Name:______________________________________ Period _______ Date _______________________
Section 3.3 Notes & Practice Worksheet
Atoms, Isotopes, and Atomic Mass
Part 1
An element can come in a variety of isotopes (meaning they are made up of atoms with the same
atomic number but different atomic ______________ ). These atoms differ because of the number of
_____________ inside their nucleus. Take hydrogen as an example; it has three common isotopes.
Isotope notation:
Nuclear Notation?:
Hydrogen – 1
Hydrogen -2
Hydrogen - 3
1. Have you ever calculated your average in a weighted class like chemistry? Say you have a 92 in
your homework category, 85 in labs and projects, and 80 in tests/quizzes. What would you
average be in chemistry? (Knowing in chem: HWK = 20%, labs/projects 30%, Tests/quizzes 50%.)
Part 2
Like calculating you chemistry average, the average atomic mass of an element is a weighted average of
all the isotopes of an element. The atomic mass of an isotope in reality is very small (Ex: 2.657 x 10-23 g).
In order to make calculations more convenient we use relative atomic masses. Carbon – 12, which is
used to set up this scale, is assigned a mass of exactly 12 atomic mass units ( ______). All other isotopes
have been assigned an atomic mass based its relation to the carbon – 12 isotope.
*Scientists use the weighted average of all the masses of an atom’s isotopes on earth to report an
average atomic mass used on the periodic table. If we need to use the periodic table to find an average
atomic mass, we will only record the number to _______ decimal places.
Complete the following Problems: What is its average atomic mass…
2. of a sample of cesium if it’s 75% 133Cs , 20% 132Cs, and 5% 134Cs ?
_____ amu
3. of a sample of hydrogen if it’s 99%
1H,
0.8%
2H
and 0.2%
3H
?
__________
4. 98% 12C , 2% 14C ?
Part 3 – Some more challenge problems
5. Naturally occurring Europium (Eu) consists of two isotopes with a mass of 150.92 amu (48.03%)
and 152.92 amu (52.97%). What is the average atomic mass of Eu?
6. Naturally occurring Boron consists of two isotopes with an atomic mass of 10.01 and 11.01 amu.
Estimate the abundance (%) of these isotopes. The average atomic mass of Boron is 10.81 amu.
7. Naturally occurring iron is 85% iron – 56 (56.00 amu) and 15% of another isotope iron - ?. What
is the atomic mass of the 2nd isotope in order to account for the 55.85 amu average atomic mass
of iron?
Remember: although isotopes have different masses, they do not differ significantly in their
_______________ properties.