If earth assembled out of space dust, how come we find heavy elements like gold, silver, uranium and bunch of others that are heavier than iron on the surface?
I mean silicon (Si mass 28.084) being the crust makes perfect sense as it is lighter than iron (Fe mass 55.845).
Shouldn't heavy elements been sank by gravity to the 'bottom' of the Earth when it was only starting to become a planet 4.54 billion years ago?
Answer
The key to answering this question is the Goldschmidt classification of elements. Thirteen of the long-lived elements are siderophilic; they preferentially bind to iron. Those thirteen elements are significantly depleted in the Earth's crust compared to their prevalence on meteors, asteroids, and the Sun.
This list of thirteen does includes rhenium to gold, but it stops at gold. Eighteen of the long-lived elements are chalcophiles; they readily bind with a chalcogen other than oxygen (e.g., sulfur). Copper and silver, along with mercury to polonium fall in this category. These chalcophilic elements have a reduced tendency to migrate toward the center of the Earth because of their greater affinity for combining with lighter elements.
That list of thirteen siderophilic elements also does not include the lanthanides or actinides. These elements, along with a number of lighter elements, are lithophiles. The lithophiles have a very marked tendency to combine with oxygen, and with other lithophiles. These lithophilic elements have an even more strongly reduced tendency to migrate toward the center of the Earth than do the chalcophiles.
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