# Lesson Explainer: Radioactivity Science • Third Year of Preparatory School

In this explainer, we will learn how to describe the radiation produced by decaying atomic nuclei.

Everything around us is made of atoms. Single atoms are far too small for us to be able to see. But if we have enough atoms, they can join together in different ways to form objects large enough for us to see.

Atoms consist of three types of particles: protons, electrons, and neutrons. We call these particles “subatomic particles,” because they are smaller than an atom.

In an atom, the protons and neutrons are packed tightly together, into a cluster at the center of the atom. This is called the nucleus. The nucleus contains most of the atom’s mass, even though the volume of the nucleus is very small. Because of this, we say that the nucleus is very dense.

In an atom, the electrons are not in the nucleus. Instead, they are outside the nucleus.

The structure of an atom is shown in the following diagram.

In the above diagram, the protons are labeled with a “” because they are positively charged. The electrons are labeled with a “” because they are negatively charged. Neutrons are neutral.

Both attractive forces and repulsive forces act on the particles in a nucleus. Attractive forces pull the particles in the nucleus toward each other. Repulsive forces push the particles in the nucleus apart.

In the nucleus, the particles are arranged in a way that means these forces cancel each other out. The effect of the repulsive forces is exactly balanced by the effects of the attractive forces. We call this equilibrium. When the nucleus is in equilibrium, the distances between the protons and neutrons stay the same.

### Example 1: Forces in the Nucleus

Which of the following statements correctly describes the forces that act on the particles in the nucleus of an atom?

1. There are no forces on the particles.
2. There are only repulsive forces on the particles.
3. There are attractive and repulsive forces on the particles.
4. There are only attractive forces on the particles.

The answer is C. There are attractive and repulsive forces on the particles.

If there were only repulsive forces on the particles, the particles would not be able to form a cluster. Instead, the particles would be pushed apart from each other, and the distances between each particle would be very large.

If there were only attractive forces on the particles, the particles would be pulled together too strongly that the nucleus would collapse.

In a nucleus, there are both attractive forces and repulsive forces acting on the particles. When the particles arrange themselves in a certain way, these forces cancel each other out—the effect of the attraction is exactly as strong as the effect of the repulsion.

When the forces cancel, the particles are in equilibrium. This means that there is no overall force on the particles. But this equilibrium is achieved by the actions of both attractive forces and repulsive forces.

Now that we have understood the structure of an atom, we can compare the nuclei of two different atoms. Consider the following diagram, which shows the nuclei of two atoms.

Recall that the nuclei of atoms do not contain any electrons. In this diagram, we can see that each nucleus contains a different number of protons. The nucleus on the left-hand side contains 3 protons, and the nucleus on the right contains 4 protons.

Because the nuclei of these atoms contain different numbers of protons, these must be atoms of different elements. An element is a material that is made up of only one type of atom. This means that an element consists only of atoms, which are atoms of that element.

To know which element our atom is of, we need to know how many protons are in the atom’s nucleus. The number of protons in the atom’s nucleus is called the atomic number. Each element has a unique atomic number. So, if we know how many protons are in the nucleus of an atom, we know which element the atom is of.

For example, the metals silver and gold are both elements. Silver is only made up of silver atoms, and gold only contains gold atoms.

The atomic number of silver is 47. This means that all silver atoms have 47 protons in their nuclei. Every atom with 47 protons in its nucleus must be a silver atom.

The atomic number of gold is 79. This means that all gold atoms have 79 protons in their nuclei. Any atom with a different number of protons must be an atom of a different element.

Now, we will discuss the number of neutrons in a nucleus. Consider the following diagram, which shows the nuclei of two atoms.

Recall that the nuclei of atoms do not contain any electrons. In this diagram, both nuclei contain the same number of protons. This means that these nuclei are of atoms of the same element. However, the nucleus on the left contains three neutrons. The nucleus on the right contains 4 neutrons.

The number of neutrons in the nucleus does not tell us anything about which element the atom is of. Atoms of the same element can contain different numbers of neutrons. For example, some silver atoms contain 60 neutrons, but some other silver atoms contain 62 neutrons.

When atoms of the same element contain different numbers of neutrons, we say we have different isotopes of that element.

For example, we have already mentioned two isotopes of silver. One isotope contains 60 neutrons, and the other contains 62 neutrons. Both isotopes must still contain 47 protons, or else they would not be silver atoms.

We have now seen how the structure of the nuclei of different elements and isotopes compare. Let us now consider how the structure of an atomic nucleus can change.

Some atoms are stable. This means that their nuclei will only change if something happens outside of the nucleus to cause the change.

Some atoms are unstable. This means their nuclei can change, even when there is nothing outside the nucleus making the change occur. We cannot make this change happen, and we cannot predict when this change will happen. So, we call these changes spontaneous.

When a nucleus spontaneously changes in this way, it is called nuclear decay.

