# ISOTOPES

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```					Isotopic Notation
Chemistry 11
Definition of an Isotope

   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
Understanding Isotopic Notation
46

21   Sc
46 = mass number
(the total number of protons (21) + neutrons (25)
21 = atomic number
(the total number of protons (21))
Sc = element symbol

In a neutral atom, the number of electrons (21) is equal to the
number of protons.
Practice these:
15N

7                8                  7
# protons = ____ # neutrons= ____ # electrons = ___
35P

15
# p+ = ____          20
# n = ____           15
#e- = ___

62Cu2+

29
# p+ = ____         33
# n = ____           27
#e- = ___

76Se3-

34
# p+ = ____         42
# n = ____            37
#e- = ___
Practice these
1.   Write the symbol for the atom with an atomic number of 21 and
a mass number of 48. 48
Sc

2.   Give the complete chemical notation for the atom with 23
protons, 26 neutrons and 20 electrons. 49 3+
V
3.   Write the isotopic notation for     110Pd
52Cr3+
a. Z = 46               A = 110
b. An atom containing 24 protons, 28 neutrons, and 21
electrons
c. Titanium-50 50Ti
Practice Problems
1. 196 Pt4+
78         118
# p = _____ # n = _____            74
# e- = _____

196
mass number = ________                             78
atomic number = _______

195.1 amu
atomic mass = ________                             platinum
name of element = _______

2. Indicate the appropriate atomic mass of an element with 30
protons, 30 neutrons, and 28 electrons. 65.39 amu
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:
   1H protium, 2H deuterium, 3H tritium.
Calculating Atomic Mass
   If you look at your periodic table, for hydrogen, the
atomic mass is 1.0079 amu (atomic mass units).
   The atomic mass is calculated by adding the % of 1H
mass found in nature to the % of 2H mass found in
nature plus the % of 3H mass.
   % 1H + % 2H + % 3H = average mass (atomic mass)
   Generally the formula used is:
% X + % Y + % Z… = atomic mass.
Mass Spectrometer
   An instrument called the mass spectrometer
is generally used to determine the percentages
and individual masses of each isotope.
Sample Problem
   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?
Solution
The equation to use is:
%X + % Y = average atomic mass
And remember to convert your percentage amounts into
fractions (by dividing by 100) before you begin anything!

(0.5182) 106.904 amu + (0.4818) 108.905 amu = mass
55.398 amu + 52.470 amu = 107.868 amu

Now look at the periodic table to verify the answer.
Try This:
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.

20.17 amu
Group Problems
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 number of neutrons in an atom of    47   Ag is:
a) 47
b) 108
c) 155
d) 61
27
3. The number of electrons in an ion of   13 Al3+ is:
a) 13
b) 10
c) 27
d) 14

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