What causes the difference in reactivity between alkali metals?

[Calpalned]

We know that lithium, sodium and potassium tarnish when exposed to air, but rubidium and cesium catch on fire. Why is there such a huge jump in reactivity between Rb and K?

 

[BvU]

Good question. Because going down in the periodic system, the ions get bigger and bigger, ionization energies become smaller. So it's easier to start a reaction with the bigger guys.
Does that make it clearer ?

 

[James Demers]

BvU has it right. There's not really a "huge jump" in reactivity ... it's just a matter of reaching a threshold in the rate of energy release, to a point where things proceed differently. Expose K to hot air, or to a higher concentration of oxygen, and it will behave like Rb.