Chlorine (CL) would be the answer
The atmosphere is a envelope of gases surrounding another planet and the troposphere is the lowest part of the atmosphere
<em>FeCl2</em><em> </em><em>+</em><em> </em><em>Na2CO3</em><em> </em><em>=</em><em> </em><em>FeCO3</em><em> </em><em>+</em><em> </em><em>2</em><em>N</em><em>a</em><em>C</em><em>l</em>
c well actually I remember being tought this but I cant remember it 100