Chapter 21 - Nuclear Chemistry

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 discovered that certain minerals were constantly producing penetrating energy rays like X-rays not related to fluorescence 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 !!!

The Curies 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”

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)

Electroscope Electroscope Electroscope

+ +++++

+++ +++

Whencharged, charged,the themetal metal When foilsspread spreadapart apartdue duetoto foils likecharge chargerepulsion repulsion like

Ionizing radiation When exposed ionizing When exposed totoionizing radiation,the theradiation radiation radiation, knockselectrons electronsoff offthe the knocks airmolecules, molecules,which whichjump jump air ontothe thefoils foilsand and onto dischargethem, them,causing causing discharge themtoto them dropdown. down. drop

When charged, the metal foils spread apart due to like charge repulsion.

Ionizing radiation knocks electrons off the air molecules, which jump onto the foils and discharge them.

Rutherford (1871-1937) Discovered Three Types of Radiation

Rutherford’s Experiment ++++++++++++

γ

β α

--------------

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 Penetrating Ability of Radioactive Rays Rays

α

γ

β

0.01 mm

1 mm Pieces of Lead

Pieces of Lead

100 mm

Penetrating Ability of Radioactive Rays

Penetrating Ability of Radioactive Rays

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)

Valley of Stability

For for Z =Z1 =1-20, ⇒ 20, stable N/Z stable N/Zratio ≈1 =1 For =⇒ 20-40, for Z =Z20 40, stable N/ZN/Z approaches 1.25 stable ratio ≈1.25 for Z = 40 ⇒ 80, For Z = 40-80, stable N/Z approaches 1.5

stable N/Z ratio ≈1.5

for Z Z >> 83,84, For there are are no nuclei there nostable stable nuclei

Mass Defect (m) He nucleus 2 neutrons + 2 protons = 6.69510 × 10–27 kg Mass of 4He = 6.64465 × 10–27 kg m = 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 Isotopes and “Nuclides” are represented symbolically:

mass number atomic number

A Z

X

symbol

mass # = protons + neutrons

Radioactivity Unstable radioactive nuclei spontaneously decompose into smaller nuclei through “radioactive decay.” PARENT NUCLIDE ————> DAUGHTER NUCLIDE(S) PARTICLE(S) and/or ENERGY

All nuclides with 84 or more protons are radioactive

Important Atomic Symbols Important Atomic Symbols Particle

Symbol

proton

p+

neutron

n0

electron

e-

alpha

α

beta

β, β

positron

β, β+

Nuclear Symbol

Transmutation Atoms of one element are changed into atoms of a different element.

Nuclear Equations

The number of protons inwith the nucleus •  we describe nuclear processes nuclear changes. equations •  use the symbol of the nuclide to represent the nucleus •  atomic numbers and mass numbers are conserved We describe the process with nuclear equations.

 use this fact to predict the daughter nuclide if you know parent and emitted particle

on’s Atomic Theory statement 3 bites the dust Nuclear Equations

er for one element to change into another, the equations, atomic numbers and r In ofnuclear protons in the nucleus must Nuclear Equations change mass numbers are conserved.

we describe nuclear processes with nuclear equations se the symbol of the nuclide to represent the nucleus tomic numbers and mass numbers are conserved

 use this fact to predict the daughter nuclide if you know parent and emitted particle conservation of nucleons

238 = 234 + 4 92 = 90 + 2 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 types Alpha Emission

icle contains 2 protons utrons

nucleus

izing, but least penetrating n alpha particle means

number decreases by 2

•  loss of an alpha particle means Alpha Emission  atomic number decreases by 2 “Radium-222 decays by alpha emission”  mass number decreases by 4

Loss of an 𝛂 particle means atomic number decreases by 2 mass number decreases by 4

proton

p+

neutron

n0

electron

e-

alpha

α

beta

β, β

positron

β, β+

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

 atomic numberBeta increases by 1 Emission  mass number remains the same “Thorium-234 decays by beta emission”

  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

Gamma Emission

Gamma (𝛄) rays are highareenergy photons. •  gamma (γ) rays high energy photons of light

•  no loss of particles from the nucleus •  particles no change in the composition of the nucleus No loss of from the nucleus  Same atomic number and mass number

•  least ionizing, but most penetrating generally occurs after the nucleus No change• in composition of the nucleusundergoes some other type of decay and the remaining particles rearrange

Occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange

“Least ionizing”, but “most penetrating”

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

Positron has a charge of +1 and•  negligible mass positrons appear to result from a proton changing into a neutron

Appears to result from a proton changing into a neutron

 atomic number decreases by 1 Positron Emission

positrons appear to result from a proton “Sodium-22 decays by positron emission” changing into a neutron

When an atom loses a positron from its nucleus, atomic number decreases by 1 mass number remains the same

Electron Capture

Be-7

Li-7

Electron Capture

An inner orbital electron is pulled into the nucleus No particle emission, but the atom changes

eutron

 mass number stays the same “Ruthenium-92 undergoes electron capture”  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?”

𝛂 β 𝛂 β 238 234 234 234 230 U ➝ Th ➝ Pa ➝ U ➝ Th 92 90 91 92 90

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