This element’s concentration is measured with the Clark electrode. Preston Cloud used banded iron formations to date a high (10[1])concentration of this element to 2.4 billion (10[2])years ago. (10[2])A bond between atoms of this element is broken in the Chapman cycle. A decrease in this element’s concentration occurs during eutrophication (10[1])from algal blooms. (-5[1])By mass, this is the most (10[1])abundant (-5[1])element in the Earth’s crust. The Montreal Protocol banned CFCs to mitigate how a compound of this element was depleted over the South Pole, leading to the formation of a namesake (10[1])“hole” of that compound. For 10 points, name this element that makes up ozone and about 21% of the atmosphere with nitrogen. (10[1])■END■