Earth’s Materials and Processes-Part 4 Radioactive Decay and Absolute Dating

An unstable nucleus releases energy to become more stable Radioactive Decay Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. There are numerous types of radioactive decay. The general idea: An unstable nucleus releases energy to become more stable

Chemistry Terms! (They’re Back!) The Atomic Number: the Number of Protons in the nucleus of an Atom. Nucleus: It’s where the Protons and Neutrons are located in an Atom. Protons: Positively Charged Particles in the Nucleus of the atom. Mass = (approx) 1 AMU Neutrons: Neutrally charged particles in the nucleus of an atom Mass = (approx) 1 AMU Mass Number of an atom: Number of Protons + Number of Neutrons in the nucleus of an atom.

Alpha Decay Type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or ‘decays’ into an atom with a mass number that is reduced by four and an atomic number that is reduced by two.

Example

Beta Decay Type of radioactive decay in which a beta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus.

Example

Gamma Decay Type of radioactivity in which some unstable atomic nuclei dissipate excess energy by a spontaneous electromagnetic process.

Alpha particles cannot escape a sheet of paper. Beta particles cannot escape a sheet of aluminum. Gamma particles can only be reduced by a much larger object, such as a piece of lead.

Types of Radioactive Decay

Sources of Radioactivity Primordial - from before the creation of the Earth Cosmogenic - formed as a result of cosmic ray interactions Human produced - enhanced or formed due to human actions (minor amounts compared to natural)

Sources of Radioactivity Naturally Occurring Sources: Radon from the decay of Uranium and Thorium Potassium -40 – found in minerals and in plants Carbon 14 – Found in Plants and Animal tissue Manmade Sources: Medical use of Radioactive Isotopes Certain Consumer products –(eg Smoke detectors) Fallout from nuclear testing Emissions from Nuclear Power plants

Radiation Sources

The Alpha, Beta and Gamma particles all add energy to the body’s tissues. The effect is called the Ionizing Energy. It can alter DNA. Even though Alpha particles are not very penetrative if the decaying atom is already in the body (inhalation, ingestion) they can cause trouble.

Radioactive Decay: Summed Up During radioactive decay, a “parent” atom releases energy and particles as it changes to a new kind of atom, a “daughter” atom.

Radioactive Decay Radioactive elements happen naturally in igneous rocks Scientists use the rate of decay to calculate the rock’s age As the radioactive element in the rock decays it changes into another element Composition changes slowly over time Amount of radioactive element goes down and amount of new element goes up

Half-Life of Radioactive Atoms The time it takes for one half of the radioactive atoms to decay.

Half-Life Example Rate of decay is constant for each radioactive element Scientists can measure the rate of decay experimentally

Radiometric Dating Table lists several common radioactive elements and their half-lives. Carbon-14 has a half-life of 5,730 years and is useful in dating plants and animals that lived up to about 50,000 years ago. (It has a short half-life…) Potassium-40 decays to a stable argon-40 and has a half-life of 1.3 billion years. It is useful in dating most ancient rocks because of its long half-life.

In radioactive dating first measure the amount of radioactive element in a rock (“parent” element) Then measure the amount of stable element that the unstable radioactive element decays into (“daughter” element) By calculating the ratio of radioactive element to the stable element you can determine the age of the rock

Earth’s Age…Estimated Earth’s ever changing processes makes it difficult for scientists to determine Earth’s absolute age Radioactive dating shows that the oldest rocks found on Earth are about 4.0 billion years old The oldest fossils of living things are about 3.5 billion years