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 amuessentially 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 RaysPenetrating 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.
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
x NA = 6.80 x 1011 J/mol
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 symbolmass number
atomic numbermass # = 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 + 492 = 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 number
• least ionizing, but most penetrating
• generally occurs after the nucleus undergoes some
other type of decay and the remaining particles
rearrange
Gamma (𝛄) rays are high energy photons.
No loss of particles from the nucleus
“Least ionizing”, but “most penetrating”
Positron has a charge of +1 and negligible mass
Appears to result from a proton changing into a neutron
Positron Emission
Positron Emission • positron has a charge of +1 c.u. and
negligible mass anti-electron
• when an atom loses a positron from the
nucleus, its mass number remains the same
atomic number decreases by 1
• positrons appear to result from a proton
changing into a neutron
“Sodium-22 decays by positron emission”
Positron Emission • positron has a charge of +1 c.u. and
negligible mass anti-electron
• when an atom loses a positron from the
nucleus, its mass number remains the same
atomic number decreases by 1
• positrons appear to result from a proton
changing into a neutron
When an atom loses a positron from its nucleus, atomic number decreases by 1 mass number remains the same
Positron Emission
An inner orbital electron is pulled into the nucleus
No particle emission, but the atom changes
Electron Capture
“Ruthenium-92 undergoes electron capture”
Electron Capture • occurs when an inner orbital electron is pulled
into the nucleus
• no particle emission, but atom changes same result as positron emission
• proton combines with the electron to make a neutron mass number stays the same
atomic number decreases by one
Proton combines with electron to make a neutron Mass number stays the same Atomic number decreases by 1
The result is the same as positron emission !!
Patterns of Nuclear Stability
The majority of elements in the universe are small (Z<40) and have
even numbers of protons.
Certain numbers appear to be “magic” with regard to number of
neutrons or protons: 2,8,20,50,82,4,126
Stable nuclei are found in a band of stability surrounded by a band of
instability.
#Neutrons#Protons
even
evenodd
odd
The number of stable nuclides having even of odd numbers of
neutrons and protons:
157
50
53
4
Patterns of Nuclear Stability
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
The “Band of Stability” Expanded
Predictability of Nuclear DecayNuclei that lie above the band of
stability are neutron rich. These nuclei tend to decay so that the final n/p ratio is closer to that found in the
band of stability.
Nuclei that lie below the band of stability are proton rich. These nuclei
tend to decay so that the final n/p ratio is closer to that found in the
band of stability.
➝14
6C 14 7N + 0
-1e
➝29 15P
29 14Si + 0
+1e
7 4Be
0 -1e+ ➝ 7
3Li
➝14
6C 14 7N + 0
-1e
➝29 15P
29 14
Si + 0 +1e
7 4Be
0 -1e+ ➝ 7
3Li
Predictability of Nuclear Decay
188Re is formed by the decay of 188W. Write a balanced equation describing the decay process. Why doesn’t 188W decay by electron capture or positron emission?
188 73Ta
188 74 W
0 -1 e+
188 75
Re188 74
W 0 -1
e+
188 73Ta
0 +1 e
188 74 W +
n/p = 114/74 = 1.54 n/p = 113/75 = 1.51
n/p = 115/73 = 1.58
Selective Types of Radioactive Emissions
Nuclear Decay Series
Nuclear Decay Series
In nature, one radioactive nuclide often changes into another radioactive nuclide.
All of the radioactive nuclides that are produced one after another until a stable nuclide is made is called a
decay series.
“What is the product formed when 238U goes through one alpha decay followed by two beta emissions and then another alpha decay?”
238 92
U 234 90
Th 234 91
Pa 234 92
U➝ ➝ ➝ ➝230
90Th
𝛂 β β 𝛂
A Natural Radioactive Decay Series for U-238
U-238
Decay Series α
β
β
α
α
α
α
β
α
β
α
β
β
α
or α
β
α
β
β
α
β
or other
combinations
