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?)

Section 2 b) Key Words

Electrostatic attraction: Attraction between two charged particles, e.g. nuclei and electrons

Group 1: The first group of the periodic table. Each element has 1 valence electron and is extremely reactive. Examples include lithium, sodium, potassium, caesium, francium

Lithium: Li, the least reactive group 1 element. Has two shells of electrons, 3 protons, and a relative atomic mass of 7.

Potassium: K, the third element of group 1. Has four shells of electrons, 19 protons and a relative atomic mass of 39.

Sodium: Na, the second element of group 1. Has three shells of electrons, 11 protons and a relative atomic mass of 23.

Valence electrons: Outer shell electrons.

Section 2 b) Specification

2.6 describe the reactions of these elements with water and understand that the reactions provide a basis for their recognition as a family of elements

Lithium, Li: Fizzes vigorously, moves around the surface of the water.
Sodium, Na: Fizzes violently, moves quickly on the water, forms a ball shape & yellow sparks.
Potassium, K: Produces a purple flame, violently moves around the surface and pops with the force of hydrogen production.

These elements each react vigorously with cold water (due to their similar electronic configurations), an indication that they are the most reactive group of elements. They produce hydrogen gas as well as a metal hydroxide.

2.7 describe the relative reactivities of the elements in Group 1

The reactions start small at the top of the table, but gradually increase in violence as you travel down the table as the atoms are bigger and less able to hold onto their valence electron. Francium reacts extremely violently and explosively when reacting with cold water, whereas lithium just fizzes.

2.8 explain the relative reactivities of the elements in Group 1 in terms of distance between the outer electrons and the nucleus.

The elements get more reactive as you go down the group because the increase in distance between the valence electron and nucleus is increased due to increased number of shells, resulting in a weaker force of electrostatic attraction and a more reactive element.