Nuclear Chemistry

Radioactivity

The process by which materials give off such rays radioactivity; the rays and particles emitted by a radioactive source are called radiation.

Ionizing vs. Nonionizing RadiationIonizing radiation – radiation with enough energy to produce ions by knocking electrons off some atoms of a bombarded substance. Alpha, beta, gamma, and X-rays are examples of

ionizing radiation. Ionizing radiation can cause changes to living cells.

Nonionizing radiation – not capable of ionizing matter. Radio waves and visible light are forms of

nonionizing radiation.

Radioisotopes

Radioisotopes are isotopes that are radioactive because they have unstable nuclei.

Nuclear StabilityThe stability of the nucleus depends upon its ratio of neutrons to protons. Too many or too few neutrons lead to an unstable nucleus.

When the number of protons in stable nuclei is plotted against the number of neutrons, a beltlike graph is obtained. This stable nuclei cluster over a range of neutron-proton ratios is referred to as the band of stability.

Stable isotopes fall within the band of stability and have neutron to proton ratios of nearly 1:1 at the lower range and nearly 1.5:1 at the upper range. Such isotopes tend to be stable. Radioactive isotopes fall outside of the band of stability.

Band of Stability

Would you expect Helium-4 to be a stable isotope? Why or why not?Yes, the neutron to proton ratio is 1:1. (falls within the band of stability)Would you expect Carbon-14 to be a stable isotope? Why or why not?No, the neutron to proton ratio is 1.3:1 (falls above the band of stability)

What happens if a nucleus is unstable?

Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons.

In this process, they give off large amounts of energy and increase their stability.

Reminder: The identity of an element changes when the number of protons changes.

Types of Radioactive Decay

During radioactive decay, unstable atoms lose energy by emitting one of several types of radiation.

The three main types of radiation are alpha, beta, and gamma.

Properties of the Three Most Common Types of Radiation

Radiation Alpha Beta Gamma

Composition

alpha particles(helium nucleus)

beta particles(electron)

form of electromagnetic radiation

Symbol or

Mass 4 amu nearly 0 amu(0.0055 amu) 0 amu

ElectricCharge 2+ 1- 0

Penetrating

Power

stopped by paper, wood, cloth, etc.

stopped by aluminumor other metals

stopped by lead

He24 e

1-0

1-0

00

Alpha Emission (Decay)An alpha particle ( ) is composed of two protons and two neutrons bound together.

Alpha emission is restricted almost entirely to very heavy nuclei. In these nuclei, both the number of neutrons and the number of protons need to be reduced in order to increase the stability of the nucleus.

All nuclei with atomic numbers greater than 83 are radioactive. A majority of these undergo alpha emission.

Alpha Emission (Decay)

In a balanced nuclear equation, the total of the mass numbers and atomic numbers on each side of the equation must be equal. Describe the change in the mass number. The mass number decreases by 4. Describe the change in atomic number. The atomic number decreases by 2.

Beta Emission (Decay)

Beta emission ( ) is the emission of electrons from the nucleus when a neutron is converted to a proton and an electron. Beta emission occurs when the nucleus of an element has too many neutrons. This is true of elements that fall above the band of stability.

Beta Emission (Decay)

Describe the change in the mass number. The mass number stays the same. Describe the change in atomic number. The atomic number increases by 1.

What happens to the neutron to proton ratio?The neutron to proton ratio decreases.

Gamma Emission (Decay)Gamma rays ( ) are high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground state. Gamma rays are produced when nuclear particles undergo transitions in energy levels. Gamma emission usually follows other types of decay that leave the nucleus in an excited state.

Gamma Emission (Decay)

Positron Emission (Decay)

Positron emission ( ) occurs when a proton is converted into a neutron. A positron is a particle that has the same mass as an electron, but has a positive charge and is emitted from the nucleus during some kinds of radioactive decay. Positron emission occurs when elements have too many protons to be stable. This is true of elements that fall below the band of stability.

Positron Emission (Decay)

Describe the change in the mass number. The mass number stays the same. Describe the change in atomic number. The atomic number decreases by 1.

What happens to the neutron to proton ratio?The neutron to proton ratio increases.

Electron Capture

Electron capture ( ) occurs when an inner orbital electron is captured by the nucleus of its own atom. The inner orbital electron combines with a proton and a neutron is formed. Electron capture also occurs in atoms with too many protons.

Electron Capture

Describe the change in the mass number. The mass number stays the same. Describe the change in atomic number. The atomic number decreases by 1.

What happens to the neutron to proton ratio?The neutron to proton ratio increases.

Complete the following nuclear equations and identify the type of radioactive decay.1. → + 2. →3. → +4. → +

beta decay electron capture

alpha decaypositron emission

Write nuclear Decay equations for each of the following.1. alpha decay of polonium-210

2. beta decay of copper-66

3. oxygen-15 undergoes positron emission

4. argon-37 undergoes electron capture

Write nuclear Decay equations for each of the following.5. potassium-38 undergoes positron emission

6. silver-106 undergoes electron capture

7. beta decay of zirconium-91

8. alpha decay of thorium-230

Radioactive Decay Series

A radioactive decay series is a series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus.

Radioactive Decay Series

Decay Series

Write the series of reactions that represent the decay series for uranium-238.

TransmutationTransmutation – conversion of an atom of one element to an atom of another element.Transmutation may occur through radioactive decay. Induced Transmutation may also occur when high

energy particles (protons, neutrons, or alpha particles) bombard the nucleus.

Transuranium elements – elements in the periodic table with atomic numbers above 92. These elements have been synthesized in nuclear reactors and nuclear accelerators; which accelerate the bombarding particles to very high speeds.

Transmutation

The nuclear equation for the induced transmutation of aluminum-27 into phosphorus-30 by alpha particle bombardment is written below.

Which particle is emitted from the aluminum atom? neutron

Write the balanced nuclear equation for the induced transmutation of aluminum-27 into sodium-24 by neutron bombardment. An alpha particle is released in the reaction.

Write the balanced nuclear equation for the alpha bombardment of plutonium-239. One of the reaction products is a neutron.

Nuclear Fusion

In nuclear fusion, light nuclei combine to form a heavier, more stable nucleus.

Nuclear fusion occurs in the sun where hydrogen nuclei fuse to make helium nuclei.

Nuclear Fusion as a Power SourceNuclear fusion produces more energy per

gram of fuel than nuclear fission.Potential fuels (hydrogen-2, hydrogen-3) are

inexpensive and readily available.Fusion products are usually not radioactive.Unfortunately fusion requires high

temperatures to initiate the reaction and once started no known structural materials can contain the reaction.

Nuclear Fission

In nuclear fission, fissionable isotopes split when bombarded with neutrons.

The isotopes release neutrons that cause a chain reaction.

Nuclear Fission as a Power SourceUranium-235 is typically used as the

source of fuel in controlled fission reactions that produce large amounts of energy.

A major problem associated with nuclear fission is the issue of how to contain, store and dispose of the nuclear waste produced.