Explain why the core of the earth is nickel / iron and why it is highly radioactive?

Question

Explain why the core of the earth is nickel / iron and why it is highly radioactive?

Answer 1

The interior of the Earth is so hot it is molten.  When you have liquids with different densities the heaviest liquid sinks to the bottom.

Nickel and Iron are dense - as is uranium.  These metals as well as other dense metals sink to the bottom - the center of the Earth.

Answer 2

When the earth and other rocky planets formed, the heavier material gravitated towards the centre.  Why there was so much iron and nickel around is a consequence of the fact that our solar system was formed partly from the porducts of old stars that ended their development as supernovae.

INormal stars, like our own sun, produce energy from hydrogen fusing to produce helium, plus large amounts of energy.  When the Hydrogen is used up (a few billion years yet, for our sun), the helium will then start to fuse to form heavier elements like lithium.  But when a normal star starts to form carbon as a result of its nuclear chain reaction, the fusion actions cease; because there is not enough energy to produce elements heavier than carbon.

In a larger star however; the effect of further gravitational collapse is great enough to fuse carbon and heavier elements.  But even a massive star cannot produce elements above iron in the periodic table.  This is because the amount of energy required to fuse iron is greater than the energy the reaction would release, so the chain reaction stops.

For a very massive star, when the iron rich core collapses, it produces so much gravitational energy that the star explodes in a massive supernova, and so much energy is released that elements all the way up to Uranium are produced in an instant.  

Other stars might eventually form from the products of these explosions (these are called 'second generation stars, and our sun is one of them).  This explains why there was a fair bit of iron and nickel around for our solar system to condense from.  As planets formed, this iron, through gravitational contraction moved towards the centre.  The energy of the gravitational contraction was not great enough to cause nuclear fusion, but was great enough to melt the iron and nickel.  This made it even easier for the molten heavy metal to be concentrated towards the centre.  Lighter magnesium and silica rich materials moved towards the outside.

Hence the earth has an iron-nickel rich core, and a silica rich mantle and crust.

The core is radioactive because small amounts of heavy radioactive uranium and thorium also were concentrated towards the centre. and crust.ded starsit stops