chapter 19 - nuclear chemistry nuclear stability and modes...

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.

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 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

Be-7 Li-7

“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 !!

The “Band of Stability” Expanded

Predictability of Nuclear Decay

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