User talk:2001:56A:70A7:A900:C04E:E3F2:C726:E836

Observable universe - section: "Matter content—number of atoms"
First sentence in this section includes:

"...and assuming all atoms are hydrogen atoms (which are about 74% of all atoms in our galaxy by mass, see Abundance of the chemical elements),..."

Strictly speaking this is correct and accords with Table 1 for Milky Way in the "Abundance of the Chemical Elements" article. However the wording maybe somewhat misleading. Note that Table 2 in the "Abundance of the Chemical Elements" article for the Solar System includes both mass fraction and atom fraction. Suggest include both columns in Table 1 as well. Easy to calculate one from another. 74% hydrogen by mass should come to about 92% by number of atoms. Then the above could be written:

"...and assuming all atoms are hydrogen atoms (which are about 92% of all atoms in our galaxy, see Abundance of the chemical elements),..."

Which I feel reads a little more clearly.

Note also that atomic weights for any given chemical element are usually (but not always) weighted averages over the various stable isotopes for that chemical element.

It is also possible to calculate weighted averages for other mixtures. This is done for air (28.966 grams per mole) which is a mixture of molecular nitrogen and oxygen, atomic argon etc.

The same can be done for the overall composition of ordinary matter in the universe, galaxy or solar system. The result is a value of approximately 2 grams per mole.