Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Atomic Mass
Atomic Mass
Every element has its own characteristic atomic mass, meaning the mass of a single atom of that element — a genuinely tiny quantity that is not practical to measure directly or to work with in grams (for example, a single hydrogen atom has a mass of about 1.6736 x 10^-24 g). To get around this, chemists agreed, by international convention (IUPAC, 1961), to measure every atom's mass RELATIVE to the mass of a single carbon-12 atom, which is taken as the reference standard and is assigned a mass of exactly 12.00000 atomic mass units (amu); every other element's atomic mass is then expressed relative to that standard. One amu (also written 'u') is accordingly defined as exactly one-twelfth of the mass of a single carbon-12 atom — a value that was later experimentally established to equal 1.66056 x 10^-24 g. More …
Worked out. Problem: the mass of one atom of oxygen is given as 26.56896 x 10^-24 g; find its atomic mass in u. Solution (own words): since 1 u is defined as 1.66056 x 10^-24 g, dividing the oxygen atom's mass in grams by the gram-value of 1 u converts it onto the u scale: 26.56896 x 10^-24 g divided by 1.66056 x 10^-24 g/u works out to 16.0 u. (By the identical method, one hydrogen atom's mass works out to 1.0080 u.) This shows how any atom's mass in grams can be converted onto the standard atomic-mass-unit scal …