Daddy, how many atoms are in me?

So asked my 8 year old daughter tonight. Never one to discourage curiosity or budding scientific inquiry, I answered “Lots and lots! Now finish your dinner.”

The look she shot back told me she had been hoping for a little more precision.

So, needless to say, we both now know the answer, and soon you will too!

Caution: unstable at high pressure.

A precious, if hazardous, material.

1. What elements am I made of?

We humans are mostly water, and since water is mostly oxygen, so are we! Here are the approximate mass percentages of elements in the body I used in the calculation.

2. How much of those elements am I lugging around?

Multiplying the percent by mass of each element by the body’s mass (in Kg) times one thousand gives the weight of each element in you in grams.

3. What does the weight of elements have to do with atoms?

Well, there’s a reason we bothered to switch into grams. The molar mass of elements is a conversion between the mass (usually in grams) and the number of moles of that element. For example, there are 15.999 grams per mole of oxygen.

A “mole” is a unit like a “dozen” or a “pair”, just a name given to a specific number of things. Like there are 2 things in a pair and 12 things in a dozen, there are 6.0221415 × 1023  things in a mole. The things in this case are atoms!

So when we added up the number of moles of each element and multiplied that by 6.0221415 × 1023, we had our answer!

There are roughly 2.57 × 1027 atoms in my little girl. So, I was right. Lots and lots. And lots.

Trying to put that massive number into perspective is another matter altogether. Here’s an attempt: if all 7 billion people on Earth each had a billion dollars, you would still need 367,248,242 Earths to have as many dollars as atoms. Or, if your atoms were the size of m&ms, you would be 2.4 times bigger than the Earth.  It didn’t really work. If anyone can suggest a better way to get a picture of a number that big, I’d love to hear it.

They grow up so fast...

In the meantime, the weight:atoms relationship is linear (if Human was an element, its molar mass would be ~6.325 g/mol), so if you want to know how many atoms you contain, multiply your weight by 9.52125  × 1025 (Kg) or 4.31877  × 1025 (lbs).

~ I’m about 5.53 × 1027 atoms (Which is how I shall report my weight the next time someone asks).

Oh, don’t forget to celebrate Mole Day on Sunday, October 23!

This entry was posted in Explanations, stories and tagged , , , , . Bookmark the permalink.

One Response to Daddy, how many atoms are in me?

  1. Darolyn says:

    Awesome information! Your way of explaining this is funny and interesting… and we love the added pictures and Mole Day video.

    Keep up the great work! We’re all better for it.


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