1. 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

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

4. 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

5. 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
E+27
55.85
E+25 2
E+25 Mg 17
1.0149E+27
24.305
E+25
E+25 Si
12.6
7.5222E+26
28.09
E+25 4
E+26 Ni
2.7
1.6119E+26
58.69
E+24
E+24 Ca
0.6
3.582E+25
40.08
E+23
E+24 Al
0.44
2.6268E+25
26.98
9.7361E+23 3
E+24
Sum of electrons =
E+26 O
27.8
E+27 16
E+26 -2
E+26 S
32.07
E+24
E+25
E+26

6. 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?

7. ***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.

8. Example: Nickel
Is the following reaction favored?
Fe  Fe e- Eored = – 0.43 V
Ni e-  Ni Eored = – 0.25 V
Eored = – 0.25 V – (-0.43 V)

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