## How Many Atoms

A typical human weighs about 12 stone of 170 pounds or 77 kg.
People are made up of atoms - mostly oxygen, carbon, hydrogen, nitrogen - arranged into molecules, some very big and some self replicating. Typically these atoms have about 10 neutrons or protons, each with a mass of
$1.66 \times 10^{-27} kg$
, so have a mass of about
$1.66 \times 10^{-26} kg$
.
The number of atoms in a human body is then about
$\frac{85}{1.66 \times 10^{-26}}=5.1 \times 10^{27}$
.
If this sounds big, it is only a tiny fraction of the number of atoms in the Earth. The Earth has a mass of about
$5.98 \times 10^{24} kg$
, so there are about
$\frac{5.98 \times 10^{24}}{1.66 \times 10^{-26}}=3.60 \times 10^{50}$
atoms in the Earth.
The Sun has a mass of about
$1.99 \times 10^{30} kg$
, so there are about
$\frac{1.99 \times 10^{30}}{1.66 \times 10^{-26}}=1.20 \times 10^{56}$
atoms in the Sun.
These numbers all shrink compared to the number of atoms - mostly hydrogen in the Universe, believed to be about
$10^{79}$
.

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