Dr. WilliamsJCHS

PART I: CHAPTER 25RADIOACTIVE DECAY

& HALF LIFE

A nucleus that consists of protons and neutrons is called a nucleon.

A nucleus is characterized by two numbers, represented by letters A & Z:

A: the atomic mass number (the total number of nucleons)

Z: the atomic number (the number of protons)

THE NUCLEUS

How big is a nucleus?

Atoms are a few angstroms, but most of the atom is empty space.

angstrom ang·strom or ång·strom (āng'strəm) n.

Abbr. A, Å, angst A unit of length equal to one hundred millionth (10- 8 ) of a

centimeter, used especially to specify radiation wavelengths.

Since the nucleus is smaller than atom, it is a few femtometers. (one quandrillionth of a

meter).

NUCLEUS SIZE

What holds the nucleus together?

Protons are close together; thus the gravitational force attracting them to each other is much smaller than the

electrical force repelling them… so what is keeping them together?

The strong force, which only works over small distances.

A strong force is an attractive force fro protons and neutrons separated by a few femtometers. It does not

work over larger distances.

NUCLEAR FORCES

There is a tug-of war between the attractive force of the strong nuclear force and the repulsive electrostatic

forces between protons.

As Z gets larger, stable nuclei will have more neutrons than protons. A point will be reached where stability is

compromised.

Bismuth, with 83 protons and 126 neutrons is the largest stable nucleus.

Nuclei with more than 83 protons are all unstable and will eventually break up into small pieces.

When Nuclei break up, this is known as radioactivity.

NUCLEUS STABILITY

Einstein’s equation related energy and mass.

E= mc^2

With his equation you can convert mass to energy.

In a nucleus there is binding energy, the energy needed to split the nucleus into individual protons and neutrons.

To calculate binding energy add the mass of individual protons and neutrons and subtract the mass of the nucleus, then plug

into Einstein’s equation.

Delta massc^2.

NUCLEUS SPLITTING

Remember isotopes…

• Atoms of the same element that have different numbers of neutrons

Most isotopes are stable – those that are radioactive have an unstable ratio of protons and neutrons in their nucleus • Emitting a particle from their nucleus creates a

more stable ratio• It also changes the nucleus into a different

element!

Reactions involving the decay of the nucleus and

changing into a new element are

called transmutation

reactions • Natural transmutation • Elements that naturally emit

energy without the absorption of energy from an outside source

• Artificial transmutation • Causing an otherwise stable

nucleus to become radioactive• Nuclear fission and nuclear

fusion

Two kinds of transmutation

reactions:

TRANSMUTATION

Transmutation can occur by radioactive decay

Transmutation can also occur when particles

bombard the nucleus of an atom.

TRANSMUTATION

There are several types of particles that an unstable nuclei can emit to

attain a more stable atomic configuration

Table O – Symbols used in Nuclear Chemistry – shows us particles that can be emitted

Note - gamma radiation is not a particle it is only energy

+ charge

0 charge

Each radioisotopes only goes through one type of decay mode.

The three decay modes are: • Alpha decay • The release of an alpha particle from the nucleus

• Beta decay • The release of a beta particle from the nucleus

• Positron decay • The release of a positron particle from the nucleus.• A positron is a particle with the mass of an electron but a

positive charge. During positron emission, a proton changes to a neutron.

CHARACTERISTICS OF RADIATION TYPESPG. 801

Alpha decay

In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable.

An example of an alpha decay involves uranium-238:

The process of transforming one element to another is known as transmutation.

Alpha particles do not travel far in air before being absorbed; this makes them very safe for use in smoke detectors, a common household item.

Beta decay

A beta particle is often an electron, but can also be a positron, a positively-charged particle that is the anti-matter equivalent of the electron. If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one. An example of such a process is:

In terms of safety, beta particles are much more penetrating than alpha particles, but much less than gamma particles.

Gamma decay

The third class of radioactive decay is gamma decay, in which the nucleus changes from a higher-level energy state to a lower level. Similar to the energy levels for electrons in the atom, the nucleus has energy levels. The concepts of shells, and more stable nuclei having filled shells, apply to the nucleus as well.

Natural Transmutation • Just as a balanced chemical reaction must always

be written for a “normal” chemical reaction a balanced nuclear reaction can be written as well.

He Th U 42

23490

23892

e Xe I 0-1

13154

13153

e Ar K 01

3818

3819

Pd e Ag 10646

0-1

10647

These are examples of balanced nuclear equations

One element becomes another

The particles involved are balanced

The sum of the mass numbers and the sum of the atomic numbers on each side are equal

Natural TransmutationSteps for writing and balancing a nuclear reactionSample: Write and balance the nuclear reaction for radium-226.Step 1: Write the notation for the radioactive isotope as the reactant Step 2: Look on Table N for the decay mode of the radioisotope

radium – 226 = αStep 3: Look on Table O for the symbol of the decay mode. This is one of

the products, write the symbol of the particle as a productStep 4: The second product is determined by first balancing the mass

number and atomic number and then looking on the periodic table to determine what element has that atomic number

Ra226

88He

4

2+

222

86Rn

Write the balanced nuclear equation for the decay of iodine - 131

NATURAL TRANSMUTATION

I131

53 β0

-1+

131

54 Xe

Natural Transmutation• Neon – 19 decays by positron emission.

Write the balanced nuclear equation.

Ne19

10β0

+1+

19

9F

PRACTICE WITH ALPHA PARTICLES

Alpha decay can most simply be described like this:

1) The nucleus of an atom splits into two parts.2) One of these parts (the alpha particle) goes zooming off into space.3) The nucleus left behind has its atomic number reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons). There are other points, but the three above are enough for this class. Here is a typical alpha decay equation:

ALPHA PRACTICE

ANSWERS SET 1

ALPHA PRACTICE

ALPHA PRACTICE ANSWERS 2

ALPHA PRACTICE

ALPHA PRACTICE SET 3

BETA PRACTICE

Beta decay is somewhat more complex than alpha decay is. These points present a simplified view of what beta decay actually is:

1) A neutron inside the nucleus of an atom breaks down, changing into a proton.

2) It emits an electron and an anti-neutrino (more on this later) which go zooming off into space.

3) The atomic number goes UP by one and mass number remains unchanged.

Here is an example of a beta decay equation:

BETA PRACTICE SET 1

BETA PRACTICE SET 1 ANSWERS

BETA PRACTICE 2

BETA SET 2 ANSWERS

BETA PRACTICE SET 3

BETA PRACTICE 3 ANSWERS