Atomic weights: Your results may vary

Seems that elements can have a range of atomic weights. Why, well because:

Some elements have more than one stable (nonradioactive) isotope—variants of the same substance, but with different numbers of neutrons in their atomic nuclei that alter the mass. (The element's identity is determined by the number of protons.) In those cases the atomic mass listed on the periodic table has traditionally been defined as an average depending on how common each isotope is in nature. But that average is not the same every place on Earth, or in every situation. Just as radiocarbon dating can place a substance in time, isotopic analysis can also pinpoint its location.

So, now instead of carbon listed as being 12.0107 atomic mass units with a measurement uncertainty of about 0.0008, it has an official atomic weight of [12.0096; 12.0116], where the brackets and semicolon indicate an interval of atomic weights. The interval doesn't reflect an uncertainty in measurement precision but rather a real variation of atomic weight from substance to substance. Only 10 elements will have these new intervals, because the others have at most only one stable isotope or because upper and lower bounds have not been quantified.

Speaking of atomic weights, let me recommend the book The Joy of Chemistry: The Amazing Science of Familiar Things . I should say more later, but for now this is a great book to give a layperson a good overview of chemistry. And it includes experiments you can do at home with common household items.