Trend 1.2 of the Lithium Group (Group IA) Trends is that the reactivities of metals toward O2 and H2O increases down the column.
Reactions with water:
The group 1 metals all react with water, ranging from a light fizzing (Li) to instantaneous explosion (Cs). These reactions all create a metal hydroxide, which raises the pH of the solution. Rather than a Brønsted-Lowry Acid/Base reaction, the reaction is instead Lewis acid/base, with H2O as a Lewis base, and the Group 1 metal as a Lewis Acid. The trend of increasing reactivity down the group is due to the “softness” increasing of the Lewis acidic metal ions.
2Li(s) + 2H20(l) à 2LiOH(aq) + H2(g)
Lithium reacts comparatively slowly, yet still quite quickly. It generates the most heat out of group 1 (222kJ/mol), but not enough to make it melt, because of its high melting point (454K).
2Na(s)+2H2O(l) à 2NaOH(aq) + H2(g)
Similar to Lithium, sodium is comparatively slow (the second slowest of the group). However, it generates enough heat in a quick enough time to melt the metal (184kJ/mol), as well as enough to ignite the H2 gas, assuming it is not moving too quickly (H2 gas propels Na around the surface of the water)
Potassium reacts the same as sodium, but is a much quicker reaction, so it normally reacts explosively
Rubidium and Caesium
Rb and Cs are both more dense than H2O, therefore they sink before exploding. Caesium reactions have been known to break containers due to the violent, exothermic reaction (∆Grxn=-203kJ/mol)
Reactions with Oxygen
All of the group 1 metals react with oxygen in the air (and Lithium reacts with the Nitrogen as well). With heat, they all burn. However, just contact with the air will cause them to be oxidized, forming a layer metal oxides.
The reactions are:
4Li(s) + O2(g) à 2Li2O(s) (Lithium Oxide)
2Na(s) + O2(g) à Na2O2(s) (Sodium Peroxide)
K(s) + O2 (g) àKO2(s) (Potassium Superoxide)
Rb(s) + O2(g) à RbO2(s) (Rubidium Superoxide)
Cs(s) + O2(g)àCsO2(s) (Caesium Superoxide)
Oxygen reacts in the form of three different ions: Oxide (O2-), Peroxide (O22-), and Superoxide (O2-). All of the elements will react with the oxide to form:
4M(s) + O2 à 2M2O(s) when they are burnt.
The increasing reactivity going down the group is due mostly to the atomic radius, and shielding: the radius is increasing, thus making it easier for outer electrons to react with the oxygen; shielding causes the outer orbitals to be less penetrating, so that they are more likely to react with oxygen.