What determines how reactive an element is?

Hint In order to know what will determine the reactivity of an element, we must first know what a reactivity is. Reactivity is said to be the impetus with which a chemical substance will undergo a chemical reaction. This chemical reaction can take place within itself or with other substances.

Complete step by step solution.
Let us first understand what reactivity is. Reactivity is said to be the impetus with which a chemical substance will undergo a chemical reaction. This chemical reaction can take place within itself or with other substances. During this process there will be a release of energy.
Elements with partially filled orbital will react to form a completely filled outer shell. How easily an element will attain the stable configuration will be determined by its reactivity.
In the Group IA, the metals will be having one electron in the outermost shell. These metals will give/donate that one electron to attain the stable electronic configuration. In sodium the outermost electron is held little tightly compared to Potassium. Therefore, Potassium is more reactive compared to Sodium. Hence, we can say that the reactivity will increase on moving down the group.
The Halogens will be having seven electrons in the outermost shell. They will take/accept that one electron to attain the stable electronic configuration. The reactivity will usually decrease on moving across the period.

Note: We have to remember that elements with the completely filled outer shell are non-reactive because they do not lose or gain any electrons. Most noble gases like Helium, Argon, Neon, Krypton, Xenon and Radon are non-reactive as they have eight completely filled outer shells.