Changes

: *Used to express atomic and [[Molecule|molecular]] masses.*1 u = 1.660539040 x 10^-27 kg.*Provides a convenient scale for comparing the masses of atoms.*'''Unified atomic mass units''' may be denoted with a lower case 'u' or with an upper case 'D', followed by a lower case 'a'.: *1 '''unified atomic mass unit''' is approximately equal to the [[mass]] of a free [[nucleon]] ([[proton]] or [[neutron]]).: *A free [[nucleon]] has a [[mass]] slightly greater than 1'''u''', but when a [[nucleon]] is bound within a [[nucleus]] some of the [[mass]] of the free [[nucleon]] has been converted into [[energy]] due to the [[Mass-Energy Equivalence|mass energy equivalence]], giving it a slightly smaller [[mass]].*Atomic and [[Molecule|molecular]] masses are often expressed in [[Unified Atomic Mass Units|atomic mass units]] to simplify calculations.*The [[Unified Atomic Mass Units|atomic mass unit]] is essential for understanding stoichiometry and [[Molecule|molecular]] weight.