ATOMS & ISOTOPES
Mrs. Page
Chemistry10
Parts of presentation modified from a PowerPoint presentation prepared by
J. Crelling, Southern Illinois University
LEARNING OBJECTIVES
• You will be able to define what an isotope is.
• You will be able to determine the number of protons,
neutrons, and electrons in different isotopes of the
same element.
• You will understand that atomic mass
• You will understand what radioactivity is the average of
the naturally occurring isotopes of an element.
• You will be able to give examples of how radioisotopes
are used in industry and medicine.
ATOMS





Contain protons, neutrons, and electrons
Protons & neutrons are found in the
nucleus
The nucleus contains most of the mass of
an atom
Electrons are distributed around the
nucleus in energy levels/shells/orbitals
(which make up the electron cloud)
The outermost electrons in the shell
farthest from the nucleus are called
valence electrons
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ELECTRONS
 First
subatomic particle discovered
 1897 J.J. Thomson used the cathode ray
tube to discover the electron
 Has a negative charge (-1)
 Mass = 9.110 x 10-28 g (0.0005 amu)
 Electrons are located in energy levels
which make up the electron cloud
 Electrons in the outermost energy level
are called valence electrons
 Valence electrons are responsible the for
the reactivity of an atom.
PROTON

Observed by E. Goldstein in 1896

Rutherford is given credit for showing that atoms
contain both negatively and positively charged
particles (gold foil experiment)

Has a charge of +1

Relative Mass of 1 AMU (1.673 x 10-24 g)
NEUTRON
Third
major subatomic particle
discovered (1932 James
Chadwick)
No
charge (neutral)
Relative
Mass of 1 AMU (1.675
x 10-24 g)
ISOTOPES
 Atoms
of the same element have
the same atomic number (# of
protons) and the same chemical
properties.
 However,
atoms of the same
element may have different
numbers of neutrons (and therefore
different atomic mass)
 Isotopes
are atoms of the same
element having different number of
neutrons
NATURALLY OCCURRING ISOTOPES
 Every
element has naturally occurring isotopes
 Hydrogen
 Protium
has 3 naturally occurring isotopes
is the most abundant isotope of
hydrogen (99.985%) has 1 proton, 0 neutrons,
and 1 electron
 Deuterium (0.015%) has 1 proton, 1 neutron,
and 1 electron
 Tritium (0.0001% ?) has 1 proton, 2 neutrons, and
1 electron
ISOTOPE EXAMPLE
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
ISOTOPES OF CARBON
Naturally occurring carbon consists of three isotopes, 12C, 13C, and
14C. State the number of protons, neutrons, and electrons in each of
these carbon atoms.
12C
6
13C
6
14C
6
#P _______
_______
_______
#N _______
_______
_______
#E _______
_______
_______
SOLUTION
12C
6
13C
6
14C
6
#P __6___
_ 6___
___6___
#N __6___
_ _7___
___8___
#E __6___
_ 6___
___6___
ZINC PROBLEM
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
B.
Number of neutrons in the zinc atom
C. What is the mass number of a zinc isotope
with 37 neutrons?
ATOMIC MASS
Na
22.99

Listed on the periodic table

Gives the mass of “average” atom of each element
compared to 12C

Average atom based on all the isotopes and their
abundance %

Atomic mass is not a whole number … mass number is a
whole number
CALCULATING ATOMIC MASS

Percent(%) abundance of isotopes

Mass of each isotope of that element

Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
ATOMIC MASS OF MAGNESIUM
Isotopes
Mass of Isotope
24Mg
= 24.0 amu
25Mg
=
25.0 amu
26Mg
=
26.0 amu
Abundance
78.70%
10.13%
11.17%
(24)(.787) + (25)(.1013) + 26(.1117) =
18.888 + 2.5325 + 2.9042 = 24.3 amu
ISOTOPES
Two Categories
Unstable
– isotopes that
continuously and spontaneously
break down/decay in other lower
atomic weight isotopes

Stable – isotopes that do not
naturally decay but can exist in
natural materials in differing
proportions
USES OF RADIOISOTOPES
 Carbon

14 Dating (Geologic Time)
When the organism dies it stops taking in 14C which
disappears as it decays to 14N
 Americum-241 Used in smoke detectors
 Cesuim-137 Used to treat cancerous tumors
 Californium-252 Used to inspect luggage for
explosives
 Cobalt-60 Used to sterilize surgical equipment

Iodine-123 Used to treat thyroid disorders

Plutonium-238 Used to power NASA spaceships