And how do scientists measure the average amount of molecular mass for each atom, they cant check every atom of the particular element in the universe to determine its molecular mass , and if scientists take samples from areas rich in a particular type of isotopes, wouldnt it affect the total average molecular mass of an element.
In: Chemistry
The isotope distribution actually affects the mass of molecules build with them. For example a water molecule made with deterium (1 proton 1 neutron) has a higher mass instead of protium hydrogen (1 proton 0 neutron. Water with an high fraction of deterium water molecules has a higher density than normal occuring water (which just have a very small amount of deuterium water). That’s why it’s called heavy water.
The element weight you can find in the periodic table is a an average value of the natural occuring isotope distribution (on earth). That’s the reason why these are not integers, as you would expect for integer amount of protons and neutrons, but fractional values. The atoms in hydrogen gas has a slightly higher mass than 1 u, as these are not only protium with 1 u, but also a few deuterium atoms with 2u.
The isotope distribution of elements on earth does not vary that much, so these average values are totally fine for most applications. Depending on the source you might find slightly different numbers for the elements however (as it depends on what materials were exactly measured).
If you need really exact values for a specific sample, you can do mass spectroscopy to determine the isotope distribution of a sample, and therefore it’s average atom weights.
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