Nuclear Chemistry
Nucleons vs. Nuclide
 Nucleons: General name referring to
nucleus made up off Protons + Neutrons
 Nuclide:
Nuclear chemistry’s way of
referring to the atom

For example:
• Radium-228
or
The Nucleus

Remember that the nucleus is comprised of two
subatomic particles; protons and neutrons.
 The number of protons is the atomic number.
 The number of protons and neutrons together is
effectively the mass of the atom.
Mass Defect and Nuclear Stability
The difference between the mass of an atom and the sum of the masses
of its protons, neutrons, and electrons is called the mass defect.
4
The measured mass of 2 He , 4.002 602 amu, is:
0.030 377 amu less than the combined mass, 4.032 979 amu.
Mass Defect
 Mass
Defect: Difference between mass of atom
and sum of masses of it’s p+, n, and e For example:

Helium-4
• 2 Protons=
(2 x 1.007276)
• 2 Neutrons=
(2 x 1.008665)
• 2 Electrons=
(2 x .0005486)
______________________________
4.032979
But its actual mass is measured at 4.00260
that’s .03038 less
Where did the .03038 go?
It was converted to energy when formed
Nuclear Binding Energy: energy released
when nucleus is formed
E = mc2
Albert Einstein

Studied the mass and energy of atoms and
found the relation between the two
 Came up with the equation E = mc2
 Led to the understanding of binding energy
(energy that holds an atom together)

Band of Stability:
 Most stable nuclei are 1:1
(Proton:
Neutrons)
Isotopes

Not all atoms of the same
element have the same
mass because of different
numbers of neutrons in
those atoms.
 For example, there are
three naturally occurring
isotopes of uranium:



Uranium-234
Uranium-235
Uranium-238
All isotopes of uranium have
92 protons, but they all have
different numbers of neutrons
Stability of Isotopes





The “like” charges of the protons in the nucleus push the
particles apart from each other, threatening to push the
nucleus apart
Binding energy keeps the nucleus together
Stable atoms have a binding energy that is strong
enough to hold the nucleus together
Because some isotopes have an extra neutron (or
more), the binding energy cannot hold the nucleus
together
This makes the atom unstable

These are radioisotopes
What is radioactivity?

Radioactivity is the act of emitting radiation
spontaneously with the resulting emission of
radiation resulting in the formation of a new
nuclei


Does not need a source to travel through space and
penetrate another material
Atoms with unstable nuclei are radioactive
Usually the number of
neutrons will determine if
a nuclei is unstable
Transmutation

Elements with atomic numbers greater than 83 are
radioisotopes


Those elements with atomic numbers less than 83 have
isotopes and most have at least 1 radioisotope
Radioisotopes try to stabilize



They try to transform into a new, stable element
This is transmutation
The change occurs
due to changes in the
nucleus and results in
radioactive decay
Radioisotope Half-Life
 Radioisotopes
are unstable. They decay,
or change into new elements, over time.
 The half-life of an element is the time it
takes for half of the material you started
with to decay.

Remember, it doesn’t matter how much you
start with. After 1 half-life, half of it will have
decayed.
 Each
element has its own half life.
Half-Life Questions

What is the half-life of this
element?





Half-life is where ½ of the
element remains
Go to 50% on the y-axis
Then drop down to x-axis and
that is the half-life
1,000,000 years
What percent of the
material originally present
will remain after 2 million
years?



Go to 2 million on x-axis
Go over to y-axis
25%
Half-life calculations
mf = mi (.5)n
mf : final mass of
sample
mi: initial mass of
sample
n: number of half-lives
tf = t1/2  n
tf = total time of decay
t1/2 = half life
n = number of half-lives
Example problem:
 Chromium-48
has half-life of 21.6 hours.
How long will it take 360.0 g of Cr-48 to
decay to 11.25 g?
108 hours
 If the half-life of tungsten-190 is 30.0
minutes, how much tungsten-90 will
remain after 114 minutes if I’m originally
given a 400.0 gram sample?
28.7 g
Types of Radioactive Decay

Spontaneous breakdown of a nucleus resulting
in release of energy and matter
 Type of radiation emitted by radioactive
materials
 Radioactive decay and transmutation occur
simultaneously
 Constantly releasing energy and matter as they
are transforming into a new, stable isotope




Alpha
Beta
Gamma
Positron
Alpha Decay α

Alpha decay results when an unstable nuclei
loses a Helium-4 particle (2 protons and 2
neutrons)


The new nucleus will have an atomic
number that is 2 less than the original
The new nucleus will have a mass
number that is 4 less than the original
Beta Decay β

Beta decay occurs when a neutron in an
unstable nucleus splits to make a proton and a
electron


The atomic number increases by 1 because of the
extra proton
The mass number does not change since one
neutron is subtracted, but one proton is added
Gamma Ray γ

Energy is emitted from the nucleus in the form of
gamma rays




Electromagnetic waves with very high frequencies
and energy
Naturally occurring waves (identical to X-rays)
Very dangerous to life
Usually accompanies alpha or beta decay
Positron Decay β+

Opposite of beta decay
 Occurs in nuclei with too few neutrons
 Proton turns into a neutron
 Atomic number decreases by 1 but mass
number stays the same
Penetrating Ability
Nuclear Equations

Nuclear Equations show the original
radioisotope and also tell you which type of
decay that radioisotope underwent


All you have to do is add or subtract to determine
which type of decay occurred
Example:
Loss of 4 from the
mass number
Loss of 2
protons
That means it
had to undergo
alpha decay
Example problems
a)
b)
c)
d)
e)
f)
g)
Beta decay = extra proton
X = 83
Alpha decay
y = 206; x = 82
Increases by one proton
x = electron
Gamma radiation
Y = 226; x = 88
Beta decay
X = 214; y = 84
Alpha decay
X = 226; y = 88
Alpha decay
X = helium atom
h)
Gamma radiation
x = same as on left