Unit 1.3Nuclear Chemistry

1.3-1 Types of Radioactivity

By the end of this section you will be able to:

◦ Observe nuclear changes and explain how they change an element.

◦ Express alpha and beta decay in nuclear equations.

◦ Model the half life of an isotope.

◦ Explain how half life is used to date materials.

Learning Objectives

Radioactivity Alpha Particle Beta Particle Alpha Decay Beta Decay Gamma Decay Half life Radioactive Dating Radioactive Decay

Important Terms

Radioactivity is the spontaneous emission of radiation by an unstable atomic nucleus.

Discovery of Radioactivity

Chemical Reactions vs. Nuclear Reactions

Occur when bonds are broken and formed

Occur when nuclei combine, split and emit

radiation

Involve only valence electrons

Can involve protons, neutrons and electrons

Atoms keep the same identity although they gain,

lose or share electrons

Atoms of one element are often converted into

atoms of another element

Associated with small changes in energy

Associated with large changes in energy

Temperature, pressure, concentration and catalysts

affect reaction rates

Temperature, pressure, concentration and

catalysts do not affect reaction rates

Nuclear reactions involve the protons and neutrons found in the nucleus

During nuclear reactions a nucleus can gain or lose protons and neutrons.

Nuclear Reactions

Remember that the number of protons determines the identity of an element.◦Changing the number of protons

changed the element into another element.

◦During nuclear reactions atoms of one element are changed into atoms of another element

Nuclear Reactions

Different isotopes of atoms can be represented using nuclear notation.

Nuclear Notation

In your notebook write the following isotopes in nuclear notation.◦ Hydrogen-1◦ Hydrogen-2◦ Hydrogen-3

Review of Nuclear Notation

Radioactive decay is the release of radiation by radioactive isotopes.

Not all radioactive isotopes decay in the same way.◦ Different types of decay change the nucleus in

different ways.

The three types of decay are: Alpha Beta Gamma decay

Radiation causes Radioactive Decay

Alpha decay is the release of alpha particles (2 protons and 2 neutrons).

Alpha particles are helium nuclei consisting of two protons and two neutrons.

Alpha particles are represented as

or α.

Radioactive ALPHA Decay

• Alpha particles, which are large in size, collide with objects around them.

Do not penetrate very deeply Are easily stopped by a thin layer of

material.

Radioactive ALPHA Decay

◦Alpha decay causes the decaying nucleus to lose 2 protons and 2 neutrons.

◦This means: the mass # decreases by 4 (2P and 2N) The atomic # decreases by 2 Examples

◦Parent Daughter alpha particle

Radioactive ALPHA Decay

The parent element turns into a daughter element with a mass number 4 less and an atomic number 2 less than the parent!

Does this reaction demonstrate the law of conservation of matter?◦ How can we check it? Explain

Equation for Radioactive ALPHA Decay

ALPHA EmissionTwo protons and neutrons are lost

The protons and neutrons leave as an alpha particle.

+ Energy!

Write the equation for alpha decay for the following particle in your notebook.

Thorium-230

Radioactive Alpha Decay

Beta decay is the release of beta particles from a decaying nucleus.

◦A beta particle is a high energy electron with a 1- charge. Beta particles are written as β- or

Beta particles pass more easily through matter than alpha particles and require sheets of metal, blocks of wood or specialized clothing to be stopped.

Radioactive BETA Decay

The electron released during beta decay is not one of the original electrons that existed outside the nucleus.

The beta particle (electron) is produced by the change of a neutron into a proton and an electron. Mass# is same!

◦Parent Daughter Beta (add P+) (sub e-)

Radioactive BETA Decay

Equation for Radioactive BETA Decay

The parent nucleus turns into a daughter with an atomic number 1 greater.

The mass number stays the same.

BETA Emission•A neutron becomes a proton (which stays in the nucleus) and electron (which is ejected from the atom).•ADD A PROTON and LOSE an ELECTRON

+ ENERGY

Write the equations for beta decay for the following particles.

