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Chapter 4
The Atom – Part 2
Subatomic Particles
PARTICLE
SYMBOL CHARGE
MASS
(amu
or u )
LOCATION
electron
e-
-1
0
orbit nucleus
proton
p+
+1
1
inside nucleus
neutron
n0
0
1
inside nucleus
Alchemy “The Golden Rule”
The number of protons in the nucleus of the atom
(atomic number = Z) determines the atoms identity
Gold Atom
Lead Atom
Mass Number
• The sum of the
protons and
neutrons in the
nucleus of an
atom.
What element is represented below?
Carbon - 12
• This is the symbol for
the isotope carbon-12.
• Atomic number is 6.
• Mass number is 12.
Carbon has 3 Isotopes
Isotopes
• Isotopes are atoms of the same element
with different masses.
• Isotopes are atoms of an element that
have the same number of protons, but a
different number of neutrons in the nucleus.
• Isotopes therefore have the same atomic
number but different mass numbers.
Isotopes
• Isotopes are atoms of the same element
with different masses.
• Isotopes are atoms of an element that have
the same number of protons in the nucleus,
but a different number of neutrons in the
nucleus.
• Isotopes therefore have the same atomic
number but different mass numbers.
• There are 275 isotopes of the 81 stable
elements, in addition to over 800 radioactive
isotopes, and every element has known
isotopic forms.
• Isotopes of a single element possess almost
identical properties.
Carbon - 12
• Write the symbols for
carbon-13 and
carbon-14.
The Discovery of Isotopes
• John Dalton
thought all atoms
of the same
element were
identical.
• The discovery of
isotopes changed
this idea.
What is the average mass of a carbon atom?
12.01
Atomic Mass
• The atomic mass of
carbon is 12.01 amu.
• Atomic mass is the
average mass of all
the isotopes of an
atom. It takes into
account the different
isotopes of an
element and their
relative abundance.
What is the average atomic weight for chlorine if it has two
isotopes? The percent abundance for chlorine-35 is
75.53%. The percent abundance for chlorine-37 is 24.40%.
The mass for Cl-35 is 35.0 amu and for Cl-37 it is 37.0 amu.
[(0.7553)(35.0 amu)] + [(0.2440)(37.0 amu)]
= [26.4355] + [9.028]
= 35.4635
the atomic weight of Cl = 35.5 amu
What is the average atomic mass for thallium, Tl? The two stable
isotopes and their abundances are listed here.Tl-205 has a mass
of 205.059 amu with an abundance of 70.48 % and Tl-203 has a
mass of 203.059 amu with an abundance of 29.52 % .
204.5 amu
What is the average atomic mass for thallium, Tl? The two stable
isotopes and their abundances are listed here.Tl-205 has a mass
of 205.059 amu with an abundance of 70.48 % and Tl-203 has a
mass of 203.059 amu with an abundance of 29.52 % .
[(0.7048)(205.059 amu)] + [(0.2952)(203.059 amu)]
= [144.5256] + [59.943]
= 204.4686
the atomic weight of Tl = 204.5 amu
Common Uses of Isotopes
• Isotope Analysis
• Radiometric Dating
• Nuclear Medicine:
– Magnetic Resonance Imaging (MRI)
– Positron Emission Tomography (PET
scans)
Importance of Isotopes
Radiometric Dating
Half-Life
• The decay of radioactive elements occurs at a fixed rate.
• The half-life of a radioactive isotope is the time required
for one half of the material to degrade into a more stable
material.
• Although the half-life pattern is the same for every
radioactive isotope, the length of a half-life is different.
• For example, C-14 has a half-life of about 5730 years
while Ir-192 has a half-life of about 74 days.
Half-Life
Carbon Dating (2:11)
Nuclear Medicine refers to imaging techniques that use
radioactive isotopes to detect and treat disease. Magnetic
resonance imaging (MRI) and positron emission tomography (PET
scans) are examples.
MRI
PET scan
• An atom of vanadium
contains 23 electrons.
How many protons
does it contain?
• 23 p+
• An atom of silver
contains 47 protons.
What is its atomic
number?
• 47
• An “atom” of sodium
contains 11 electrons.
What is its atomic
number?
• 11
• An atom contains 37
protons. What
element is it?
• rubidium
• How many electrons,
protons and neutrons
are in an atom of
actinium with a mass
number of 221?
• 89p+
• 89e• 132n0
• How many electrons,
protons and neutrons
are in an atom of
rhodium-105?
• 45p+
• 45e• 60n0
Homework
 Chapter 4 Worksheet 1