Nuclear Chemistry –Radioactive Decay

The discovery of radiation

In 1896 Henri Becquerel made an important discovery.

He accidentally had placed a piece of uranium ore on top of an unexposed photographic plate. Later, when the plate was developed, the image of the rock was found on the plate.

Based on further experiments, he concluded that the plate had been exposed by rays given off by the uranium.

Madame Curie discovers Radium and Polonium

Following Becquerel’s discovery, Marie Sklodowska Curie and her husband, Pierre Curie, attempted to isolate the “radioactive” material from the uranium ore.

In doing so they discovered two new elements, Radium and Polonium, both of which were more radioactive than the original ore.

Ernest Rutherford

Rutherford investigated this new property of matter and discovered that, in the process of emitting radiation, atoms of one element became atoms of another element.

Today, we describe the process of an atom of one element becoming an atom of a different element as transmutation.

Rutherford discovered the different types of radiation

Rutherford was the first to identify and name two different types of radiation given off when an atom of one element underwent transmutation and became an atom of another element. The two types of radiation he found were:

•The alpha particle ()

•The beta particle ()

A third type of radiation that was discovered later is called:

•Gamma radiation ()

The properties of the different types of radiation

The differences between the three types of radiation can be seen by passing them through an electric field.

Characteristics of an alpha particle

The alpha particle () is deflected to some extent toward the negative plate.

This indicated that it is positively (+) charged and has a fairly large mass.

Today we know that an particle is the same as the nucleus of a He atom.

Characteristics of a beta particleThe beta particle () is deflected toward the positive plate (+). It is also deflected more than the particle

This indicates that it is negatively (-) charged and has a much smaller mass than the particle.

Today we know that a particle is the same as an electron.

Characteristics of gamma radiation

Gamma radiation () is not deflected toward either the positive (+) or negative (-) plate.

This indicates that it has no charge.

Today we know that gamma rays are a type of electromagnetic radiation made up of photons (packets of energy).

Radiation and Table OThe Regents Reference Tables provides us with a summary of the different types of radiation:

Penetrating power of radiation

The ability of radioactive particles to pass through air and other materials is inversely related to their mass.

• Alpha particles – the least penetrating, they travel only a few centimeters through air. They can be stopped by a single sheet of paper.

• Beta particles – more penetrating, they travel several meters through air. They can be stopped by a sheet of Al or plastic.

• Gamma Rays – most penetrating, thick sheets of lead or concrete are required to stop gamma rays.

Diagram showing penetrating ability

www.epa.gov

Where does the radiation come from?

Rutherford suggested that the radiation resulted from the breakdown of the nucleus of an atom, resulting in radiation being given off, and the nucleus of the atom changing into a new element.

For instance, the fact that U-238 undergoes alpha decay (emits an particle) can be shown by this reaction:

Why does the atom break up?Remember that the nucleus of the atom is held together by the strong nuclear force. This force is normally strong enough to hold the protons and neutrons together.

However, sometimes the force of repulsion due to the protons having the same charge overcomes the strong nuclear force and the atom breaks apart.

How does beta decay occur?

Sometimes an atom will emit a particle when it breaks up.

In beta decay a neutron apparently “spits out” an electron (the particle) and becomes a proton.

Balanced nuclear equationsNuclear reactions can be represented by equations. These reactions are governed by two “laws”:

The law of conservation of mass number – the sum of the mass numbers on the reactant sides must be equal to the sum of the mass numbers on the product side.

This law applies to all nuclear equations!

Balanced nuclear equationsThe second law is:

The law of conservation of charge (atomic #) – the sum of the atomic numbers on the reactant sides must be equal to the sum of the atomic numbers on the product side.

This law applies to all nuclear equations!

Predicting products in alpha decay

The Law of Conservation of Mass Number and the Law of Conservation of Charge allows us to predict products in a nuclear reaction.

For instance, suppose we wanted to predict the atom produced when Radon-222 undergoes alpha decay.

Predicting productsBased on the two laws we can predict:

The mass number of particle X must be 218.

The charge (atomic #) of particle X must be 84.

The symbol of the element can then be determined from the Periodic Table.

How about beta decay?

It works the same way. Let’s look at the beta decay of Strontium-90.

Remember the sum of the mass numbers and atomic number on both sides MUST be the same.

So atom X must be:

Using balanced nuclear equations to identify the type of radioactivity.

