NSCI 1321 Lecture Notes - Lecture 3: Mass Spectrometry, Carbon-12, Mass Spectrum

• School: Fordham University
• Department: Natural Science
• Course: NSCI 1321
• Professor: Marie Thomas

Document Summary

A ce(cid:374)tral feature of dalto(cid:374)"s ato(cid:373)ic theory was the idea that a(cid:374) ato(cid:373) of a(cid:374) ele(cid:373)e(cid:374)t has a characteristic mass. A naturally occurring element may be a mixture of isotopes, each isotope having its own characteristic mass. However, the percentages of the different isotopes in most naturally occurring elements have remained essentially constant over time and in most cases are independent of the origin of the element. What dalton actually calculated were average atomic masses. What he could do was find the average mass of one atom relative to the average mass of another. We will refer to these relative atomic masses as atomic weights. To obtain the atomic weight of oxygen (relative to hydrogen), you need to know the relative numbers of hydrogen atoms and oxygen atoms in water. Dalto(cid:374)"s hydroge(cid:374)-based atomic weight scale was eventually replaced by a scale based on oxygen and then, in 1961, by the present carbon-12 mass scale.