Lesson 1 Reading

Atoms and Radiation

Atoms: The Building Blocks of All Matter

Before we can understand radioactivity, we must first understand some basic facts about ourselves and the world around us. All matter, even our own bodies, is composed of atoms. There are more atoms than you can possibly imagine. The ink dot at the end of this sentence, for instance, has more than a thousand billion atoms.

Atoms are the fundamental building blocks of everything in the universe. They are the smallest particles of an element that have all the chemical properties of that particular element. Atoms are so small that it takes billions of them to make a speck of dust.

We know that at least 92 different kinds of atoms occur in nature. Many of the 92 naturally occurring atoms have variations called isotopes.

The Makeup of Atoms

As small as atoms are, they are made of even smaller particles. There are three basic particles in atoms — protons, neutrons, and electrons.

Protons carry a positive electrical charge. Neutrons are electrically neutral; they have no electrical charge. Protons and neutrons together make up a bundle at the center of an atom. This bundle is called the nucleus (in the plural, nuclei).

Electrons have a negative electrical charge and move around the nucleus. Normally, an atom has the same number of protons and electrons, but the number of neutrons can vary.

The number of protons an atom has determines its chemical element. For instance, an oxygen atom has 8 protons in its nucleus. A carbon atom has 6 protons in its nucleus; iron has 26; gold has 79; lead, 82; uranium, 92, and so on. The 92 elements found in nature make up everything on our planet, including ourselves.

A Microscopic Universe

As incredibly small as an atom is, the inside of one is mostly empty space. It’s a microscopic “universe” unto itself, occupied by a relatively small number of negatively charged electrons revolving around the central mass, the nucleus.

The nucleus, which is millions of times smaller than the entire atom, accounts for nearly all of the atom’s weight. This is because protons and neutrons are thousands of times more massive than electrons. So the nucleus, with its protons and neutrons, provides nearly all the weight of the atom.

It’s mind-boggling to realize that atoms are millions of times smaller than the tiniest speck we can see with the unaided eye — and yet the inside of an atom itself is mostly empty space, with the nucleus being millions of times smaller than the complete dimension of the atom as a whole.

Atoms Combined Make Molecules

When atoms of one element join with each other or with atoms of another element, a molecule is formed. If the molecule combines atoms of two or more elements, the new substance is called a compound.

An atom of sodium (Na) combines with an atom of chlorine (Cl) to make a molecule of sodium chloride (NaCl), or table salt. Two atoms of hydrogen combine with one atom of oxygen (H2O) to make a molecule of water.

What kind of molecule is composed depends on the kind and number of different atoms that combine. This combining of atoms is called a chemical reaction. In chemical reactions, atoms do not change; instead, they combine with or separate from other atoms. Their own atomic structures remain the same, however.

For example, gold is an element, and a bar of pure gold contains only atoms of one element — gold. Hydrogen and oxygen are also pure elements. However, when two hydrogen atoms (H₂) combine with one oxygen atom (O), they become one molecule of a different substance — H₂O, or water.

Similarly, one atom of sodium (symbol = Na) combined with one atom of chlorine (Cl) makes a molecule of sodium chloride (NaCl), or simple table salt, the kind we put on our fries and popcorn.

As another illustration, the molecules of wood and coal contain many carbon atoms. If enough oxygen is present with enough heat, wood and coal will burn. The chemical reaction involved produces molecules of one carbon atom joined with two oxygen atoms, as a gas called carbon dioxide (CO₂). (The prefix di- comes from Latin and Greek and means “twice,” so “dioxide” means an oxygen atom twice.)

Radiation and Radioactivity

Radiation

Put quite simply, radiation is energy traveling through space. It can take the form of waves or of extremely small, fast-moving particles. Radiation is everywhere — in, around, and above the world we live in. It is even within our own bodies.

All living beings require certain types of radiation just to live. Light and heat, for example, are two basic forms of radiation necessary, either directly or indirectly, for all life on planet Earth. Light is a form of electromagnetic radiation, while heat is thermal radiation.

We don’t often think about radiation, but we are constantly experiencing it in one form or another — like the flash of a bicycle reflector, the glow of a full moon, or the sounds of a favorite song carried by radio waves through the air.

Types of Radiation

Depending on how much energy it has, radiation can be described as either non-ionizing (low energy) or ionizing (high energy). Both kinds are part of everyday life.

