Chemical element whole number. For example, the relative atomic mass of chlorine is 35.453 u, which differs greatly from a whole number as it is an average of about 76% chlorine-35 and 24% chlorine-37. Whenever a relative atomic mass value differs by more than 1% from a whole number, it is due to this averaging effect, as significant amounts of more than one isotope are naturally present in a sample of that element. Chemists and nuclear scientists have different definitions of a "pure element". In chemistry, a pure element means a substance whose atoms all (or in practice almost all) have the
Westward the Course of Empire Takes Its Way in the Rocky Mountains" that was part of the 1898 Trans-Mississippi Issue, and reprinted a century later. A Currier and Ives print from 1868 uses the same title and theme for a very different print, showing a railroad crossing a new settlement as the train goes west. A and a stereograph by Alexander Gardner, both of an 1867 end-of-track frontier construction train, were titled "Westward The Course of Empire Takes Its Way". David Foster Wallace named one of his short stories "Westward the Course of Empire Takes Its Way" in his 1989 collection "Girl with Curious Hair". Early revisions of