If you mean the most common or 'ubiquitous' element, then it is hydrogen (H).
Thallium is a naturally occurring element in the Earth's crust. It is not a man-made element.
The average of all the naturally occurring isotopes of a particular element are an element's atomic Mass.
Uranium, number 92 is the last of the naturally occurring elements and, of course, it has 92 electrons.
j
The known weighted-averagemass of all the naturally occurring* isotopes for an element is the atomic mass of the element.____________________*This is not the same as "all the known isotopes", becausemost elements have known isotopes that are not naturally occurring.
Aluminum is an naturally occurring element
Synthetic
Oxygen is a naturally occurring element.
Uranium and Plutonium
Xenon is a naturally occurring element.
By taking the wieghted averages of naturally occurring isotopes of that element. :)
If big means large atomic number and naturally occurring means if the world lacked humans to make nuclear tests or deposits, then Plutonium, element 94, is the answer. Some time ago plutonium was discovered in uranium ore, therefore making it the biggest, since technetium, francium, promethium, protactinium, neptunium, and other elements with atomic numbers inside the range 1 - 94 including endpoints have been found as natural, Plutonium is the biggest. Why not Americium or above? Some people say it's Californium, 98, to be the largest but alas that has been wrong since Americium all the way to Californium are only natural with pesky rocket scientists messing with fusion. Hope that helps.