It depends. If the decay contains a particle with mass, then the nucleus' mass number must decrease. If the decay involves the emission of a massless particle (like a gamma photon), then the mass number is unchanged. If the reaction (not technically a decay) involves the nucleus absorbing a particle with mass (like U-235 absorbing a neutron in a fission chain reaction) then it is a transmutation and not a natural decay. The mass number must increase.
Radioactive atoms can give off several different particles. There are three different "types" of radiation, beta, gamma, and alpha. Beta decays which give off electrons/positrons as well as gamma rays (usually). Gamma emmitters are generally meta stable particles that omit a gamma ray in order to stabilize the nuclears, and their are alpha decays which eject a alpha particle (a helium nucleus). On rarer occasions radioactive particles can also be classified as neutron emitters.
The name for the emissions of rays and particles by a radioactive material are called radioactive decay. There are many different types of radioactive decay that emit different rays and particles.
radioactive decay
If an atom were to change the number of protons it had than it would change the atomic number and therefore become a different element. Radioactive decay is one example of this, for example, alpha decay is when a radioactive nucleus emits an alpha particle (2 protons + 2 neutrons) and in doing so, becomes a nucleus of a different element. Polonium-212 decays to Lead-208 in this way.
A parent atom is a term used to describe the original state of an atom or element before it undergoes a chemical change. For instance, it may be used when discussing radioactive decay. The original element would be the parent atom, and the element that results from the radioactive decay would be the daughter atom. It may also be used when discussing ions. The parent atom would be the element before it undergoes ionization, and the daughter atom would be the resulting cation or anion.
nucleus
When a hydrogen-3 nucleus undergoes radioactive decay, it emits a beta particle (specifically an electron) and an anti-neutrino to transform into helium-3.
Alpha decay occurs when thorium-231 undergoes radioactive decay to form protactinium-231. In alpha decay, a nucleus emits an alpha particle (two protons and two neutrons) to transform into a nucleus with a lower atomic number.
When an unstable magnesium nucleus undergoes gamma decay, it remains as a magnesium nucleus. Gamma decay does not change the atomic number or mass number of the nucleus, only releasing a gamma photon to reduce excess energy.
When an oxygen-19 nucleus undergoes beta decay, a nitrogen-19 nucleus is formed. In beta decay, a neutron is converted into a proton, causing the atomic number to increase by one while keeping the mass number the same.
Atomic nuclei that are unstable and decaying are said to be radioactive. Radioactive decay involves alpha, beta and gamma particle emissions.
Nuclear decay.
It is the nucleus of the atom that undergoes change during radioactive decay.
Particles or electromagnetic radiation are emitted.
Radium-226 undergoes radioactive decay to become more stable. During this process, the nucleus emits an alpha particle, decreasing its atomic number and mass number. This transformation helps the nucleus achieve a more balanced and stable configuration.
Phosphorus-32 is the radioactive isotope that undergoes beta decay to produce sulfur-32. During beta decay, a neutron in the nucleus of phosphorus-32 is converted into a proton and an electron, resulting in the formation of sulfur-32.
A nucleus that starts to decay is called a radioactive nucleus or atom. It decays with a known and unique half life by several processes including but not limited to beta decay, alpha decay, electron capture decay, and positron emission.