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