Explain the relationship between the reactivity of an element and the likelihood of it existing as an uncombined element.

Reactivity is a measure of the reaction potential of an element. In other words, reactivity of an element tells us the likelihood of an element reacting with another element. For example, an element that has a high reactivity with oxygen, is likely to react with it, on exposure to oxygen or air. 

An element, that has a high reactivity, is more likely to exist in the combined form than uncombined form. For example, if an element has a high reactivity with oxygen, it is more likely to exist as oxide than as pure element, in nature. Nonreactive elements (such as noble gases), on the other hand, are more likely to occur in the pure form, since they are unable to combine with other elements. 

Thus, reactivity of an element is a deterrence to its existence in uncombined or pure form, in the nature.

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