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by Ron Kurtus (updated 8 February 2022)

Radioactivity is a property of some forms of matter where radiation and particles are emitted from the nucleus.

In general, the nucleus of the various atoms is stable. In some cases—especially with the larger, heavier atoms—the nucleus can be unstable and will decay into smaller, stable particles. Often in this process, high energy gamma radiation and other particles are emitted. This process is called radioactivity.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

What makes a nucleus unstable?

The number of neutrons in an atom's nucleus seems to determine whether or not it will be unstable. Usually, there are more neutrons in a nucleus than protons. Elements may have a varied number of neutrons in their nuclei. These are called isotopes, and many of them are unstable. Measurement of their instability is usually designated as the half-life of the isotope.

Model of the nucleus

The basic model of the nucleus of an atom is that it is made up of protons and neutrons bound together in a sphere.

Since protons have a positive charge, they would tend to repel each other, but there is a powerful "nuclear glue" or force that holds them together within the nucleus. This force seems to require a certain number of neutrons within the nucleus for it to work. Modern theories state that a gluon particle acts to hold the nucleus together.

Atomic number and weight

The atomic number is the number of protons in the nucleus, and the atomic weight is a value slightly less than the sum of the number protons and neutrons in the nucleus. Thus Oxygen is number 8 and has an atomic weight of approximately 16, because it has 8 protons and 8 neutrons in its nucleus. Uranium is number 92 and has an atomic weight of approximately 238, because it has 92 protons and 145 neutrons in its nucleus.

The reason the atomic weight is slightly less than the sum of the protons and neutrons is because some weight is lost to the energy required to make the "nuclear glue" force that holds the atom together. Since electrons weigh approximately 1/1200 of a neutron or proton, their contribution to the atomic weight is negligible.

There are usually an equal number or slightly more neutrons than protons in the nucleus. With the heavier atoms, the ratio goes up to about 1.5 neutrons per proton.


With many atoms, there are often variations on the possible number of neutrons in the nucleus. These are called isotopes of the atom. For example, Uranium exists with weights of 235 and 238. In U-238, there are 3 more neutrons than in U-235.

Unstable isotope

The nucleus of an isotope that has too many or too few neutrons as compared to the number of protons, is unstable. An atom with such a nucleus often doesn't occur naturally or remain in existence for very long. Such an unstable isotope will give off particles and radiation, in order to become a stable nucleus. Material that gives off particles and radiation are considered radioactive.

Radiation harmful

The radiation given off usually consists of gamma rays, which are more powerful and dangerous than high energy x-rays in the electromagnetic spectrum. That is why radioactive material can be harmful to living beings and plants.

What happens to an unstable nucleus?

If an unstable nucleus gives off any excess neutrons, it simply becomes a stable version of the element. If the nucleus gives off any protons, it becomes a different element, since it would then have a different atomic number. It is even possible for an unstable nucleus to give off an electron. In such as case, a neutron is changed to a proton and an electron, so the material also changes its atomic number.

Besides giving off neutrons, protons, or electrons, unstable nuclei can give off a larger particle such as a helium nucleus. Some large unstable nuclei, such as Uranium, will actually split or explode into several smaller parts. In such a case, the atom's nucleus will split and break into stable nucleus configurations, along with various particles and radiation.

Half-life measurement

Although an unstable nucleus will decay into a stable nucleus--or even several stable nuclei—it doesn't necessarily do that immediately. Some may take a fraction of a second, while others may take years before they decay. The rate at which a radioactive material decays is called its half-life. That is the amount of time it takes half of the material to decay.

Dangers of radioactivity

A radioactive substance can give off high-energy beta particles (electrons), protons, neutrons, alpha particles (Helium nuclei), and gamma rays. All of these can cause damage to living cells, as well as to cause other particles to become radioactive.

Gamma rays--which are electromagnetic waves of higher energy than X-rays--are especially damaging to living cells. Besides the immediate damage, exposure to radiation may result in the person getting cancer years later.

High amounts of radiation from a nuclear explosion can kill living beings instantly or painfully after several days of radiation sickness.

Uses of radioactivity

Some applications of the characteristics of radioactivity are to determine the age of fossils, to identify certain items, and to fight cancer.

Carbon dating

The age of fossils and ancient living things can be determined by measuring how much radioactive Carbon is in the material. Life forms are constantly renewing their supply of Carbon. Once an animal or plant dies, the amount of Carbon remains constant.

Since Carbon has a small amount of a radioactive isotope Carbon-14 mixed with it, by measuring the amount of C-14 remaining in a formerly living object, scientists can estimate the age of a fossil up to 50,000 years. This is because the half-life of C-14 is 5730 years. Thus, if there is 1/2 of the C-14 left in a fossil, its age is about 5730 years old. If there is only 1/4 left, the age is about 11,640 years old.

This method is also used to determine the age of ancient rocks, using Uranium-238, which has a half-life of 4500 million years.

Identifying items

By using a small, safe amount of radioactive material, scientists or engineers can detect or identify items that absorb the substance.

For example, a radioactive solution can be spread on a metal beam on a bridge. Some would collect in any tiny cracks in the beam. By using a Geiger counter to detect the radiation, they can find where the cracks are, so they can repair them before greater damage is done.

Fighting cancer

One method to fight cancer is to implant a radioactive pellet in a cancerous tumor. It seems that cancer cells are more sensitive to radioactivity than normal cells. The cancer cells are killed by the radiation from the pellet, while only a few of the normal cells are killed. 


Too many neutrons in the nucleus can make it unstable, such that it will decay into stable particles and give off radiation. Radioactivity can be dangerous to living beings, such that it can kill them or cause cancer. Uses of radioactivity include measuring the age of very old material and fighting cancer.

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