1. Isotope Notation
Isotope Notation uses a symbol to convey information about an
isotope of a particular element. 23 Na 11

2. Isotope Notation Mass Number 23 Na 11 Atomic Number
Is the number of protons plus the number of neutrons. This number tells us the kind of isotope. This number is NOT on the periodic table.
Atomic Number
Is the # of protons (p+) in the atom
AND
The # of electrons (e-) IF the atom is electrically neutral
( i.e., not an ion).
This number is on the periodic table.

3. Isotope Notation 23 Na 11 23 Na+ 11 For this isotope 11 p+ + 11e-
0 net charge
For this isotope
11 p+
+ 10e-
1+ net charge

4. Isotope Notation Mass Number 23 Na 11 Atomic Number
The # of protons + the # of neutrons
Calculate the number of neutrons for this isotope.
23 (neutrons + protons) protons = 12 neutrons

5. Atomic Mass Is the weighted average mass of all the isotopes
of one element in atomic mass units (amu).
In other words, it is the mass of one average atom*.
Two measurements are needed in order to calculate this average.
The percent abundance (% abundance,) also known as how often the isotope occurs in nature.
The mass of one atom of one isotope.
Atomic mass is also known as
Weighted Average Atomic Mass
Average Atomic Mass

6. Atomic Mass Here is the formula you need to know in order to calculate
the average atomic mass.
Average Atomic Mass =
[(Mass of Isotope 1) x (% abundance of Isotope 1)]
+ [(Mass of Isotope 2) x (% abundance of Isotope 2)]
+ and so on for the 3rd and remaining isotopes.
The average atomic mass is written on the periodic table

7. Mass Number vs. Atomic Mass
Mass number = # protons + # neutrons
(This number is specific for one isotope.)
Atomic mass = weighted average mass of all the isotopes
of one element in atomic mass units (amu).
(The atomic mass is on the periodic table.)