I know that we can find how old the Earth is by knowing how much uranium ultimately decayed into lead. Uranium-238 has a very long to decay, about 4.5 billion years. So that is how we are able to understand how old the Earth is.
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It would protect best against C) Alpha radiation, as Beta radiation is stopped by lighter metals such as aluminium, and Gamma radiation can only be stopped by heavier metals such as lead.
Answer: 177g
Explanation:
Aw 12 = 6.02214076*10^23 atoms
mass = 12*88.70^23/6.022*10^23