Suppose we know that a particular atom undergoes radioactive decay and we are able to identify the atom that is produced.

Using the Laws of Conservation of Mass # and Charge, we can identify the type of radiation given off.

For instance, Iodine-131 is known to form Xenon-131 when it decays. What radioactive particle must it emit?

Particle X must be a particle:

Another type of radioactive decay

Some atoms undergo a decay process that produces a positron. A positron has the same mass as an electron, but is positively charged.

Symbols for the positron include:

Positrons are a form of anti-matter. Antimatter is made up of particles with the same properties as normal matter, but are opposite in charge.

Positrons are also listed in Table O

Positron EmissionWe can use our ability to balance nuclear equations to predict what will be given off when Potassium-37 undergoes positron emission.

There’s only one atom that will work, and that’s Argon-37.

Your turn!Using the Laws of Conservation of Mass # and Charge, write balanced nuclear equations for the following nuclear reactions:

1. Beta decay of Phosphorus-32

2. Alpha decay of U-238

3. Positron decay of Iron-53

4. Decay of Oxygen-17 into Nitrogen-17

5. Decay of Potassium-42 into Calcium-42

6. Decay of Plutonium-239 into Uranium-235

Half-lifesThe rate at which a particular radioisotope decays is described by its half-life.

The half-life is defined as the time that it takes for one half of a sample of a radioactive element to decay into another element.

The half-life of a radioisotope is dependent only on what the radioisotope is.

Table N provides us with a list of various nuclides, their decay modes, and their half-lifes.

Using Table N, what is the decay mode and half-life for Radium-226?

Using Table N

Table N indicates that Radium-226 undergoes alpha decay.

Based on this we can write a balanced nuclear equation to represent this reaction:

This tells us that for every atom of Radium that decays an atom of Radon is produced.

Using Half-lifeTable N also tells us that Radium-226 has a half-life of 1600 years.

Starting with a 100g sample, after 1 half-life (or 1600 years), 50g remain.

After another 1600 years, half of the 50g will remain (25g).

Carbon-14 Dating

The age of objects that were once alive can be determined by using the C-14 dating test. In this test, scientists determine how much C-14 is left in a sample and from this determine the age of the object.

From Table N we can determine that C-14 undergoes decay:

Where does the Carbon-14 come from?

C-14 is created in the atmosphere by cosmic rays.

It becomes part of living things through photosynthesis and the food chain.

When the plant or animal dies, the C-14 begins to decay.

Using C-14 to Age ObjectsBy comparing the amount of C-14 left in a sample to the amount that was present when it was alive, and using the half-life of 5700 years (Table N), one can determine the age of a sample.

Uranium-238 SeriesThe Uranium-238 Decay Series is used to determine the age of rocks.

In this series, the ratio of the U-238 to the Pb-206 is used to determine the age of the rock.

“Parent-daughter” Relationship

Aging moon rocks

NASA astronauts have retrieved 842 pounds (382 kg) of moon rocks (in many missions), which have been closely studied. The composition of the moon rocks is very similar to that of Earth rocks. Using radioisotope dating, it has been found that moon rocks are about 4.3 billion years old.

Sample Half-life Problem 1A 10 gram of sample of Iodine-131undergoes decay, what will be the mass of iodine remaining after 24 days?

From Table N, the ½ life of iodine is determined to be approximately 8 days.

That means that 24 days is equivalent to 3 half-lifes.

The decay of 10 grams of I-131 would produce:

1.25 grams of I-131 would remain after 24 days.

Sample Half-life Problem 2A sample of a piece of wood is analyzed by C-14 dating. The percent of C-14 is found to be 25% of what the original C-14 concentration was. What is the age of the sample?

First, let’s analyze how many half-lives have taken place.

Two half-lives have gone by while the sample decayed from the original C-14 concentration to 25% of that concentration.

Based on Table N, the half-life of C-14 is 5730 years, so…

Your turn!

On a sheet of paper, answer the following questions from your textbook. Indicate how you arrived at your answer and turn in your work for a homework/quiz grade.

Page 670

•Questions 34 (a and b), 36, 37, 38, 41, 42.

Page 671

• Questions 50, 58, 59

The End

• This is the end of the first slide show on nuclear reactions. You may continue learning about nuclear reactions by viewing the second show:

Nuclear Chemistry:

Fission and Fusion