Chemical Formation of the Earth Cloud of elements condenses into planets As they do so, they react to form compounds They differentiate chemically and by density

Chemical Makeup of the Earth Two questions to answer: Why are there unreacted metals? Why are the metals in the core?

Primary Differentiation of the Earth The biggest redox reaction in the world metals + nonmetals  compounds Focus on “metals” and “nonmetals” Metal = elements that loses electrons Nonmetals = elements that gain electrons

Chemical Makeup of the Earth Bulk Earth Fe 35.4% Mg 17.0% Si 12.6% Ni 2.7% Ca 0.6% Al 0.44% O 27.8% S 2.7% All others: 3.5% Element % mass in Earth (g) molar mass mol element charge mol electrons Fe 35.4 2.11338E+27 55.85 3.78403E+25 2 7.56806E+25 Mg 17 1.0149E+27 24.305 4.17568E+25 8.35137E+25 Si 12.6 7.5222E+26 28.09 2.67789E+25 4 1.07116E+26 Ni 2.7 1.6119E+26 58.69 2.74646E+24 5.49293E+24 Ca 0.6 3.582E+25 40.08 8.93713E+23 1.78743E+24 Al 0.44 2.6268E+25 26.98 9.7361E+23 3 2.92083E+24 Sum of electrons = 2.76511E+26 O 27.8 1.65966E+27 16 1.03729E+26 -2 -2.0746E+26 S 32.07 5.02619E+24 -1.0052E+25 -2.1751E+26

Primary Differentiation of the Earth 1. So, we end up with excess iron. 2. It stays in the metallic state and is dense, so it sinks to the center of the earth, forming the metallic core. 3. Next question: what other elements go with Fe to the core? Which do not?

***If Fe can reduce a cation of the other element Reaction potential = Eoreaction = Eored(cathode) - Eo(anode) that is, = Eored (species reduced) - Eored (species oxidized) If Eoreaction is positive, the reaction is favored. ***If Fe can reduce a cation of the other element to its neutral state, that element goes to the core.

Example: Nickel Is the following reaction favored? Fe  Fe2+ + 2 e- Eored = – 0.43 V Ni2+ + 2 e-  Ni Eored = – 0.25 V Eored = – 0.25 V – (-0.43 V)

Deficient Elements: log(cosmic/crust) H 6.58 He 14.17 C 3.22 N 5.25 Ne 10.46 Compare to O: 0.79