Average Atomic Mass
Chemistry 6(D)
Average Atomic Mass
Lesson Objective
• Calculate average atomic mass of an element using
isotopic composition
Contents of the Nucleus
• Nucleus – center of the atom, where protons and neutrons are
found
– Neutron ( n 0 ) – neutrally charged subatomic particle
• Mass of approximately 1 amu
– Proton ( p + ) – positively charged subatomic particle
• Mass of approximately 1 amu
• Atomic number – number of protons of an element
– Number of protons determines the identity of an element
– Number of neutrons can vary without changing the element
Isotopes
• Isotopes – atoms of the
same element that
contain different
numbers of neutrons
Ex) What is the mass number
of the isotope of carbon
that has 6 neutrons?
– Neutrons have mass
• Different isotopes of an
element have different
masses
– Isotopes are
distinguished using mass
numbers
Mass number = n 0 + p+
Mass number = 6 n0 + 6 p+ = 12
Carbon-12
Isotopic Notation
Isotopic notation
Ex) How many neutrons
does 17 O contain?
8
Mass
Number
n0 = 17 − 8 p+ = 9 n0
Atomic
Number
Element
Symbol
Isotopic Composition
• Isotopic composition represents the relative amounts of
each isotope of an element
Average Atomic Mass
Ex) Use the data table to find the
average atomic mass of silver
1.
2.
3.
Change the percent
abundance to decimal form
Multiply the mass of each
isotope by its percent
abundance in decimal form
Add the values found in
Step 2 together
Isotope
Percent
Abundance
Atomic Mass
(amu)
Silver-107
51.839%
106.91
Silver-109
48.161%
108.90
51.839 % =
51.839
= 0.51839
100
48.161 % =
48.161
= 0.48161
100
Silver-107 = 106.91 amu × 0.51839 = 55.421 amu
Silver-109 = 108.90 amu × 0.48161 = 52.447 amu
52.421 amu + 55.447 amu = 107.87 amu
Average Atomic Mass
Lesson Objective
• Calculate average atomic mass of an element using
isotopic composition