nuclear chemistry the discovery of radiation nuclear reactions are different from other types of...
TRANSCRIPT
Nuclear Chemistry
The Discovery of Radiation
• Nuclear reactions are different from other types of reactions.
• Nuclear chemistry is concerned with the structure of atomic nuclei and the changes they undergo.
• Wilhelm Roentgen discovered x-rays in 1895.
• Marie Curie and her husband Pierre isolated the first radioactive materials.
Nuclear Reactions Nuclear reactions are different
from other types of reactions. Involve changes in the nucleus
of the atom protons (p+) and neutrons (nº)
The location of the atom (which compound it is in) has no effect
Differences from Chemical reactions
Unaffected by changes in conditions temperature and pressure
Speed of reaction cannot be changed “Actuarial tables”
Mass is not conserved
Radioactivity The process where the nucleus of an atom undergoes changes and releases energy, particles, or both
Radioisotopes Isotopes of an element that are
radioactive due to unstable nuclei
Undergo radioactive decay Nucleus falls apart, not grows larger
• Unstable nuclei emit radiation to attain more stable atomic configurations
Why do radioisotopes emit radiation?
A. to balance charges in the nucleus
B. to release energy
C. to attain more stable atomic configurations
D. to gain energy
Radioactivity
Types of radiationAlpha particlesBeta particlesGamma radiation “Positron” emission at times
Types of Radiation
• The ability of radiation to pass through matter is called its penetrating power.
• Gamma rays (like x-rays) are highly penetrating because they have no charge and no mass.
Alpha particles Symbolized as:
42He
4
2 Charge of +2 Mass ~4 amu
Heaviest of the particles
Types of Radiation
• Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.
Beta particles Symbolized as
0-1e-
O
-1 Charge of -1 Mass ~ 0.00055amu
Actually an electron emitted from the nucleus
Types of Radiation
• Beta radiation is a stream of fast moving particles with greater penetrating power—a thin sheet of foil will stop them.
Gamma radiation
Not a particle High energy electromagnetic
radiation (“light”) Also known as a type of “photon” Symbolized as No charge, no mass
Neutrino Symbolized as 00 Have no charge and extremely little
mass Carry energy away from the reaction Hypothesized to exist to explain the
wide variance of energy released in some nuclear reactions
Penetratin
g power~0.05m
m~4mm Very
highShielding Paper,
clothes, dead skin
Metal foil Pb, concrete
(incomplete)
energy ~5 MeV ~1 MeV
~1 MeV
X rays are most similar to what type of nuclear emissions?
A. gamma rays
B. alpha particles
C. beta particles
D. delta waves
Nuclear reactions The total of all the mass numbers
of the reactants equals the total of all the mass numbers of the products
The same holds true for the atomic numbers
Actual mass is not conserved Matter is transformed into energy
Symbols in nuclear reactions
Alpha particle 4
2He or 42
Beta particle 0
-1e or 0-1
Gamma radiation 0
0 Neutrino
00
Proton 1
1p+ or 11H
Neutron 1
0no
Electron 0
-1e
Positrons 0
+1e
Writing Nuclear reactions
1) The nuclide that decays is the one on the left-hand side of the equation.
2) The order of the nuclides on the right-hand side can be in any order.
3) The neutrino symbol is the Greek letter "nu." 00
4) The mass number and atomic number of a neutrino are zero.
Alpha ( ) Decay1) The nucleus of an atom splits into
two parts.2) One of these parts (the alpha
particle) goes zooming off into space.3) The nucleus left behind has its atomic
number reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons).
Types of Radiation
• Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.
Alpha ( ) Decay
Beta ( ) Decay 1) A neutron inside the nucleus of
an atom breaks down, changing into a proton.
2) It emits an electron and an anti-neutrino which go zooming off into space.
3) The atomic number goes UP by one and mass number remains unchanged.
Types of Radiation
• Beta radiation is a stream of fast moving particles with greater penetrating power—a thin sheet of foil will stop them.
Beta ( ) Decay
Types of Radioactive Decay (cont.)
Positron (01+e)
Emission 1) Something inside the nucleus of an
atom breaks down, which causes a proton to become a neutron.
2) It emits a positron and a neutrino which go zooming off into space.
3) The atomic number goes DOWN by one and mass number remains unchanged.
Positron (01+e)
Emission
Electron Capture Electron capture is not like any
other decay - alpha, beta, or position. All other decays shoot something out of the nucleus. In electron capture, something ENTERS the nucleus
Electron Capture 1) An electron from the closest energy
level falls into the nucleus, which causes a proton to become a neutron.
2) A neutrino is emitted from the nucleus. 3) Another electron falls into the “empty
space” in the electron cloud and so on causing a cascade of electrons falling.
4) The atomic number goes DOWN by one and mass number remains unchanged.
Writing electron capture equations
1) The nuclide that decays is the one on the left-hand side of the equation.
2) The electron must also be written on the left-hand side.
3) A neutrino is involved , It is ejected from the nucleus where the electron reacts, so it is written on the right-hand side.
Electron Capture
Types of Radioactive Decay (cont.)
Nuclear Chemistry
What causes nuclear instability?
The “wrong” number of neutrons
Atoms 1 – 20, p+ to no ratio is ~ 1:1
Atoms 21 and up, the ratio approaches 1:1.5
Nuclear Stability (cont.)
• The strong nuclear force acts on subatomic particles that are extremely close together and overcomes the electrostatic repulsion among protons.
Nuclear Stability (cont.)
• As atomic number increases, more and more neutrons are needed to produce a strong nuclear force that is sufficient to balance the electrostatic repulsion between protons.
• Proton to neutron ratio increases gradually to about 1:1.5
What if there are too many protons and neutrons?
Lose some of both Alpha emission (4
2)
Remember- an alpha particle is two protons and two neutrons
Mass number goes down by 4 atomic number goes down by 2
What if there are too many n0?
Convert a no into a p+
Beta emission Mass number stays the same, but atomic number goes up by 1
What if there are too few no?
Either 1) Positron emission: Convert a proton into a neutron
158O 15
7N + 01+e + 0
0
or 2) e- capture: A proton merges with an electron to become a neutron
Both: #p+ by 1 and #n0 by 1
Credits….
Portions of this presentation were adapted from the ChemTeam website