Section 2 b) Summary

The elements of group 1 are:

• Li, Lithium
• Na, Sodium
• K, Potassium
• Rb, Rubidium
• Cs, Caesium
• Fr, Francium

They each have 1 valence electron, making them quite reactive. As you go down the group, from lithium to francium, the reactivity increases. This is because the number of shells of electrons increases, making the distance from nucleus to valence electrons further and decreasing the strength of the forces of electrostatic attraction. Because of these weaker forces, the atoms are able to lose their valence electrons more easily, making them more reactive. 

Group 1 elements react quickly and vigorously with cold water, indicating just how reactive they are. They must be stored under oil so they are unable to react with water or air while in storage. 

Reactions with cold water:

Lithium floats on the surface of the water due to its low density, gently fizzing and giving off hydrogen. It gradually reacts, forming an ionic compound that dissolves in water to form a lithium hydroxide solution. 

Sodium, like lithium, floats on the surface of the water. The heat from the reaction is enough to melt it, so as it moves around it forms a sphere, gradually decreasing in size. A white trail of sodium hydroxide follows this ball as it moves (due to hydrogen production propelling it), then dissolves to form a sodium hydroxide solution. It lets of yellow sparks as it reacts. 

Potassium reacts more violently than both elements before it. The reaction is faster, and the hydrogen produced is caught fire to, burning with a purple flame due to potassium contamination. Its product, potassium hydroxide, also dissolves. 

Rubidium, Caesium and Francium all react too violently to be demonstrated in a school, so this video shows how they (and the other ones too, with inaccuracy about francium which is unable to be obtained to react with water) react. 

Interesting (but unnecessary) information on Francium here (Francium is less reaction than caesium?)