9-1: What Is Radioactivity?
Tuesday, August 18, 2015
5:53 PM
Key terms:
1. Radioactivity – the process by which an unstable nucleus emits one or more particles or energy in the form of
electromagnetic radiation.
2. Nuclear radiation – the particles that are released from the nucleus during radioactive decay.
3. Alpha particle – a positively charged atom that is released in the disintegration of radioactive elements and that
consists of two protons and two neutrons.
4. Beta particle – a charged electron emitted during certain types of radioactive decay, such as beta decay.
5. Gamma ray – the high-energy photon emitted by a nucleus during fission and radioactive decay.
6. Half-life – the time required for half of a sample of a radioactive substance to disintegrate by radioactive decay or by
natural processes
I. Nuclear Radiation
Radioactive materials have unstable nuclei, which go through changes to become more stable. This
results in a different isotope of the same element being created or a different element forming.
a. There are different types of nuclear radiation
i. 4 types of nuclear radiation
1. Alpha particles
2. Beta particles
3. Gamma ray
4. Neutron emission
ii. When a radioactive nucleus decays, the nuclear radiation leaves the nucleus.
b. Alpha particles consist of protons and neutrons
i. Equivalent to a helium nucleus.
ii. Alpha particles are positively charged and more massive than any other type of nuclear radiation.
iii. They do not travel far through materials; barely able to travel through a sheet of paper.
iv. Positive charge removes electrons from matter as it moves through it (ionizes it). This causes the alpha
particle to lose energy and to slow down more.
c. Beta particles are electrons produced from neutron decay
i. Equivalent to a fast moving electron
ii. Neutrons decay to form a positively charged proton and a negatively charged electron. The proton can
stay in the nucleus, but the electron is emitted.
iii. Faster and less massive than alpha particles. They easily travel through a piece of paper but are stopped
Ch9 Nuclear Changes Page 1
iii. Faster and less massive than alpha particles. They easily travel through a piece of paper but are stopped
by 3mm of aluminum or 10mm of wood.
iv. Also ionize other atoms as they travel through them, causing the beta particle to lose energy and slow
down.
d. Gamma rays are very high energy
i. Not made of matter and do not have an electric charge, but can easily ionize matter.
ii. Consist of a form of electromagnetic energy called photons.
iii. Can penetrate up to 60mm of aluminum or 7cm of lead. Not stopped by clothing or building materials.
iv. Greater danger to health than either alpha or beta particles.
e. Neutron radioactivity may occur in an unstable nucleus
i. Neutrons can be emitted from an unstable nucleus.
ii. No charge, so they do not ionize matter like alpha and beta particles.
iii. Because they do not ionize matter, they can travel farther through materials. Can penetrate up to 15cm
of lead.
II. Nuclear Decay
Nuclear decay equations are similar to equations for chemical reactions. The nucleus before decay is the
reactant and goes on the left, and the products are placed on the right.
a. A nucleus gives up two protons and two neutrons during alpha decay
b. A nucleus gains a proton and loses a neutron during beta decay.
In all cases of beta decay, the mass number before and after the decay does not change, but the atomic
number of the product nucleus increases by 1.
Ch9 Nuclear Changes Page 2
III. Radioactive Decay Rates
Although it is impossible to predict the moment when any particular nucleus will decay, it is possible to
predict the time it takes for half the nuclei in a sample to decay (half-life). After a half-life has passed,
half the sample remains unchanged.
Ch9 Nuclear Changes Page 3
a. Half-life is a measure of how quickly a substance decays
i. Different radioactive isotopes have different half-lives, ranging from nanoseconds to billions of years.
ii. Scientists can use half-lives to predict how old an object is.
b. Carbon-14 is used to date materials
i. Archaeologists use the half-life of radioactive carbon-14 to date materials such as animal remains or
fibers from ancient clothing.
ii. By measuring the ratio of carbon-14 to carbon-12 the age of an object can be estimated.
Ch9 Nuclear Changes Page 4
Quiz:
1. What happens in nuclear decay?
2. The half-life of iodine-131, a radioactive isotope of iodine, is 8.1 days. What fraction of a sample of iodine-131 would
remain unchanged after 16.2 days?
Ch9 Nuclear Changes Page 5