I. ATOMIC MASS AND AVERAGE ATOMIC MASS
Atomic Mass = used to numerically indicate the mass of an atom in its ground state, it is expressed in
the non SI unit of u
u = refers to unified atomic mass unit (formerly known as atomic mass unit or amu)
1 amu = 1/12 the mass of carbon-12 atom,
therefore the mass of C-12 atom is made equal to 12amu
Carbon-12 atom is an isotope of carbon
1 amu = 1.66 x 10-24 g
Note that: Atomic mass of 12C = mass of p + mass of n + mass of e
Mass of e = 1/1800 of mass of p and n so it is negligible making the equation
Atomic mass of 12C = mass of p + mass of n
Atomic mass vs. Average atomic mass
• For carbon it is 12 u not 12.01 u • Used to relate the fact that the numerical value assigned to each
element in the periodic table reflects the average abundances of the
atoms that compose a naturally occurring element
• Related to isotopes
• For carbon it is 12.01 u
•
Chemists often will use the term “atomic mass” when they are
actually referring to average atomic mass of an atom.
II.RELATIVE ATOMIC MASS AND ATOMIC WEIGHT
Relative atomic mass and Atomic weight = different meaning from atomic mass but synonymous with
each other although different historical origins.
Relative atomic mass = refers to the “ratio” of the average mass of the atom to the unified atomic mass,
and it is dimensionless
Atomic weight = the name Dalton used in the early 19th century to numerically describe the weight of
atoms relative to each other

III.MOLAR MASS AND ATOMIC MASS
• Molar mass = provides a bridge between mass and amount
• Mass = expressed by grams
• Amount = moles
• Mass and amount are both visible quantities
• Molar mass is usually defined as the mass of 1 mole of substance, SI unit is g/mol
• Molar mass = MA = m/n = mass/chemical amount
• Molar mass vs. atomic mass
relates to macroscopic level ( unit is grams/mole) relates to atomic level ( subatomic which is neutrons and
protons) expressed in u
IV. EQUIVALENT WEIGHT = precursor to molar mass, early terms chemists gave to the study
of combining ratios of elements and is based on how much an element weighs relative to
another element.
V.MOLECULAR WEIGHT, RELATIVE MOLECULAR MASS, AND RELATIVE MOLAR MASS = all
involve relative quantities
RELATIVE MOLECULAR MASS, Mr = defined by IUPAC as the “ratio of the mass of a molecule
to the unified atomic mass unit”
MOLECULAR WEIGHT = in the CRC Handbook of Chemistry and Physics is described as “the
ratio of average mass per molecule or specified quantity of substance to 1/12 of the mass of
nuclide 12C”
• Molecular weight, relative molecular mass, and relative molar mass are considered
to be synonymous with each other but not synonymous with MOLAR MASS.
• Units: Carbon’s molecular weight, relative molecular mass and relative molar mass
are 12.01 and are dimensionless
• MOLAR MASS of Carbon is 12.01 g/mol
V. FORMULA WEIGHT – conceptually similar to molecular weight but with one key difference,
referring to ionic compounds instead of molecular compounds.