Section 4-4

Section 4.4 Unstable Nuclei and Radioactive Decay

• Explain the relationship between unstable nuclei and radioactive decay.

element: a pure substance that cannot be broken down into simpler substances by physical or chemical means

• Characterize alpha, beta, and gamma radiation in terms of mass and charge.

Section 4-4

Section 4.4 Unstable Nuclei and Radioactive Decay (cont.)

radioactivity

radiation

nuclear reaction

radioactive decay

alpha radiation

Unstable atoms emit radiation to gain stability.

alpha particle

nuclear equation

beta radiation

beta particle

gamma rays

Section 4-4

Radioactivity

• Nuclear reactions can change one element into another element.

• In 1895, Wilhelm Roentgen found that invisible rays were emitted when electrons bombarded the surfaces of certain materials.

• These rays had high-energy and caused photographic plates to darken. Roentgen named these rays Xrays.

• A French scientist Henri Becquerel was studying minerals that were phosphorescent….that emitted light after being exposed to sunlight.

• Becquerel wanted to see if phosphorescent minerals also emitted Xrays, so he studied uranium salts.

• He found that uranium salts made emissions that would darken photographic plates even when they weren’t exposed to light.

• Marie Curie and husband Pierre found that the darkening of the photographic plates was due to rays emitted from the uranium atoms in the mineral sample that Becquerel used.

• Marie Curie named this process, by which some substances spontaneously emitted radiation, radioactivity.

• The rays and particles emitted by a radioactive source are called radiation.

• A reaction that involves a change in an atom's nucleus is called a nuclear reaction.

Section 24-1

The Discovery of Radiation

Section 4-4

Radioactive Decay

• Atoms that contain too many or two few neutrons are unstable and lose energy through radioactive decay to form a stable nucleus.

• Few exist in nature—most have already decayed to stable forms.

Section 4-4

Radioactive Decay

• Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay.

• Unstable radioactive elements undergo radioactive decay thus forming stable nonradioactive elements.

• Isotopes of atoms with unstable nuclei are called radioisotopes.

• The three most common types of radiation are alpha, beta, and gamma.

Section 4-4

Radioactive Decay (cont.)

• Alpha radiation is made up of positively charged particles called alpha particles.

• Each alpha particle contains two protons and two neutrons and has a 2+ charge.

•No electrons

• Alpha particle ()– helium nucleus

2+paperHe4

2

Section 24-1

Types of Radiation (cont.)

• Alpha radiation is not very penetrating—a single sheet of paper will stop an alpha particle.

Section 4-4

Radioactive Decay (cont.)

• Beta radiation is radiation that has a negative charge and emits beta particles.

• Each beta particle is an electron with a 1– charge.

• Beta particle (-)– electron

1- foil

e0-1

Section 24-1

Types of Radiation (cont.)

• Beta radiation is a stream of fast moving particles with greater penetrating power—a thin sheet of foil will stop them.

Types of Radiation (cont.)

• Gamma rays almost always accompany alpha and beta radiation.

• Gamma rays account for most of the energy lost during radioactive decay.

• Gamma rays are high-energy radiation with no mass and are neutral.

• The ability of radiation to pass through matter is called its penetrating power.

• Gamma rays are highly penetrating because they have no charge and no mass.

Section 4-4

Radioactive Decay (cont.)

• Gamma ()– high-energy photon 0 concrete

Section 4-4

Radioactive Decay (cont.)

A. A

B. B

C. C

D. D

Section 4-4

A B C D

0% 0%0%0%

Section 4.4 Assessment

A reaction that changes one element into another is called what?

A. chemical reaction

B. beta radiation

C. nuclear reaction

D. physical reaction

A. A

B. B

C. C

D. D

Section 4-4

Section 4.4 Assessment

A B C D

0% 0%0%0%

Why are radioactive elements rare in nature?

A. They do no occur on Earth.

B. Most have already decayed to a stable form.

C. They take a long time to form.

D. They are too hard to detect.