Physical Science 7.4f -The Decay of Uranium

Main Ideas

Uranium-238’s Composition

A uranium-238 nucleus contains 92 protons and 146 neutrons, for a total mass number of 238. Its atomic number, 92, identifies it as uranium.

First Step in Its Decay

Uranium-238 decays into thorium-234, emitting an alpha particle and a gamma ray.

• Thorium-234 has 90 protons and 144 neutrons.
• An alpha particle is a helium nucleus containing 2 protons and 2 neutrons.
• The numbers balance: the mass numbers total 238 on both sides (238 = 234 + 4), and the atomic numbers total 92 (92 = 90 + 2).

Radiation and Energy

Alpha particles and gamma rays are forms of radiation. The narrator describes a small amount of mass being converted into energy, associated with gamma radiation, and warns that radiation can be dangerous—particularly gamma rays.

Radioactive Decay Chain

Thorium-234 is also radioactive. It continues decaying through a sequence of about 16 stages, ultimately producing stable lead, at which point the decay chain ends.

Probability and Half-Life

It is not possible to predict exactly when an individual uranium atom will decay, but the overall rate of decay is statistically predictable. Uranium-238 has a half-life of about 4.5 billion years:

• After one half-life, half of the original uranium-238 remains.
• After another half-life, half of what remains decays, and so on.

Emissions from Many Atoms

Although uranium-238 decays slowly at the level of an individual atom, even a small sample contains an enormous number of atoms. The video states that half a pound of uranium emits about 3 million alpha particles per second. The narrator cautions against keeping uranium on a desk because it is hazardous.

Speakers and Sources

• One narrator/instructor, identified with the video channel as Derek Owens.
• No other speakers or sources are featured.