Explain the relationship between the reactivity of an element and the likelihood of its existence as an uncombined element. Give examples of elements which never exist as uncombined elements and those which are often found as uncombined elements.
There is a correlation between the reactivity of an element and its ability to be found as an uncombined element in nature. Elements that are highly unreactive can be found as uncombined elements in nature but elements that are highly reactive are never found as uncombined elements in nature. Perhaps the ultimate example of unreactive elements are the noble gasses (group 18 on the far right side of the periodic table). These gasses have an octet of electrons in their valence shells, which is a highly stable electronic configuration. As such they are very unreactive and are only found as pure gasses in nature. Most chemical elements form compounds in order to achieve an octet of electrons through chemical bonds. As far as an example of a highly reactive compound never found in nature as an uncombined element, sodium is the perfect example. Pure sodium is a metal but it is extremely reactive and readily ionizes to form the sodium cation. As a result, sodium metal is never found in nature and sodium only exists naturally as various ionic salts with other anions.
Typically the greater the reactivity of a metallic element, the lesser the likelihood of its existance as an uncombined element. There isn't a pattern of reactivity to uncombined forms in non-metallic elements. For examples: Gold (Au) has an extremely low reactivity, so it is commonly found uncombined with other elements. In fact Gold is a "Noble" metal. Like the "Noble" gases, it doesn't naturally form compounds with other elements. As you go up the reactivities to Iron, Copper and Aluminum, they can exist for a time as uncombined elements, but ultimately their reactivities lead to them combining into compounds; Iron to rust, Copper to Cupric Oxide (The color of the Statue of Liberty), and Aluminum to Aluminum oxide (a transparent micron thick coating). When you get to the Alkali Earth Metals, and the Alkaline metals, (Mg and Ca former, Na and Li latter) you have elements that never exist as uncombined elements, they always react quickly to form compounds.