Non-Ionizing Radiation

All our lives, perhaps without knowing it, we have reaped many benefits from non-ionizing radiation. For example, radio and television waves provide news and entertainment in the home; microwave ovens help us heat food faster; and the light from electric light bulbs takes away the night. These are some forms of non-ionizing radiation. Nonionizing radiation has enough energy to be useful, but not so much as to change the basic structure of atoms or molecules it passes into or through.

Ionizing Radiation

Some forms of radiation have enough energy to knock electrons out of atoms and molecules, thus creating electrically charged particles called ions. Radiation this energetic is called ionizing radiation.

This graphic illustrates the general concept of electrons (c) orbiting around the much more massive nucleus, which is made of protons (a) and neutrons (b). The illustration is not to scale, of course, because the nucleus of an atom, while thousands of times heavier than its electrons, is millions of times smaller than the entire space encompassed by the overall atom. An atom is mostly empty space.

Ionizing radiation has many benefits, but some potential health risks as well. This is the form of radiation most people mean when they use the words “radiation” or “radioactive.” Therefore, unless otherwise stated, when we use the term “radiation” in these lessons, it will be the ionizing type we are concerned with.

Ionizing radiation exists in the form of pure waves of energy of extremely short wavelength (X-rays and gamma rays) or as particles of matter that carry energy. When a substance consists of atoms that spontaneously emit radiation, it is called radioactive, and the atoms are called radionuclides. Some of the 92 naturally occurring elements include radioactive nuclides; most do not.

Benefits of Radiation

Radiation can be used for many beneficial purposes. Medical and dental X-rays, for example, are controlled forms of ionizing radiation that help doctors create pictures to see inside our bodies and diagnose illness or injury. Radiation can also be used to help cure some diseases.

A radioactive substance (americium) provides the sensitivity that makes most smoke detectors do their job. The mantles (wicks) of many, but not all, types of portable gas-burning camp lanterns also contain a radioactive substance (thorium).

Color television sets and computer monitors emit tiny amounts of ionizing radiation (very low-level X-rays). Even the food we eat exposes us to radiation — because it contains naturally occurring radionuclides. For instance, all living things on our planet absorb radioactive carbon-14 through the food chain.

Carbon-14, a radioisotope of carbon, which normally has an atomic weight of 12, enters the planetary food chain through complex chemical interactions between cosmic radiation and gases in the Earth’s atmosphere. Every creature or plant that lives — or died within the last 50,000 years — on planet Earth has residual amounts of carbon-14 in his, her, or its body. (See the activity entitled “Using Radioisotopes to Date Materials,” in Lesson 3 of this unit.)

We consume radioactive carbon-14 in everything we eat that comes from any plant or animal. In addition, potassium, an essential nutrient for a healthy human diet, is not available in food without its radioisotope (potassium-40). Potassium, too, is found in all plants and animals, but we need to eat enough of the right foods to get sufficient potassium in our diets. In other words, nature requires us to eat radioactive potassium, and to consume radioactive carbon just by eating. (See the activity entitled “Internal Exposure from Radioactivity in Food,” in Lesson 3 of this unit.)

Potential Health Risks of Radiation

Just by existing on Earth, we are constantly absorbing ionizing radiation, which, in high enough doses, can displace electrons in the atoms that make up the molecules that compose the living cells of our bodies. This can damage or kill our cells. But our bodies are constantly repairing damage to cells, too. Even if radiation knocks electrons out of the atoms and molecules to the point of actually killing cells, our bodies usually will just make new ones to take their place.

Too much ionizing radiation, however, can cause serious negative health effects. This possibility increases if we are exposed to an extremely high amount of radiation in a short period of time, or if we are exposed to excessive amounts when we are very young.

Cosmic radiation from outer space is ionizing, too. Everyone on Earth receives some ionizing radiation from space. The higher we live above sea level, or the more often we fly in a jet plane, the more cosmic radiation we are exposed to. This is because the higher up we are, the thinner the Earth’s atmosphere is. Being thinner at higher altitudes, the atmosphere higher up offers us less protection from cosmic radiation.

The many ways we are exposed to radiation make it one of the most thoroughly studied subjects in modern science. Radiation is useful and necessary, but if it is ionizing, it can also have health risks.

Most of our attention throughout this unit will be focused on ionizing radiation — what it is, where it comes from, how it can help or hurt us. In the final lesson of the unit, we will focus on why spent nuclear fuel and other high-level radioactive waste require careful planning, handling, and disposal.