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The two major constituents of the Earth's core are iron and nickel. In documentaries and scientific conversations, iron gets more attention than nickel, probably because it makes up a bigger percentage of the core.

But before I consider this scenario—where it was nickel that makes up the bigger percentage instead of iron—I must ask:

Why does iron make up the majority of Earth's core as opposed to nickel? Does either metal have an advantage over the other? If so, which advantage? If nickel is the number-one metal of the core instead of iron, how would the magnetic field be affected?

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    $\begingroup$ "Does either metal have an advantage over the other?" Advantage for whom? Unless you believe that the earth was created by some divine being with some kind of an agenda, it doesn't make sense to ask about advantages. The earth's core wasn't made the way it is to realise some advantage: it just happened that way. $\endgroup$ – David Richerby Dec 14 '15 at 9:20
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    $\begingroup$ @DavidRicherby: Arguably, the scenario that led to the Earth's formation was advantageous for success, over other contemporary possibilities that might have otherwise occurred instead. This is the same way we talk about evolutionary advantages: in the "competition" to form the core of a planet, Iron seems to have won out. I'm not saying that's actually the case or not here, but I believe it's what the OP is asking. $\endgroup$ – Lightness Races with Monica Dec 14 '15 at 12:09
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Simply put, the amount of iron in the solar system/universe is higher than nickel due to a process called nucleosynthesis.

Basically at it's origin the universe comprised only of hydrogen, through nuclear fusion heavier elements were/are being created. Fundamentally, hydrogen is fused into heavier elements in the star, our sun is currently turning massive amounts of hydrogen into helium through the proton-proton chain.

Eventually as a star gets older you start seeing fusion of heavier elements, helium, carbon, nitrogen, etc up until you hit iron at which point you hit what is known as the iron peak where fusing iron atoms together requires more energy than is release. Heavier elements are all formed during supernovas and thus we see lower level of abundance. That being said the abundance of nickel is still higher than all the other heavier metals.

https://en.wikipedia.org/wiki/Stellar_nucleosynthesis

http://physics.info/nucleosynthesis/

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    $\begingroup$ The early Universe was closer to 92% Hydrogen and 8% helium en.wikipedia.org/wiki/… Otherwise, spot on. $\endgroup$ – userLTK Dec 14 '15 at 9:00
  • $\begingroup$ @userLTK However, that early-universe (prior to the formation of stars) helium was in fact the result of nuclear fusion. It's just due to the incredible particle density and high energy of the baryogenesis phase that allowed some protons to get close enough to each other to form deuterons, tritons, and alpha particles. The latter were later able to capture electrons to become the Helium atoms in the overwhelmingly Hydrogen universe. $\endgroup$ – Monty Harder Dec 14 '15 at 18:44
  • $\begingroup$ @MontyHarder I agree. I just felt the statement "At it's origin the Universe comprised only of hydrogen" was a bit too imprecise. $\endgroup$ – userLTK Dec 15 '15 at 15:25
  • $\begingroup$ Parts of this answer aren't true. In stars, Fe-52 fuses with an alpha particle to form Ni-56. Many elements heavier than iron form by the "s-process" in stars, without requiring a supernova. en.wikipedia.org/wiki/S-process $\endgroup$ – DavePhD Dec 17 '15 at 21:54
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I can only answer the question of why is it mostly iron. Not too sure of the magnetic properties of iron versus nickel.

As said in another answer, there is simply much more iron around than nickel. But the earth has also a large amount of other metals: silicon, magnesium, calcium, aluminium. So why is the core made of iron-nickel and not the other stuff? The answer is in the oxygen.

When you take a metal and oxygen, you form an oxide - usually a solid, brittle, "earthy" material. The best example is metallic iron + oxygen = rust. But not all metals are the same - some metals "like" oxygen more than others. If you have one atom of magnesium and another of iron, and only one atom of oxygen around - it will first bond with magnesium. Considering the entire oxygen budget of the Earth, there was enough oxygen to bond with all of the silicon, magnesium, calcium, aluminium (and just about the rest of the periodic table) to form the solids we usually refer to as rocks. By the time you got to the less oxygen-loving elements, such as nickel and iron, you had very little oxygen left. You had enough to oxidise some of the iron and nickel, but not all of it. That's why there was a lot of iron and nickel that was left as metals.

During Earth's formation, the entire thing was molten. Metallic iron and nickel are denser than the oxidised stuff ("rocks"), so it sank to the bottom, forming the metallic core. The oxidised metals ended up forming the mantle on top of it.

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    $\begingroup$ Nice answer. A fun party question -- your mileage may vary depending on the sort of people at parties you go to -- is to have people guess what percentage of the mass of the crust of the earth is oxygen. People think of oxygen as a nigh-massless gas, but of course elemental oxygen is almost 50% of the mass of the crust, and almost ten times the mass of the iron in the crust. $\endgroup$ – Eric Lippert Dec 14 '15 at 15:42
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The main reason that there is more iron than nickel in the Earth's core, and in the universe generally, is that Nickel-56 beta-decays to Iron-56 (via cobalt-56).

Much nickel-56 forms in Asymptotic Giant Branch stars and supernovae. However, nickel-56 decays with a half-life of 6 days.

In nuclear fusion in stars, He-4 nuclei (alpha particles) form from hydrogen.

In large stars, fusion does not stop when helium is formed.

Instead, carbon-12 is formed from three alpha particles.

Carbon-12 nuclei fuse to yield oxygen-16, neon-20 and magnesium-24. All these nuclei have equal number of protons and neutrons.

The series continues, by fusing one after another alpha particle.

Iron-52 fuses to Nickel-56.

However, it is not energetically favorable to add another alpha particle to go from nickel-56 to zinc-60.

Nickel-56 decays to Iron-56.

So in summary, it is the combination of the fact that Nickel-56 is at the end of a series of favorable alpha particle fusions, and the fact that Nickel-56 decays to Iron-56, that makes iron particularly abundant.

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