Magnesuim-27 Sulfur-35

Radioactive BETA Decay

Gamma decay is the release of gamma rays from a nucleus.

◦A gamma ray is a high energy form of electromagnetic radiation with out a change in mass or charge.

Radioactive Gamma Decay

◦Gamma rays have high penetrating ability and are very dangerous to living cells.

◦To stop gamma rays thick blocks of lead or concrete are needed.

Radioactive GAMMA Decay

During gamma decay only energy is released!

◦Gamma decay does not generally occur alone, it occurs with other modes of decay. (alpha or beta)

Radioactive GAMMA Decay

When gamma decay is expressed in an equation it is expressed as γ.◦ Electron from beta decay is captured to cause

gamma particle to emit.

◦ The following equation shows both gamma and alpha decay occurring.

Equation for Radioactive GAMMA Decay with Beta or Alpha Decay

GAMMA Emission with Beta decay

Beta emission

Co-60 Ni-60 + Beta e- Ni-60 + gamma photon (particle of radiation)

(excited state)

PLEASE DO NOT WRITE THE QUESTIONS!Each correctly answered question is worth 1

point!1. What are the three types of decay?2. Explain what occurs to the element in each

type of decay, be specific.◦ A.

B. C.

3. Which type of decay is least harmful to living cells.

4. Which is most harmful?5. If Uranium-238 alpha decays, what would the

decay equation be?

Quiz!!

1. Alpha, beta and gamma2. Alpha- gives off alpha particle which is 2

protons and 2 neutrons. It reduces the atomic number by 2 and the mass by 4 so becomes a new elementBeta- a neutron becomes a proton and an electron and gives off the electron, it adds 1 to the atomic number but leaves the mass number the same so a new element is formed

Gamma- just a gamma ray, pure electromagnetic radiation (energy)

3. Alpha4.Gamma5 238 U -> 234 Th + 4 He 92 90 2

Answers to quiz questions

Nuclear Equations: What type of decay is Represented? Fill in the blanks

Nuclear Equations: try these!

Radiation can be detected with Geiger counters and scintillation counters.

◦Geiger counters detect ionizing radiation.

◦Scintillation counters register the intensity of radiation by detecting light.

Radioactive Decay

It is impossible to predict when a specific nucleus in a sample of radioactive material will undergo decay.

The rate of overall decay is constant so that it is possible to predict when a given fraction of a sample will have decayed.

Rate of Radioactive Decay

Half-life is a term used to describe the time it takes for half of a given amount of a radioactive isotope to decay.◦Half-life varies greatly depending on the isotope

Half-Life

Half-life: How long is it?

Radioactive decay has provided scientists with a technique for determining the age of fossils, geological formations and human artifacts.

◦Four isotopes are commonly used for dating objects Carbon-14 Uranium-238 Rubidium-87 Potassium-40

Half-Life and Radioisotope Dating

Carbon-14 Dating◦All organisms take in carbon during their

lifetime.◦When organisms die they stop taking in carbon.

Most carbon that organisms take in is stable (Carbon-12 or Carbon-13).

About one atom in a million is Carbon-14. While the organism is alive the amount of Carbon-

14 in its tissues remains constant. After the organism dies no more Carbon-14 is

taken in and the amount begins to decline at a predictable pace. (half-life of C-14=5730 years)

Half-Life and Radioisotope Dating;C-14

Half-Life of Carbon-14

The half-life of Carbon-14 is 5730 years.◦Objects greater than 60,000 years old cannot

be dated using this method because the amount of Carbon-14 that remains is too small to be detected. Objects greater than 60,000 years old are

dated using: Uranium-238 (t½ = 4.5 billion years) Rubidium-87 (t½ = 48 billion years) Potassium-40 (t½ = 1.25 billion years)

Half-Life and Radioisotope Dating

Radioactive Decay Series: U-238