How can we predict which elements are reactive under normal conditions and which are unreactive?

We can predict or comment about reactivity of elements by taking into consideration their electronic configuration that imparts stability or un-stability to an atom.

Explanation:

Reactivity : It refers to the affinity of a substance to combine with other substance.

Elements are mainly of two types:

Metals and Non metals

In terms of metals or non metals it can be defined as tendency to loose or gain electrons.

On the basis of it, we can easily make out or comment about the reactivity of element (metal or non metal). Like, if an element has completely filled outermost shell or we can say have stable electronic configuration then it is already stable. Therefore, it won't react with any other element. But, if its outer shell isn't complete then in order to attain stability it will combine or react with other. Hence, we can say it will be quite reactive.