Counting the atoms in a human body
The Sunday Nation
Nairobi,
28 July 2013
How many atoms are
there in a human body? Well, it depends on the size of the body. The
average adult weighs about 70kg and, even though we know that some
people can tip the scale at double that figure, we can still use it to
get a fairly good estimate – or at least a feel of the order of
magnitude.
The other quantity
needed is the average mass of an atom. Now here the variation is much
wider. The heaviest atom is over 200 times the mass of the lightest one.
The principle behind
the measurement of the mass of an atom is straightforward: you simply
fire an ion though a magnetic field of known intensity at a known
velocity and then measure the angle of deflection. Plug the numbers into
a formula and voila! The answer pops out.
It turns out that the
masses of atoms are range between
0.000,000,000,000,000,000,000,000,001kg and
0.000,000,000,000,000,000,000,000,1kg. Those numbers are extremely
small, but the latter is 100 times the former. The first one has 27
decimal places and the second one has 25.
27 decimal places may
be called a thousandth of a trillionth of a trillionth while the larger
number may be termed a tenth of a trillionth of a trillionth. Thus we
can immediately expect that number of atoms in a human body should be in
the order of thousands of trillion of trillions – a number with 27
zeroes.
Now the human body contains a lot of water – about 55 per cent of the
mass. Thus out of the 70kg, about 38kg is water. But a water molecule is
made of two atoms of hydrogen and one of oxygen – that’s why it is
written as H2O.
The mass of one water
molecule is about 30 thousandth-trillionth-trillionths of a kilogram.
Therefore, there are 1.27 thousand-trillion-trillion water molecules. We
get this by dividing 38kg by the molecular mass. But each molecule has
three atoms – two of hydrogen and one of oxygen; therefore, this makes a
total of 3.8 thousand- trillion-trillion (38 followed by 26 zeroes).
The remaining 45
percent of mass (or 22kg) mainly comprises of hydro-carbons; that is
compounds of hydrogen and carbon. Other elements are calcium (in bones),
nitrogen and phosphorous (both in DNA). All together, they make up about
another 2 thousand-trillion-trillion atoms. This brings the total to
about 6 thousand-trillion-trillions. Or, the number 6 followed by 27
zeroes.
Now that’s a huge
number. It is about 100,000 times the number of all the stars in the
observable universe. Which brings me to a strange idea: What if our
universe is just a humongous creature and the stars and galaxies are
atoms and molecules [respectively] that make up this being?
It’s not farfetched;
Ernest Rutherford did a similar extrapolation (albeit in reverse) when
he proposed the nuclear model of the atom – it looks like the solar
system with a nucleus (sun) the centre and electrons (planets) revolving
around it in orbits.
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