Explain why is sodium less reactive than potassium.

As potassium (K) has more number of orbits, valance electron is held loosely by nucleus. Thus, it is easy to take out that electron. In general rule as we go down the group, Ionisation Enthalpy decreases and thus it becomes easy to take out electrons and hence element becomes more reactive. Ionisation Enthalpy of potassium is 419 KJ/mol^–1 whereas of sodium is 496 KJ/mol^–1.