Honors Chemistry Worksheet Template Page 2
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4Average Atomic Weight
The atomic weight of an element is the weighted average of the masses of the isotopes of that
element. The weighted average is determined using the abundance and mass of each isotope. Most
elements have more than one naturally occurring isotope.
For example, there are two naturally occurring isotopes of copper, copper-63 (62.93 amu) and copper-65.
(64.93 amu). The natural abundances of the isotopes are 69.17% and 30.83% respectively.
To determine the atomic weight:
d the relative abundance (as a decimal). The mass of the
Step 1: Multiply the mass number an
electron is insignificant in this calculation and is not used.
isotope name
atomic mass
x
abundance (as a decimal)
=
result
copper-63
62.93 amu
x
0.6917
=
43.53
copper-65
64.93 amu
x
0.3083
=
20.02
Step 2: Add up your results.
Atomic Weight =
63.55 amu
The mass of a proton is approx. 1.008 amu. The mass of a neutron is approximately 1.009 amu. The mass
of the electron is insignificant. When elements are formed from the individual subatomic particles a large
amount of energy in the form of heat is released. This loss of mass is called the mass defect and this
2
conversion of mass into energy is seen in the equation: E = mc
.
Homework:
1. Which part of Dalton’s atomic theory was proven to be incorrect with the discovery of subatomic particles?
2. Fill in the chart below
Subatomic
Symbol
Location
Charge
Mass
Particle
neutron
electron
proton
3. What makes an atom neutral?
4. What is the relationship between the atomic number of an element and its number of protons?
5. What is the ratio of the mass of a proton to the mass of an electron?
6. What is the relationship between the number of protons & neutrons and an atom’s mass number?
7. What is an isotope?
8. The existence of isotopes, proved which part of Dalton’s atomic theory incorrect?
9. Write the isotope symbol and isotope name of a bismuth isotope that has 126 neutrons?
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