Journal 10/16/14
What does AMU mean?
Objective
Tonight’s Homework
To become more familiar with
the structure of an atom
Finish the practice problems.
Structure of the Atom
Atoms are made of 3 particles:
Positively-charged, heavy
protons, found in the middle,
or nucleus.
Neutral, heavy neutrons,
also found in the nucleus.
Very light negatively-charged
electrons, found far out from the
nucleus.
Structure of the Atom
If we look at the nucleus of an atom, we have
protons and neutrons.
Protons determine the identity of an element.
You can change neutrons or electrons without
changing what element an atom is, but if you
change protons, you change what it is.
Carbon always has 6 protons.
Copper always has 29.
Sulfur always has 16.
Gold always has 79.
Structure of the Atom
We call the number of protons than at element
has it’s “atomic number”.
Structure of the Atom
Since atoms are so small, they weigh almost
nothing. So instead of measuring their weight in
grams, we measure it in AMU (atomic mass
units).
Scientists defined the weight of hydrogen as 1
AMU since it’s the lightest element.
An element’s AMU tells us how many times
heavier than hydrogen that element is.
To measure an element’s AMU, we add together
the number of protons and neutrons in the
element’s nucleus. For most light elements, the
number of neutrons and protons are about equal.
Notes on Reading the Periodic Table
We can do some simple math to figure stuff out
about isotopes.
Atomic mass = (M1)(P1)+(M2)(P2)+(M3)(P3)+ …
Where:
M1 = mass (protons + neutrons) of an isotope
P1 = percent that this isotope is of all atoms of
this element
We can repeat this for other isotopes. (M2 and
P2. M3 and P3. Etc, etc
Notes on Reading the Periodic Table
Example:
Carbon has 2 isotopes:
- C-12, with a mass of 12.00 amu, makes up
98.89% of all carbon
- C-13, with a mass of 13.00 amu, makes up
1.11% of all carbon.
What's the average atomic mass?”
Notes on Reading the Periodic Table
Example:
Carbon has 2 isotopes:
- C-12, with a mass of 12.00 amu, makes up
98.89% of all carbon
- C-13, with a mass of 13.00 amu, makes up
1.11% of all carbon.
What's the average atomic mass?”
Atomic
Atomic
Atomic
Atomic
mass
mass
mass
mass
=
=
=
=
(M1)(P1)+(M2)(P2)
(12)(0.9889)+(13)(0.0111)
(11.87)+(0.14)
12.01
Atomic Mass Practice
1. The element copper has naturally occurring isotopes with mass numbers
of 63 and 65. The relative abundance and atomic masses are 69.2% and
30.8% respectively. Calculate the average atomic mass of copper.
2. Calculate the average atomic mass of sulfur if 95.00% of all sulfur
isotopes are Sulfur-32, 0.76% are Sulfur-33 and 4.22% are Sulfur-34.
3. The four isotopes of lead are shown below, each with its percent by mass
abundance and the composition of its nucleus. Using the following data, first
calculate the approximate atomic mass of each isotope. Then calculate the
average atomic mass of lead.
82p
122n
1.37%
82p
124n
26.26%
82p
125n
20.82%
82p
126n
51.55%
4. Calculate the average atomic mass of bromine. One isotope of bromine
has an atomic mass of 78.92amu and a relative abundance of 50.69%. The
other major isotope of bromine has an atomic mass of 80.92amu and a
relative abundance of 49.31%.