13 I S O T O P E S

1. ISOTOPIC NOTATION
isotopes are atoms with the same number of protons but
different number of neutrons
A
Z X
A = mass number
(the total number of protons + neutrons)
Z = atomic number
(the total number of protons)
X = element symbol

3. READING ISOTOPIC NOTATIONREADING ISOTOPIC NOTATION
46
21 Sc
46 = mass number
(the total number of protons (2121) + neutrons (2525)
21 = atomic number
(the total number of protons (2121))
Sc = element symbol
In a neutral atom, the number of electrons
(21) is equal to the number of protons.

4. PRACTICE PROBLEMS

15
N
# protons = ____ # neutrons= ____ #electrons = ___
35
P
# p = ____ # n= ____ #e- = ___
62
Cu
2+
# p = ____ # n= ____ #e- = ___
76
Se
3-
# p = ____ # n= ____ #e- = ___
7 8 7
15 20 15
29 33 27
34 42 37

5. Writing ISOTOPIC NOTATION
1.Write the symbol for the atom with an atomic
number of 21 and a mass number of 48.
3.Give the complete chemical notation for the
nuclide with 23 protons, 26 neutrons and 20
electrons.
5.Write the isotopic notation for
a.Z = 46 A = 110
b.An atom containing 24 protons, 28 neutrons,
and 21 electrons
c.Titanium-50
48
Sc
49
V
3+
110
Pd
52
Cr
3+
50
Ti

6. PRACTICE PROBLEMS
1.
196
Pt
4+
# p = _____# n = _____ #e- = _____
mass number = ________ atomic number = _______
atomic mass = ________ name of element = _______
2. Indicate the appropriate atomic mass of an element with 30
protons, 30 neutrons, and 28 electrons.
78 118 74
196 78
195.1 amu platinum
65.39 amu

7. Atomic Mass
•The atomic mass of an element represents the
average mass of all the isotopes found in nature. No
element exists with only one possible isotope.
Hydrogen has the smallest number of isotopes:
1
H
protium,
2
H deuterium,
3
H tritium. Its atomic mass is
1.0079 amu (atomic mass units). The atomic mass is
calculated by adding the % of
1
H mass found in
nature to the % of
2
H mass found in nature plus the
% of
3
H mass.
•%
1
H + %
2
H + %
3
H = average mass (atomic mass)
•Generally the formula used is:
% X + % Y + % Z… = atomic mass.
An instrument called the mass spectrometer is generally
used to determine the percentages and individual
masses of each isotope.

8. Atomic Mass
•Silver is found to have two stable isotopes, one has an atomic
mass of 106.904 amu and the other weighs 108.905 amu. The
first isotope represents 51.82 % of the mass of the element and
the second represents 48.18 %. What is the atomic mass of the
element silver?
The equation to use is %X + % Y = average
And remember to turn your percents into fractions before
multiplying.
(0.5182) 106.904 amu + (0.4818) 108.905 amu =?
55.398 amu + 52.470 amu =?
107.868 amu !!
Now look at the periodic table to verify the answer.

9. PRACTICE PROBLEMS # 8
1. A sample of neon contains three isotopes, neon-20 (with an
isotopic mass of 19.9924 amu), neon-21 (20.9939 amu) and
neon-22 (21.9914 amu). The natural abundances of these
isotopes are 90.92%, 0.257 %, and 8.82 %. Calculate the
atomic weight of neon.
2. There are only two naturally occuring isotopes of copper,
63
Cu
and
65
Cu. Copper has an atomic mass of 63.55 amu. What is
the natural abundance of each isotope?
3. There are only two naturally occuring isotopes of gallium, 69Ga
and 71Ga. What is the natural abundance of each isotope?
20.17 amu
65
Cu = 30% &
63
Cu = 70%
69
Ga = 60% and
71
Ga = 40%

11. GROUP STUDY PROBLEM #8
_______1. The element with atomic number 53 contains
a) 53 neutrons b) 53 protons C) 26 neutrons & 27 protons d) 26 protons & 27 neutrons
_______2. The mass of one atom of an isotope is 9.746 x 10
-23
g. One atomic mass unit
has the mass of 1.6606 x 10
-24
g. The atomic mass of this isotope is
a) 5.870 amu b) 16.18 amu c) 58.69 amu d) 1.627 amu
108
_______3. The number of neutrons in an atom of
47
Ag is
a) 47 b) 108 c) 155 d) 61
27
_______4. The number of electrons in an ion of
13 Al
3+
is
a) 13 b) 10 c) 27 d) 14
_______5. What is the relative atomic mass of boron if two stable isotopes of boron have
the following mass and abundance:
10.0129 amu (19.91%) & 11.0129 (80.09%)
a) 10.81 amu b) 10.21 amu c) 10.62 amu d) 10.51 amu