CHEM 1A03 Lecture Notes - Lecture 3: Amedeo Avogadro, Carbon-12, Molar Mass

• School: McMaster University
• Department: Chemistry
• Course: CHEM 1A03
• Professor: David Brock

Document Summary

A specific compound always contains the same elements in definite proportions by mass. All atoms of the same element have the same mass. Scientists reacted elements with a standard element (assigned a particular mass) and determined a relative mass based on the definite proportion that they combine with. Relative atomic mass is the mass of an element that would react with a fixed mass of a standard element, currently carbon-12. By assigning carbon a value of 12, nearly all other elements are close to whole numbers. An atomic mass unit (u) is used for the relative atomic mass of elements and molecules and is defined as 1/12 the mass of a carbon-12 atom. (1 u = 1. 660 539 x 10-27 kg) Isotopic abundance is the percentage of an isotope in a sample of an element. Amadeo avogadro (1776-1856) defined a number for grouping small entities such as atoms, ions, and molecules and this value is known as avogadro"s constant (na).