The trefoil symbol is used to indicate radioactive material.

Radioactive decay is the process where the nucleus of an atom changes into another type of nucleus and releases a particle at the same time. Nuclei that change like this are called radioactive or unstable. Most atoms on earth are not radioactive and are stable. But, atoms that have too few or too many neutrons than a stable atom can be radioactive.

For example, most carbon atoms in the world have six protons and six neutrons in their nucleus. This carbon is called carbon-12, because 12 is the number of protons plus the number neutrons in the carbon-12 nucleus (six protons + six neutrons = 12). Carbon's atomic weight is 12. If two more neutrons are added to carbon-12, it becomes carbon-14. Carbon-14 still acts chemically like carbon, because carbon is defined by having six protons and six electrons. Carbon-14 attracts six electrons no matter how many neutrons it has. In fact, carbon-14 exists in all living things that contain carbon; all plants and animals contain carbon-14. However, carbon-14 is radioactive and so it can be detected. Carbon-14, in the small amounts found about us in nature, is harmless.

Alpha decay, beta decay and gamma decay are the most common types of radioactive decay. They are different from each other because different types of decay produce different particles. The starting radioactive nucleus is called the parent nucleus and the nucleus that it changes into is called the daughter nucleus. The high-energy particles produced by radioactive materials are called radiation.

## Nuclear transformations and energy

Radioactive decay changes an atom from one that has higher energy inside its nucleus into one with lower energy. The change of energy of the nucleus is given to the particles that are created. The energy released by radioactive decay may either be carried away by a gamma ray electromagnetic radiation (a type of light), a beta particle or an alpha particle. In all those cases, the change of energy of the nucleus is carried away. And in all those cases, the total number of positive and negative charges of the atom's protons and electrons sum to zero before and after the change.

## Alpha decay

During alpha decay, the atomic nucleus releases an alpha particle. Alpha decay causes the nucleus to lose two protons and two neutrons. Alpha decay causes the atom to change into another element, because the atom loses two protons (and two electrons). For example, if Americium were to go through alpha decay it would change into Neptunium because Neptunium is defined by having two protons fewer than Americium. Alpha decay usually happens in the most heavy elements, such as uranium, thorium, plutonium, and radium.

Alpha particles cannot even go through a few centimeters of air. Alpha radiation cannot hurt humans when the alpha radiation source is outside the human body, because human skin does not let the alpha particles go through. Alpha radiation can be very harmful if the source is inside the body, such as when people breathe dust or gas containing materials which decay by emitting alpha particles (radiation).

## Beta decay

There are two kinds of beta decay, beta-plus and beta-minus.

In beta-minus decay, the nucleus gives out a negatively charged electron and a neutron changes into a proton:

$n^0 \rightarrow p^+ + e^- + \bar{\nu}_e$.
where
$n^0$ is the neutron
$\ p^+$ is the proton
$e^-$ is the electron
$\bar{\nu}_e$ is the anti-neutrino

Beta-minus decay happens in nuclear reactors.

In beta-plus decay, the nucleus releases a positron, which is like an electron but it is positively charged, and a proton changes into a neutron:

$\ p^+ \rightarrow n^0 + e^+ + {\nu}_e$.
where
$\ p^+$ is the proton
$n^0$ is the neutron
$e^+$ is the positron
${\nu}_e$ is the neutrino

Beta-plus decay happens inside the sun and in some types of particle accelerators.

## Gamma decay

Gamma decay happens when a nucleus produces a high-energy packet of energy called a gamma ray. Gamma rays do not have electrical charge, but they do have angular momentum. Gamma rays are usually emitted from nuclei just after other types of decay. Gamma rays can be used to see through material, to kill bacteria in food, to find some types of disease, and to treat some kinds of cancer. Gamma rays have the highest energy of any electromagnetic wave, and gamma ray bursts from space are the most energetic releases of energy known, even more energetic than supernovas.