Transcript Atomic Mass Notes

Atomic Mass
Masses in amu
• Only carbon-12 has an atomic mass
exactly equal to its mass #
• One atomic mass unit (amu) is defined
as 1/12 the mass of a carbon-12 atom
• However, protons and neutrons do not
have a mass of exactly 1 amu
• Example: The mass of chlorine-37 is
actually 36.966 amu
Isotopes
• To find the atomic mass of an element,
the masses of all the different isotopes
must be considered
• Example: Chlorine exists as 2 isotopes,
chlorine-35 (mass = 34.969 amu) and
chlorine-37 (mass = 36.966 amu)
Fractional Abundance
• Different isotopes are not equally
abundant in nature
• Isotopes are typically found in constant
ratios called fractional abundances
• Example: Chlorine-35’s fractional abundance
is 75.53%. Chlorine-37 makes up the
remaining 24.47% of the chlorine atoms.
Calculating Atomic Masses
• Due to fractional abundances of isotopes,
the atomic mass of an element must be a
weighted average.
• Is the atomic mass of chlorine closer to 35 or
37?
• 35
• Why?
• Chlorine-35 is the more abundant isotope
Calculating Atomic Mass:
Practice
Mass
Element
#
Mass
(amu)
Fractional
Abundance
35
34.969
75.53%
37
36.966
24.47%
Chlorine
Average
Atomic
Mass
Calculating Atomic Mass:
Practice
Mass
Element
#
35
Mass
(amu)
Fractional
Abundance
34.969
75.53%
Chlorine
Average
Atomic
Mass
35.45 amu
37
36.966
24.47%
Calculating Atomic Mass:
Practice
Mass
Element
#
Mass
(amu)
Average
Fractional
Atomic
Abundance
Mass
12
12
(exactly)
98.89%
13
13.003
1.11%
Carbon
Calculating Atomic Mass:
Practice
Mass
Element
#
12
Mass
(amu)
12
(exactly)
Average
Fractional
Atomic
Abundance
Mass
98.89%
Carbon
12.01 amu
13
13.003
1.11%