic radii of alkali metals are the largest in their respective periods.

Each alkali metal atom is the first element of its period. As one moves from the left to right in a period, the differentiating electrons are added in the same electron shell and the nuclear charge increases with increase in the atomic number. Thus, in going from left to right in a period, the number of shells remains the  same but nuclear charge increases with each succeeding element. 

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Thus, the electrons in the valence shell experience a greater pull towards the nucleus. This results in the successive decrease of the atomic and ionic radii with increase in the atomic number. This is why the atomic and ionic radii of alkali metals are the largest in their respective periods. 

The atomic and ionic radii of alkali metals increase on moving down the group i.e. they increase in going from Li to Cs.

As one moves from Li to Cs in group 1, a new electron shell is added at each element and the nuclear increases in the atomic number. The addition of an electron shell at each element tends to increase the size of the atom but the increase in the nuclear charge has a tendency to decrease the size of the atom or ion. Thus, the two factors oppose each other. 

The increase in the number of shells increases the screening effect of the inner electrons on the valence s-electron. This results in the expansion of the electron cloud. As the screening effect is quite  large, it over weighs the contractive effect of the nuclear charges with increase in the atomic number. The net result is an increase in the atomic and ionic radii of the alkali metals in going from Li to Cs.

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