7.1 continued: Radioactive Decay

A brief review of last class…

https://www.youtube.com/watch?v=IVtwfSBSLtI

Radioactive Decay• Radioactivity results from having an unstable nucleus.• When these nuclei lose energy and break apart, decay occurs.

Radioactive decay releases energy from the nucleus as radiation. Radioactive atoms release energy until they become stable, often as

different atoms.• An element may have only

certain isotopes that are radioactive. (C-12 & C-13are stable, C-14 is unstable)

• These are called radioisotopes

See page 293Radioisotope uranium-238 decays in several

stages until it finally becomes lead-206

Three Types of Radiation

• Rutherford identified three types of radiation using an electric field. Positive alpha particles were attracted to the negative plate. Negative beta particles were attracted to the positive plate. Neutral gamma particles did not move towards any plate.

See page 294

Alpha Radiation• Alpha radiation is a stream of alpha particles, .

Positively charged, and are the most massive of the radiation types. Alpha particles are essentially the same as a helium nucleus. Alpha particles are represented by the symbols .

Because it has two protons, it has a charge of 2+. The release of alpha particles is called alpha decay.

Alpha particles are slow and penetrate materials much less than the other forms of radiation. A sheet of paper will stop an alpha particle.

See page 294 - 295

24 or 2

4 He

Radium-226 releases an alpha particle and becomes Radon-222. Radon has two less protons than radium.

88226 Ra 86

222 Rn + 24

or

88226 Ra 86

222 Rn + 24 He

Beta Radiation• Beta radiation, , is an electron (from the nucleus.)• Negatively charged, and are fast moving.• Beta particles are represented by the symbols .

Electrons are very tiny, so beta particles are assigned a mass of 0. Since there is only an electron, a beta particle has a charge of 1–.

• Beta decay occurs when a neutron changes into a proton + an electron. The proton stays in the nucleus, and the electron is released. It takes a thin sheet of aluminum foil to stop a beta particle.

See page 296

-10 or -1

0e

Iodine-131 releases a beta particle and becomes Xenon-131. A neutron has turned into a proton + the released electron.

53131I 54

131 Xe + –10

or

53131I 54

131 Xe + –10e

Gamma Radiation• Gamma radiation, , is a ray of high energy, short-wavelength

electromagnetic radiation.• Gamma radiation has no charge and no mass, .• Gamma radiation is the highest energy form of electromagnetic radiation.

It takes thick blocks of lead or concrete to stop gamma rays.• Gamma decay results from energy being released from a high-energy

nucleus.

• Often, other kinds of radioactive decay will also release gamma radiation.

Uranium-238 decays into an alpha particle and also releases gamma rays.

See page 297

2860 Ni* 28

60 Ni + 00

92238 U 90

234 Th + 24 He + 2

00

Summary of the 3 forms of radiation…

https://www.youtube.com/watch?v=5oUagoF_viQ

Radiation Summary

T R Y : Workbook p.125

If finished, read text p.293-297.

Nuclear equations for radioactive decay

• Nuclear equations are written like chemical equations, but represent changes in the nucleus of atoms.1. The sum of the mass numbers should be equal.2. The sum of the atomic numbers should be equal.

ThTh HeHe RaRa230230

9090 22

44 226226

8888

CsCs eeBaBa137137

5555 -1-1

00137137

5656

LrLr MdMd HeHe256256

103103 101101

252252 44

22

αα2244

αα2244

ββ-1-100

230 = 4 + ____230 = 4 + ____22622690 = 2 + ____90 = 2 + ____8888

Radioactive Decay Summary

Homework: Workbook p.125-128