Is there ever an accurate atomic mass number for each element?
Answer:
The atomic mass of an element is really a whole number. It is found by adding the number of protons and neutrons found in an atom. (each proton and each neutron have a mass of 1 amu) However, not all atoms of an element have the same number of neutrons. For example Carbon-12 and Carbon-13 are both Carbon. But Carbon-13 has an extra neutron. The number given on the Periodic table is an average based on how common each variety is found.
Rounding to the nearest whole number will give you the correct atomic mass for the most common variety.
The second chart rounded it. I have one that rounds to 5 decimal places. I would use up to 2 decimals, but in this case, i guess it's rounded to 16. Maybe use 15.99.
Atomic mass is pretty much the number of protons and neutrons in the nucleus. Similar elements have the same number of protons in them. However elements sometimes have different number of neutrons called isotopes. Some isotopes are more abundant in nature.
The atomic number you see on the periodic table is the weighted average of how abundant is in nature. (for example, if 25% of element X has a mass of 20 and 75% had a mass of 21, than on the periodic table it would read 21.75).
That would explain elements like Chlorine with a atomic mass of 35.45.
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