Chemistry 1027A/B Lecture Notes - Lecture 12: Unified Atomic Mass Unit, Mole Fraction, Molecular Mass

• School: Western University
• Department: Chemistry
• Course: Chemistry 1027A/B
• Professor: Tsorng- Yeh Lee

Document Summary

Approx. same mass as neutrons (equal to one atomic mass unit: neutron. Approx. same mass as protons (equal to one atomic mass unit: electron. Mass equal to about 1/2000th of the mass of a proton and neutron. The number of protons in an element corresponds to atomic number. The mass number is the total number of protons and neutrons in an atom. Mass number = number of protons + number of electrons. Atoms with same number of protons and different numbers of neutrons. Same atomic number but different mass number: ex. Chlorine has two isotopes: cl-35 and cl-37. Elements can have more than two isotopes. Sometimes there is a large quantity of one isotope and only a small amount of another isotope. Isotopic abundance: percent by number of each of the isotopes of an element, there are three isotopes of silicone and their abundances: Average atomic mass = percent isotopic abundance x isotopic mass.