What is Radioactivity? Radioactive Decay. Nucleus contains protons and neutrons Electron circles the...
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Transcript of What is Radioactivity? Radioactive Decay. Nucleus contains protons and neutrons Electron circles the...
![Page 1: What is Radioactivity? Radioactive Decay. Nucleus contains protons and neutrons Electron circles the nucleus in orbits Proton: +1 charge, mass number.](https://reader035.fdocuments.net/reader035/viewer/2022081506/56649e185503460f94b03d60/html5/thumbnails/1.jpg)
What is Radioactivity?
Radioactive Decay
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• Nucleus contains protons and neutrons
• Electron circles the nucleus in orbits
• Proton: +1 charge, mass number (MN) of 1
• Neutron: 0 charge, MN of 1
• Electron: -1 charge, MN nearly zero
Quick Review
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Standard Atomic Notation (SAN)
B95• SAN tells us all
we need to know about an atom
• For boron, # of protons = 5# of electrons = 5
• Mass number = 9# neutrons = 4
Atomic Symbol
Atomic Number (number of protons)
Mass Number
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Use your periodic table to state the following for each atom:
Atomic number # of protons # of electrons rounded atomic mass # of neutrons Standard Atomic Notation
1) barium 2) uranium 3) chlorine
Try these…
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Barium Uranium Chlorine
Atomic number
56 92 17
# of protons 56 92 17
# of electrons 56 92 17
Mass number 137 238 35
# of neutrons 81 146 18
Standard Atomic Notation
Ba137
56 U238
92 Cl35
17
Answers
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O168 O178and
O188and
All three are forms of the oxygen atom found in nature but each have different numbers of neutrons. has 8 protons and 8 neutrons
has 8 protons but 9 neutrons
has 8 protons and 10 neutrons
O168
O178
O188
Isotopes can also be written as Oxygen-18 for
O188
What about these?
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• Isotopes are the same atoms with different mass numbers
• The different masses in the three oxygen isotopes are from extra neutrons in the nucleus
• Isotopes appear in all the same compounds as the element
• Isotopes have different but similar properties, e.g., ordinary water (water with atoms) boils at 1000 C but heavy water (water with atoms) boils at 101.420 C
They are isotopes
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• Isotopes mean that the nucleus has changed (added neutrons)
• This is new because in chemical reactions the nuclei of the reactants and products remain unchanged
• A reaction that changes the nucleus of an atom is called a nuclear reaction
A new type of reaction
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• Elements with atomic number <83 [except for technectium (43) and promethium (61)] have one or more stable isotopes
• All elements above atomic number 83 have isotopes with unstable nuclei
• The nuclei of these elements spontaneously disintegrate
• The spontaneous disintegration of a nucleus is called radioactivity
• The unstable nucleus that disintegrates is said to be radioactive
Unstable nuclei
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• When a nucleus disintegrates it can emit:• alpha particles • beta particles (electrons or positrons)• gamma rays• energy
• The new nucleus that remains may be radioactive or stable
Nuclear disintegration
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• Given the Greek symbol for alpha, (first letter of the Greek alphabet)
• Fast moving, high energy• Relatively heavy, loses energy quickly• Stopped by a piece of paper• Travel 2.5 cm in air• Penetrate skin only 0.3 mm• Consist of 2 protons and 2 neutrons• Rutherford found an alpha particle to be a helium
atom,
Alpha particles
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• When an unstable nucleus emits an alpha particle it undergoes alpha decay
• The resulting new nucleus is an isotope
Example: Uranium-238 undergoes alpha decay
Uranium-238 decays to make a new isotope - thorium and an alpha particle
Nuclear Equation
Alpha Decay
* Notice the conservation of mass holds (no particles are destroyed).
Alpha particle
U238
92 Th234
90 He4
2
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• Given the Greek symbol for beta, β (second letter of the Greek alphabet)
• Very fast • Very light• Stopped by 0.1 mm of lead• Can travel 4.5 m in air• Penetrate skin 17 mm• Beta particles may be positively or negatively charged
Beta particles
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Two types of beta decay are known:
β – Decay• Neutron in the nucleus is converted into a proton• Atomic number increases by 1 (new element)• During this conversion an electron and an antineutrino are
ejected from the nucleus• Also known as electron emission• An example of β- decay is:
Electron Antineutrino
Th234
90 Pa234
91e0-1 v
__
Beta Decay
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β + Decay (positron emission)• Proton in the nucleus is converted into a neutron• Atomic number decreases by 1 (new element)• During this conversion a positron and a neutrino are
ejected from the nucleus• Also known as positron emission• An example of β+ decay is:
Positron Neutrino
Na 22
11Ne
22
10e0
1 v
Beta Decay
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• Given the Greek symbol for gamma, γ (third letter of the Greek alphabet) • Type of ionizing electromagnetic radiation• Most penetrating form • Highest frequency and shortest wavelength in the electromagnetic spectrum• Produced when the nucleus of an atom is in an excited state and then
releases energy, becoming more stable • When a nucleus emits an or β particle, the daughter nucleus is sometimes
left in an excited state. It can then jump down to a lower level by emitting a gamma ray
Gamma Rays
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• Nuclear reactions are just like regular chemical reactions• Mass is conserved• Nuclear reactions show the starting material, the type of
radiation given off and the end products• Example:Radioactive atom new radioactive element particle
emitted
Nuclear Reaction Equations
Electron
Alpha particleUranium atom New isotope
U238
92 Th234
90 He4
2
Thorium atom New isotope
Th234
90 Pa234
91 e0-1