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Chapter 19 - Nuclear Chemistry Nuclear Stability and Modes of Decay
History and
Discovery of Radioactivity
The Discovery of Radioactivity (1896)
Antoine-Henri Bequerel designed experiment to determine whether phosphorescent minerals also gave
off X-rays.
Bequerel determined that the minerals contained uranium
uranic rays Production of uranic rays did not
require exposure to outside energy.
Energy was being produced from nothing !!!
Bequerel discovered that certain minerals were constantly producing
penetrating energy rays
like X-rays not related to fluorescence
Marie Curie (1867-1934) broke down these minerals and used an
“electroscope” to detect uranic rays.
She discovered the rays were emitted from specific elements.
She also discovered new elements 1. radium named for its green phosphorescence 2. polonium named for her homeland
She coined the name “radioactivity”
The Curies
Other Properties of Radioactivity
Can “ionize” matter (cause matter to become charged)
(basis of Geiger Counter)
High energy
Can penetrate matter
Can cause phosphorescent chemicals to glow
(basis for the scintillation counter)
ElectroscopeElectroscope + + + +
+ +
When charged, the metal
foils spread apart due to like charge repulsion
When exposed to ionizing
radiation, the radiation knocks electrons off the
air molecules, which jump onto the foils and
discharge them, causing
them to drop down.
Electroscope
+ + + +
+ +
When charged, the metal
foils spread apart due to like charge repulsion
When exposed to ionizing
radiation, the radiation knocks electrons off the
air molecules, which jump onto the foils and
discharge them, causing
them to drop down.
Ionizing radiation knocks electrons off the air molecules, which jump onto the foils and
discharge them.
When charged, the metal foils spread apart due to like charge
repulsion.
Ionizing radiation
Rutherford (1871-1937) Discovered Three Types of RadiationRutherford’s Experiment
++++++++++++
--------------
α
γ β
Types of Radioactive Rays
“Alpha” Rays (𝛂)
charge of +2 and mass of 4 amu essentially the nucleus of a helium atom
“Beta” Rays (β)
charge of -1 and negligible mass high-energy electrons
“Gamma” Rays (𝛄) electromagnetic radiation, not 𝛂 or β
Penetrating Ability of Radioactive Rays Penetrating Ability of Radioactive
Rays
α β γ
0.01 mm 1 mm 100 mm
Pieces of Lead Pieces of Lead
Nuclear Chemistry
Nuclear reaction – process that alters the number of neutrons and protons in the
nucleus of an atom.
Radionuclide – an unstable nuclide that undergoes radioactive decay.
Radioactive decay – the spontaneous disintegration of unstable particles
accompanied by the release of radiation.
Binding Energy and
Nuclear Stability
What Causes Nuclei to Break Down?
The particles in the nucleus are held together by a very strong attractive force between nucleons,
the strong force, which acts only over very short distances.
Neutrons and Protons are Held Together by the “Strong Force.”
Neutrons play an important role in
stabilizing the nucleus. They add to the strong
force, but don’t repel each other like protons.
The “Valley of Stability” and the N/Z Ratios (neutrons/protons)
For Z =1-20, stable N/Z ratio = 1
For Z = 20-40, stable N/Z ratio ≈1.25
For Z = 40-80, stable N/Z ratio ≈1.5
For Z > 84, there are no stable nuclei
Valley of Stability
for Z = 1 ⇒ 20,
stable N/Z ≈ 1
for Z = 20 ⇒ 40,
stable N/Z approaches 1.25
for Z = 40 ⇒ 80,
stable N/Z approaches 1.5
for Z > 83,
there are no stable nuclei
Mass Defect (Δm)
He nucleus
2 neutrons + 2 protons = 6.69510 × 10–27 kg
Mass of 4He = 6.64465 × 10–27 kg
Δm = 5.045 × 10–29 kg
E = mc2
BE = 4.54 x 10-12 J/atom
BE = 1.13 x 10-12 J/nucleon
Binding Energy
Binding Energy
Unstable Nuclei and
Modes of Radioactive Decay
Review of Nuclear Structure
Every atom of an element has the same number of protons
designated by the atomic number “Z”
Atoms of the same element may have different numbers of neutrons
called “isotopes” have different atomic masses
XAZ symbol mass number
atomic number mass # = protons + neutrons
Isotopes and “Nuclides” are represented symbolically:
Radioactivity
Unstable radioactive nuclei spontaneously decompose into smaller nuclei through “radioactive decay.”
PARENT NUCLIDE ————> DAUGHTER NUCLIDE(S)
All nuclides with 84 or more protons are radioactive
PARTICLE(S) and/or ENERGY
Important Atomic SymbolsImportant Atomic Symbols Particle Symbol Nuclear
Symbol
proton p+
neutron n0
electron e-
alpha α
beta β, β
positron β, β+
Transmutation
Atoms of one element are changed into atoms of a different element.
The number of protons in the nucleus changes.
Nuclear Equations
• we describe nuclear processes with nuclear equations
• use the symbol of the nuclide to represent the nucleus
• atomic numbers and mass numbers are conserved use this fact to predict the daughter nuclide if you know
parent and emitted particle
We describe the process with nuclear equations.
Transmutation
• Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of
a different element - transmutation
Dalton’s Atomic Theory statement 3 bites the dust
• in order for one element to change into another, the number of protons in the nucleus must change
Nuclear Equations
In nuclear equations, atomic numbers and mass numbers are conserved.Nuclear Equations
• we describe nuclear processes with nuclear equations
• use the symbol of the nuclide to represent the nucleus
• atomic numbers and mass numbers are conserved use this fact to predict the daughter nuclide if you know
parent and emitted particle
238 = 234 + 4 92 = 90 + 2
conservation of nucleons
conservation of charge
Alpha Emission
An 𝛂 particle contains 2 protons and 2 neutrons. a helium nucleus
The “most ionizing”, but “least penetrating” of radiation typesAlpha Emission
• an α particle contains 2 protons and 2 neutrons
helium nucleus
• most ionizing, but least penetrating
• loss of an alpha particle means atomic number decreases by 2
mass number decreases by 4
“Radium-222 decays by alpha emission”
Alpha Emission • an α particle contains 2 protons
and 2 neutrons
helium nucleus
• most ionizing, but least penetrating
• loss of an alpha particle means atomic number decreases by 2
mass number decreases by 4
Alpha Emission
Loss of an 𝛂 particle means atomic number decreases by 2 mass number decreases by 4
Beta Emission
A beta particle is like an electron
moves much faster (has more energy) produced in the nucleus
In β decay, a neutron changes into a proton
Important Atomic Symbols Particle Symbol Nuclear
Symbol
proton p+
neutron n0
electron e-
alpha α
beta β, β
positron β, β+
Beta Emission
“Thorium-234 decays by beta emission”
Beta Emission
• a β particle is like an electron moving much faster
produced from the nucleus
• when an atom loses a β particle its atomic number increases by 1
mass number remains the same
• in beta decay, a neutron changes into a proton
Loss of an β particle means atomic number increases by 1 mass number remains the same
Gamma Emission
No change in composition of the nucleus
Occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange
Gamma Emission
• gamma (γ) rays are high energy photons of light • no loss of particles from the nucleus • no change in the composition of the nucleus
Same atomic number and mass