What happens when an unstable nucleus give off particles and change its number of protons?

Answer 1

Fundamentally, the number of protons in a nucleus determines its chemical identity. Carbon, for instance, has six protons, and that makes it carbon, regardless of how many neutrons or electrons there might be. Uranium, on the other hand, has 92 protons. So, the simple answer is that, when an unstable nucleus, i.e. a radionuclide, changes its number of protons, it becomes a different element.

Now, how that happens is much more interesting. There are several decay processes that involve a change in the number of protons.

Beta-

In beta- decay, the weak interaction causes a down quark in a neutron to change to an up quark. The neutron becomes a proton, and a W- boson is emitted, which subsequently decays to an electron and an electron antineutrino. This increases the atomic number by one, and leaves the Atomic Mass number unchanged. Carbon-14, for instance, becomes nitrogen-14 by beta- decay.

Beta+

In beta+ decay, an up quark in a proton is changed into a down quark. The proton becomes a neutron, and a positron and electron neutrino is emitted. This reaction requires energy, and will only happen if there is a certain amount of excess energy in the nucleus or if there is an inner shell (K or L) electron available to add energy in the process of electron capture, also known as K capture. This decreases the atomic number by one, and leaves the atomic mass number unchanged. Americium-238, for instance, becomes Plutonium-238 by beta+ decay, one of its decay schemes.

Electron Capture

Electron capture is where an inner (K or L) shell electron is assimilated into the nucleus, contributing its energy, and changing a proton into a neutron. The atomic number decreases by one while the atomic mass number stays the same. In some cases, electron capture results in beta+ decay. In some cases, the positron emission is suppressed and you only get the electron neutrino, along of course with the proton conversion, depending on energy.

Alpha and other fission

In fission decay, the nucleus is split into two parts. You get two new elements.

Alpha decay is a specific example of fission. In this case, a helium nucleus (two protons and two neutrons) is split off; and the atomic number is decreased by two, while the atomic mass number is decreased by four. Uranium-238, for instance, becomes Thorium-234 by alpha decay, one of its decay schemes.

As far as fission decay in general, the actual split point is determined by complex relationships that I will leave out of this somewhat simple explanation.

Photon emission

In many cases of an underlying decay process, such as described above, the nucleus and/or electron cloud is left in an excited state. When this happens, it "wants" to shed its excess energy and return to ground state. This results in the emission of a photon with energy representing that energy change. If this process originates in the nucleus, the photon is called a gamma ray; if it originates in the electron cloud, the photon is called an x-ray.

Delayed photon emission

Usually, if there is going to be a gamma ray, it occurs very quickly after the initiating event, typically within 1 x 10-12 seconds. Sometimes, in what we call a metastable state, the gamma emission is delayed, sometimes for a very long time. Technetium-99m, for instance, can participate in beta- decay, but the secondary gamma ray is delayed with a half-life of six hours, making it very useful in the medical imaging field.

Answer 2

It decays into a new element.