What controls how different atoms/ions combine together?
What are isotopes and why they are important?
It is the number of protrons in the nucleus that determines what element an atom is. The number of neutrons can vary. Isotopes are different versions of an element with different atomic weights due to more or less neutrons than other versions. Isotopes can be either stable or unstable. The latter produce radioactivity as they decay. Hydrogen as the isotopes deuterium and tritium, with one and two neutrons respectively. Both are unstable, and are what makes 'heavy water.'The amount and ratios of isotopes in geologic material can be very informative.
What determined the chemical make-up of the earth?
The broad details are fairly clear. The earth formed from meteorites, comets and other early solar system debris circling the early sun. So certain types of meteorites (chondrites) give us a clue as to what the earth formed from, and what primitive compositions were. What determined the chemical make-up of the early solar system debris? Ultimately it is the stars we must look to. Stars are the thermo-nuclear furnaces of the elements. For some elements super-nova reactions are thought to be required. There are more details, if you want to track them down, of what elements are produced by what types of stellar reactions and stars.
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