What do many elements found in nature exist as?
Elements that are discovered in nature appear in multiple ways:
A pure, natural form of itself, like gold or copper. Known as ore in this state. Less than 100 of the 116 or so elements are found like this.
In radioactive decay - daughter elements like uranium.
In compounds. Some elements are extremely reactive and are never found by itself, always with an accompanying element. These two (or more) elements react and create a compound. Scientists then extracts and isolates the elements within the compound. Alkaline Earth metals are found this way.
Man-made. Plutonium can occur in natural geological circumstances, but very rare, "Scientists discovered that, by allowing fast neutrons to collide with the common isotope of uranium known as U-238 in a nuclear reactor, the ‘new’ element plutonium was made".
Hope this helps :)