### Example 2: Spontaneity

Which of the following statements about spontaneous changes to the compositions of atomic nuclei of elements is correct?

1. The composition of an atomic nucleus can spontaneously change only for atoms of certain elements.
2. The composition of an atomic nucleus can spontaneously change for atoms of any element.
3. The composition of an atomic nucleus cannot spontaneously change for atoms of any element.

The answer is A. The composition of an atomic nucleus can spontaneously change only for atoms of certain elements.

An atomic nucleus can only spontaneously change if the atom is unstable. If the atom is stable, its nucleus cannot spontaneously change.

Some elements have atoms that are always stable, and some elements have atoms that are unstable. So, the composition of an atomic nucleus can spontaneously change only for atoms of certain elements.

### Example 3: Spontaneity

Which of the following statements about the prediction of radioactive decay of an unstable atomic nucleus is correct?

1. The time at which an unstable atomic nucleus will decay cannot be predicted.
2. The time at which an unstable atomic nucleus will decay can be predicted.

The answer is A. The time at which an unstable atomic nucleus will decay cannot be predicted.

Nuclear decay is spontaneous. Here, spontaneous means two things:

1. We cannot do anything to trigger a nuclear decay.
2. We cannot predict when a nuclear decay will occur.

So, we cannot predict the time at which an unstable atomic nucleus will decay.

Some elements are stable, so atoms of that element do not undergo nuclear decay. Examples of stable elements include oxygen, which is found in the air we breathe. Silver and gold are both stable elements.

Now, let us consider some unstable elements. Radon is an example of an unstable element. Radon is a colorless, odorless gas that is commonly produced in rocks and soil. The atomic number of radon is 86. This means that radon atoms have 86 protons in their nuclei. All radon atoms can undergo spontaneous nuclear decay.

Other examples of unstable elements include uranium and plutonium.

If an atom contains a higher proportion of neutrons compared to protons, it is less likely to be stable, and more likely to be unstable.

### Example 4: Stability of Nuclei

Which of the following atomic nuclei is more likely to be unstable?

1. A nucleus with a high proportion of neutrons
2. A nucleus with a low proportion of neutrons
3. A nucleus with equal numbers of neutrons and protons

The answer is A. A nucleus with a high proportion of neutrons.

A nucleus is most likely to be unstable when it has a high proportion of neutrons.

A nucleus with a low proportion of neutrons is less likely to be unstable than a nucleus with a high proportion of neutrons. A nucleus with equal numbers of neutrons and protons is the least likely to be unstable.

We have now seen that elements can be either stable or unstable, and that atoms of unstable elements undergo nuclear decay. Let us consider the changes that occur in the nucleus when an atom undergoes nuclear decay.

When a decay happens, the nucleus emits particles or electromagnetic waves. We call this nuclear radiation. We call unstable atoms radioactive, because they produce nuclear radiation when they decay.

There are three types of nuclear decay, or three ways through which a nucleus can change:

• alpha decay ,
• beta decay ,
• gamma decay .

Each type of decay produces a different kind of nuclear radiation. Gamma radiation consists of electromagnetic waves. Alpha radiation and beta radiation both consist of particles. Alpha decay emits different kinds of particles from beta decay.

In each kind of decay, the nucleus changes in a different way.

### Example 5: The Composition of Nuclear Radiation

Which of the following statements correctly describes the possible compositions of the radiation emitted by unstable atomic nuclei decaying?

1. The radiation emitted by unstable atomic nuclei decaying consists only of electromagnetic waves.
2. The radiation emitted by unstable atomic nuclei decaying includes particles and electromagnetic waves.
3. The radiation emitted by unstable atomic nuclei decaying consists only of particles.

The answer is B. The nuclear radiation emitted by unstable atomic nuclei decaying includes particles and electromagnetic waves.

The exact composition of the nuclear radiation emitted during a nuclear decay depends on what type of decay occurs.

Gamma decay emits electromagnetic waves only. Alpha decay and beta decay both emit particles, but the type of particles emitted in each decay is different.

So, overall, nuclear radiation can consist of both particles and electromagnetic waves.

Let us now summarize what has been learned in this explainer.

### Key Points

• Atoms contain a nucleus, consisting of protons and neutrons. Atoms also contain electrons, which are found around the nucleus.
• The atomic number of an atom tells us which element that atom is an atom of.
• Atoms of the same element always contain the same number of protons.
• Atoms with the same number of protons but different numbers of neutrons are isotopes of the same element.
• Atoms are unstable if their nuclei can spontaneously change. These changes are called nuclear decay.
• When an atom decays, it emits nuclear radiation. So, we call unstable atoms “radioactive.”
• Some elements are naturally radioactive, such as radon, uranium, and plutonium. Atoms of these elements spontaneously decay.

## Join Nagwa Classes

Attend live sessions on Nagwa Classes to boost your learning with guidance and advice from an expert teacher!

• Interactive Sessions
• Chat & Messaging
• Realistic Exam Questions