The Earth's core is a mixture of Fe-Ni alloy, but there are also some lighter elements in there as well (we don't know exactly what, but speculate that there may be K, Rb, Si, or some other lower-density elements).Why is the core Fe-Ni metal? Well, it all comes down to two things: density, and a process called 'planetary differentiation'.

Our solar system is ~4.56 billion years old, and while the Earth was forming, it accreted a lot of material and (heat) energy, and was hit by a multitude of meteorites (putting more energy into the earth's system). All of this heat, combined with radioactive decay of some of the elements within the Earth, basically resulted in Earth spending the first bit of its existence as a homogeneous, well-mixed, molten bleb of a planet.

Enter planetary differentiation. As the Earth cooled, different parts of it developed contrasting densities and compositions (see below picture). The denser parts of the earth (i.e., the Fe-Ni core) gravitated towards the center of our planet, while those parts of the earth (i.e., mantle, crust) separated out more towards the surface. The same concept is demonstrated by pouring canola oil, water, corn syrup, and salt water into a bottle. Over time, their contrasting densities will order themselves so that the most dense material is on the bottom, and the least dense is on the top.

Earth Core

I hope this helped better your understanding of why the Earth's core is Fe-rich. Have a great day!

Cheers,

Iron is one of the most abundant elements in our solar system because it is the heaviest element that can be fused in stars (all of the elements heavier than Fe are formed during supernovae). So there was a lot of iron around when the solar system formed and therefore, there is a lot of iron in the Earth. (Side note - the Earth and the other rocky planets have basically the same chemical composition). Iron is denser than the other major elements in the Earth like oxygen, silicon, and magnesium, so iron quite simply sank to the center of the Earth during the early stages of our planet's formation.

When the Earth formed, it was wayyy hotter than today and likely existed in a state we call a "magma ocean", so it was much more simple for the iron to settle through liquid to the center of the planet than today when the mantle is solid rock. Iron took the other heavy element, Ni, with it, so our core is made up of Fe-Ni alloy.

There are some other elements sprinkled in the core as well, and determining the exact chemical composition of the core is a very actively researched and debated topic right now. All of the other rocky planets also have an iron core, and there are several asteroids that are large enough to go through this process of core separation called differentiation.

The crust and mantle are composed mainly of silicates. While some minerals contain both iron and silicates, iron is one of the first things to separate from silicate minerals when they melt. Iron is also much denser than silicates. During the first few tens of millions of years in our planet's history, Earth melted due to heat from impacts and decay of radioactive elements. Since iron is much denser than silicates and does not mix easily with them, the iron sank down to the center of the earth.

Early in Earth's history, the molten iron sank into the core because iron is denser than rock.