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Head to NCW web page to find out more about National Chemistry Week! The Earth is 4.565 billion years old, give or take a few million years. How do scientists know that? Since there’s no “established in” plaque stuck in a cliff somewhere, geologists deduced the age of the Earth thanks to a handful of radioactive elements. With radiometric dating, scientists can put an age on really old rocks — and even good old Mother Earth. For the 30th anniversary of National Chemistry Week, this edition of Reactions describes how scientists date rocks.