I was unable to answer this question from my daughter. Is it just a coincidence or is there a connection between the following two observations: (1) the core of the earth is made of an iron (-nickel alloy) as well as the existence of iron meteorites with the fact that (2) the heaviest element that can be produced by a star (prior to supernova) is iron?
From what I know, iron is the element that has the lowest energy combination of protons and neutrons as nucleus. Heavier elements can produce energy by fission. Lighter elements can produce energy by fusion.
It is related to the production of stars (prior to supernova), as stars sustain themselves by producing energy, which they get from fusion. Once they have changed what matter they have available to iron, they can no longer produce energy to fight gravitational collapse, which may result in a variety of events depending mainly on their mass.(wikipedia)
So I guess iron is a very heavily produced element in a star lifecycle, which explains that it is found in abundance. It is also the heaviest of those produced abundantly, hence Archimedes sends it at the bottom of things.
If you look at the fusion cycle in stars energy production, you may notice that the more common elements play an important role. (wikipedia)
Elements heavier than iron are rarer than the ligter ones.