Estimate the number of atoms that make up a 70 kg human body.
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Answer
About 7,000,000,000 quintillion atoms
Almost two thirds of those atoms are hydrogen, yet by weight the body is mostly oxygen.
How to estimate it
The body is mostly water. A water molecule weighs 18 atomic units and has 3 atoms, so the average atom weighs about 6 units, meaning 6 grams hold 6 × 10^23 atoms. 70 kg is 70,000 grams, and 70,000 / 6 is about 11,700 such portions. 11,700 × 6 × 10^23 = 7 × 10^27 atoms. The accepted estimate is about 7 × 10^27 atoms.
Source: Jefferson Lab (US Department of Energy)
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