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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 7: Types of Radiation and Nuclear Equations

When Ernest Rutherford was working under J.J. Thomson (discoverer of the electron), he found that uranium gave off two distinct types of radiation. The first, which he called "alpha radiation," could only travel a few centimeters through air and could not penetrate a sheet of aluminum mere hundredths of an inch thick. The second type traveled through matter much more easily, and he termed it "beta radiation." Later research found that alpha radiation consisted of particles that are clusters of two protons and two neutrons. Alpha particles have the symbol $↖4↙2$α2+. Beta particles, like cathode rays, were found to be electrons. (This is a newly formed electron, not one of the electrons in the cloud surrounding the nucleus.) A beta particle is represented by the symbol 0β1-. A third type of radiation from uranium was found in 1900 and termed "gamma radiation." Gamma rays are a type of high-energy electromagnetic wave like X-rays. They have no mass and no charge, and are represented by the Greek letter gamma: γ.

These three types of radiation differ in their penetrating power; alpha particles penetrate the least, and gamma rays penetrate the most. A piece of ordinary paper will stop alpha particles, a piece of aluminum or wood can stop beta particles, and lead or concrete is needed to stop gamma rays. (Figure 12-13)

Types of Radiation

Figure 12-13. Types of Radiation

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Types of Radiation

Figure 12-13. Types of Radiation

Different types of radiation penetrate substances to different degrees.

Like chemical reactions, nuclear reactions can be described by equations. Nuclear equations must balance in terms of both mass and charge. For example, when an atom of polonium-210 undergoes alpha decay, the equation is:

$↖120↙84$Po → $↖206↙82$Pb + $↖4↙2$α2+

When alpha decay happens, four heavy particles are lost, so the mass number decreases by four, and the atomic number goes down by two. Other types of common nuclear decay do not affect the mass number of the element. When C-14 undergoes beta decay, the equation is:

$↖14↙6$C → $↖14↙7$N + 0β1-

Note that the mass numbers (in superscripts) balance on both sides of the arrow, similar to how the number of atoms balance in a regular chemical equation. In a beta decay, one of the neutrons in the nucleus releases a beta particle, which results in the neutron becoming a proton. So, carbon-14 became nitrogen-14; this is the main reaction that is involved in the removal of 14C from living material that is exploited for carbon dating. Even bananas contain a lot of naturally occurring potassium-40, which undergoes a beta decay; for this reason, bananas will set off a Geiger counter, a tool used for detecting radiation.

In gamma emission, the equation looks like this:

$↖{99\m}↙43$Tc → $↖99↙43$Tc + 0γ0

Because a gamma ray has no mass and no charge, the mass and charge numbers on the technetium atom don't change. The m indicates that the protons and neutrons in the technetium nucleus are in a metastable, high-energy arrangement; as the particles return to a stable state, that excess energy is released as a gamma ray. Technetium-99m is used in many radio-imaging facilities in hospitals; it is the main component of Cardiolyte, which is used to take 3D images of the heart.

Three decades later, in the 1930s, physicists reported two additional kinds of nuclear decay: electron capture and positron emission.

In electron capture, the nucleus of an atom captures one of the innermost electrons in the electron cloud. This electron combines with a proton to create a neutron. The nuclear equation looks like this:

$↖26↙13$Al + 0β1- → $↖26↙12$Mg

The captured electron leaves a vacancy in a low-level orbital. A higher-energy electron will fall into its place, and this transition will release an electromagnetic wave. Because this is a large drop in energy, the wave is a high-energy wave: an X-ray.

In positron emission, a proton turns into a neutron and a positron, as follows:

$↖11↙6$C → $↖11↙5$B + 0β1+

Though it sounds like something out of science fiction, a positron is an example of antimatter. It has the same mass as an electron, but the opposite charge: +1 instead of -1. And when a positron collides with an electron, they annihilate each other, releasing energy in the form of gamma rays. In hospitals, positron-emitting elements are used in PET (or positron emission tomography) scanning. In this technique, the gamma rays caused by the annihilation of the positrons are captured by a camera, and an image of where these elements were in the body can be created.

Glossary

Antimatter

A substance made of antiparticles, such as positrons and antiprotons, which have the same mass but opposite charge as their matter counterparts. When a particle and its antiparticle collide, both are annihilated and energy is released.

Beta particles

A high-energy electron produced in the process of nuclear decay (β-).

Electron capture

A type of nuclear decay in which a nucleus captures an electron from a low energy level.

Gamma emission

A type of nuclear decay in which a rearrangement of nuclear particles releases a gamma ray.

Gamma radiation

High-energy electromagnetic radiation.

Positron

An antimatter particle equal in mass to the electron but with a positive charge.

Positron emission

A type of nuclear decay that releases a positron.

Strong force

The nuclear force between protons and neutrons that holds a nucleus together.

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