Question

what happens when alkali metals are exposed to air and moisture ???​

Answer 1

Alkali metals react with air to form caustic metal oxides. The heavieralkali metals (rubidium and cesium) will spontaneously ignite uponexposure to air at room temperature. ... The heat produced by this reaction may ignite the hydrogen or the metalitself, resulting in a fire or an explosion...answer

Answer 2

When alkali metals are exposed to air and moisture they get tarnished due the formation of oxide , hydroxide or carbonates on their surface.

M + O2 --------> M2O (metal oxide)

M2O + H2O ----------> MOH (metal hydroxide)

MOH + CO2 ------------> M2CO3 (metal carbonate) + H2O

Now when alkali metals are heated in excess of air or oxygen they burns vigorously forming different types of oxides.

Formation of different types of oxides depends on the Nature of different alkali metals.

Lithium forms normal oxide containing peroxide ion (O^2- )

Sodium forms peroxide containing peroxide ion (O2^2-)

Others form superoxide containing superoxide ion (O2^1-)

Lattice effect theory tells that only smaller cation can stabilize tha charge if smaller anion and larger cation acn stabilize the charge of larger anion.

Lithium combines with oxide ion and due to strong positive field of lithium it pulls the electron cloud of oxide ion towards itself and does not allow oxide ion to form covalent bond with other oxygen atoms. This is the reason why lithium forms only normal oxide .

Sodium due to weak positive field can't pull the entire electron cloud of oxide ion and also can't prevent it to form covalent bond with other oxygen atom and thus form peroxide.

Others on going down the group the positive field decreases due to increase in size of atom , so the rest of the alkali metals can't prevent the oxide ion to form superoxide by combining with other oxygen atoms .