Explaining the Reactivity Trend of the Alkali Metals

1. When an alkali metal atom reacts, it loses its valence electron to form a positively charged ion.
Example
Li → Li⁺ + e
Na → Na⁺ + e
K → K⁺ + e

2. As we go down the group from one element down to the next, the atomic radius gets bigger due to an extra filled electron shell.
3. The valence electron is further and further from the nucleus. Thus the attraction force between the nucleus and the valence electron become weaker and weaker.
4. This causes the valence electron is easier to be released to form an ion when the atom takes part in a reaction.