2.2: Radiation

Learning Objectives

• To define and give examples of the major types of radioactivity.

Atoms are composed of subatomic particles—protons, neutrons, and electrons. Protons and neutrons are located in the nucleus and provide most of the mass of the atom, while electrons circle the nucleus in shells and account for an atom’s size. The following notation succinctly identifies an isotope of a particular atom:

\[\ce{^{37}_{17}Cl} \label{Eq1}\]

The element in this example, represented by the symbol Cl, is chlorine. Its atomic number, 17, is the lower left subscript on the symbol and is the number of protons in the atom. The mass number, the superscript to the upper left of the symbol, is the sum of the number of protons and neutrons in the nucleus of this particular isotope. In this case, the mass number is 37, which means that the number of neutrons in the atom is 37 − 17 = 20 (that is, the mass number of the atom minus the number of protons in the nucleus equals the number of neutrons). Occasionally, the atomic number is omitted in this notation because the symbol of the element itself conveys its characteristic atomic number. Another way of expressing a particular isotope is to list the mass number after the element name: chlorine-37.

Atomic theory in the 19th century presumed that nuclei had fixed compositions. But in 1896, the French scientist Henri Becquerel found that a uranium compound placed near a photographic plate made an image on the plate, even if the compound was wrapped in black cloth. He reasoned that the uranium compound was emitting some kind of radiation that passed through the cloth to expose the photographic plate. Further investigations showed that the radiation was a combination of particles and electromagnetic rays, with its ultimate source as the atomic nucleus. These emanations were ultimately called, collectively, radioactivity.

There are three main forms of radioactive emissions. The first is called an alpha particle, which is symbolized by the Greek letter α. An alpha particle is composed of two protons and two neutrons, and so it is the same as a helium nucleus. (We often use \(\ce{^{4}_{2}He}\) to represent an alpha particle.) It has a 2+ charge.

The second major type of radioactive emission is called a beta particle, symbolized by the Greek letter β. A beta particle is an electron ejected from the nucleus, not from the shells of electrons about the nucleus! Because a beta particle is an electron, it has a 1− charge and we can represent it as \(\ce{^0_{-1}e}\) or β⁻.

The third major type of radioactive emission is not a particle but rather a very energetic form of electromagnetic radiation called gamma rays, symbolized by the Greek letter γ. Gamma rays themselves do not carry an overall electrical charge, but they may knock electrons out of atoms in a sample of matter and make it electrically charged (for which gamma rays are termed ionizing radiation). 

Alpha, beta, and gamma emissions have different abilities to penetrate matter. The relatively large alpha particle is easily stopped by matter (although it may impart a significant amount of energy to the matter it contacts). Beta particles penetrate slightly into matter, perhaps a few centimeters at most. Gamma rays can penetrate deeply into matter and can impart a large amount of energy into the surrounding matter. Table \(\PageIndex{1}\) summarizes the properties of the three main types of radioactive emissions.

Figure \(\PageIndex{2}\): Different emissions exhibit different pentration powers. (CC BY-NC-SA 3.0; anonymous)

 

Table \(\PageIndex{1}\): The Three Main Forms of Radioactive Emissions

Characteristic	Alpha Particles	Beta Particles	Gamma Rays
symbols	α, \(\mathrm{_{2}^{4}He}\)	β, \(\ce{^{0}_{-1} e}\)	γ
identity	helium nucleus	electron	electromagnetic radiation
charge	2+	1−	none
mass number	4	0	0
penetrating power	minimal (will not penetrate skin)	short (will penetrate skin and some tissues slightly)	deep (will penetrate tissues deeply)

Key Takeaway

The major types of radioactivity include alpha particles, beta particles, and gamma rays.