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 (-5[1])2.4 (10[1])billion (10[4])years ago. (10[5]-5[2])A (10[1])bond between atoms of this element is broken in the Chapman cycle. (10[1]-5[1])A decrease in this element’s (10[1])concentration (10[1])occurs during eutrophication (10[1])from (10[1]-5[1])algal blooms. (10[7]-5[3])By mass, (10[5]-5[1])this is the most (10[3])abundant (10[1]-5[1])element (-5[2])in the Earth’s (-5[2])crust. (10[3]0[1]-5[2])The Montreal Protocol (-5[1])banned CFCs (-5[1])to mitigate how (10[1])a compound of this element (10[1])was depleted (10[1])over the South (10[1])Pole, (0[1])leading to the formation of a namesake (10[1])“hole” (10[1])of that compound. (-5[1])For 10 points, (10[1])name this element that makes up ozone (10[1])and about 21% (10[2])of the atmosphere (10[1])with (10[1])nitrogen. (10[2])■END■ (10[3])