Lect notes 3a

Chem 1048A
BHSc
Learning Goal 3A:
The Mole Concept
(Attendance at lectures is necessary to complete these notes)



Ms Brankin
Office 303
(011) 717-6729


, The Atomic Mass Scale
• Atoms have very small masses.
1H mass = 1.6735  10-24 g

16O mass = 2.6560  10-23 g



• It is convenient to use atomic mass units (amu)
when dealing with the extremely small masses of
individual atoms
Hence:
1 amu = 1.66054 x 10-24 g
1 g = 6.02214 x 1023 amu

, Relative Atomic Masses
• Definition: mass of 1 atom of 12C = exactly 12
amu
• Masses of all atoms are expressed relative to the
mass of 12C.
• Such masses are called relative atomic masses
or atomic weights. Symbol : Ar

Example:
Ar of 19F = 19.00
Hence a 19F atom has 19.00/12.00 = 1.58 the mass of 12C

, Weighted Averages
• Since atomic weights compare the mass of an
atom to the mass of one atom of 12C, atomic
weight or relative atomic mass has no units.

• Accurate values of atomic weights determined by
mass spectrometry.

• Atomic weights listed in the Periodic Table are
weighted averages of the atomic weights of the
isotopes of the elements.