nuclear chemistry nucleons vs. nuclide nucleons: general name referring to nucleus made up off...
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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.
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.
Mass Defect and Nuclear Stability
The measured mass of , 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 2 protons
Loss of 4 from the mass number
That means it had to undergo alpha decay
Example problems
a) Beta decay = extra proton X = 83
b) Alpha decayy = 206; x = 82
c) Increases by one protonx = electron
d) Gamma radiationY = 226; x = 88
e) Beta decayX = 214; y = 84
f) Alpha decayX = 226; y = 88
g) Alpha decayX = helium atom
h) Gamma radiationx = same as on left