Answer:
-the number of outer electrons
Explanation:
Outer electrons are referred to as valence electrons and they are typically what determines the property of an elements such as their reactivity.
For eg, Na (sodium) has 1 valence electron and it really wants to give it aways so it can return back to octet rule (having 8 electrons in outershell) to become stable. So when you put sodium in water (you can watch videos of it) it literally blows because of high reactivity.
