• Nuclear decay is any spontaneous process where unstable nuclei release extra energy to arrive at a more stable state. Typical decay processes are Alpha, Beta, and Gamma. Some large unstable nuclei (e.g. Plutonium-240) can sometimes decay by spontaneous fission.
• Nuclear fission is a process where certain large nuclei (e.g. Uranium-235 & Plutonium-239) absorb a neutron and then split into two smaller nuclei and a few free neutrons. Some large unstable nuclei (e.g. Plutonium-240) don't need to be hit by a neutron to fission.
• Nuclear fusion is a process where small nuclei under unusual conditions of very high temperature and very high pressure combine to form larger nuclei.

All three processes above are exothermic.

In stars nuclear fusion stops at nickel and iron (further fusion past this would be endothermic). If all we had was the above processes then that would be where the periodic table ended (therefore there could not be nuclear fission as such heavy nuclei could not exist). However stars die, and some die so spectacularly we call them supernovas.

When a supernova occurs, an intense shock wave blows all the outer layers of the star away at very high velocity. At these velocities nuclei collide so hard that normally impossible endothermic nuclear fusion reactions occur. The rest of the periodic table is filled here, including many transuranics not found naturally on earth (e.g. Americium, Californium, Berkelium).

All nuclear decay releases both energy and particles. Even gamma rays from the meta stable decay of Technetium-99m, being only photons, are particles, because a photon is considered a particle - or is it energy? - or is it mass? - hmmm? - see quantum mechanics on that one.

Also, Einsten's famous mass energy equivalence equation e = mc2 states rather plainly that energy is mass and mass is energy. That means that if nuclear decay releases energy, then it also releases mass, and vice versa. There is no way around the equivalence.

Do not misunderstand this. The equation does not mean that energy can be converted into mass or vice versa, it means that energy is mass and vice versa. Neither energy nor mass can be created nor destroyed. So, when an atomic bomb goes off and loses mass generating a high amount of energy, the mass that is lost is simply carried away with the energy.

Sorry if it seems I deviated from the topic, but I did not. This is part of reinforcing the answer and enhancing the explanation.

(Warning: This is a little long. For a summary, scroll down to the bottom.)

Depends on the kind of decay. There are many different types of possible nuclear decays:

• Alpha decay (throwing off helium-4 nuclei)
• Beta-minus decay (converting neutrons to protons, releasing electrons)
• Beta-plus decay (converting protons to neutrons, releasing positrons)
• Gamma decay (emission of high-energy photons after alpha or beta decay)
• Electron capture (an electron falls into the nucleus, converting protons to neutrons without releasing positrons)
• Spontaneous fission (the nucleus suddenly splits in two)
• Proton decay (a lone proton is thrown off)
• Neutron decay (a lone neutron is thrown off)9

Of all of those decays, beta decays and electron capture involve the weak nuclear force.

Deep inside of a proton or neutron, there are 3 fundamental particles named "quarks". In atomic nuclei, there are two kinds of quark: up and down. Up quarks have a charge of +2/3, while down quarks have a charge of -1/3 (yes, quarks have fractional charges.) Because of the strong nuclear force, quarks must gather into groups of 3.

A proton contains two up quarks and one down quark. Two up quarks (charge +4/3) and one down quark (charge -1/3) add up to the proton's net positive charge of +3/3.

A neutron contains two down quarks and one up quark. We'll let you do the math on this one, but they ultimately balance out to 0. Neutrons are heavier than protons, and, given the opportunity, they will spontaneously transform into a proton, throwing off an electron to balance the charges. A mysterious particle called an "antineutrino" is emitted (more on antineutrinoes later). This is caused by a down quark turning into an up quark via the weak nuclear force.

Beta-minus decay is simply when a neutron in a nucleus is converted into a proton, throwing off a high-energy electron. This electron is our beta-minus particle.

Beta-plus decay does not normally occur, because protons are lighter than neutrons, so they should not decay. But, in some particularly light nuclei, e.g. carbon-11, there is enough energy for a proton to transform into a neutron. This produces a high-energy particle called a positron. Positrons are basically electrons with a positive charge, instead of a negative one. A neutrino is also produced, more on these later. This is also governed by the weak nuclear force.

Electron-capture occurs in the same nuclei beta-plus decay can take place in. We'll use potassium-40 as our example. K-40 can either undergo beta-plus decay, or, there is a slighter chance one of its protons will "capture", or consume, one of its electrons. This converts the electron into a neutrino, while satisfying the nucleus, which transformed from potassium-40 into stable argon-40.

Neutrinoes are very evasive particles. They do not interact electromagnetically, hence the name, which means "small neutral one" in Italian. They are almost massless, and for a while, it was believed they were. Neutrinoes were first theorized in 1930 to explain why beta particles often had different energies, but were only found in 1955. Neutrinoes only interact via the weak nuclear force. They mainly serve a purpose as satisying the balance. There are also antineutrinoes, which are almost identical to normal neutrinoes, except for their position on the balance, explained below.

This balance is of something called "electron number". You see, in a nuclear reaction, the total number of electrons involved must be conserved, both before and after the reaction. Electrons and neutrinoes have an electron number of +1. Positrons and antineutrinoes have an electron number of -1. In beta-minus decay, we start with a neutron (electron number 0). It turns into a proton (also electron number 0), producing an electron (electron number +1) to conserve charge. In order to satisfy the balance and conserve electron number, an antineutrino (electron number -1) is released. Neutrinoes have no electrical charge, so both charge and electron number are balanced.

Alpha decay, gamma decay, and spontaneous fission do not rely on the weak nuclear force. Alpha decay is when a helium nucleus manages to escape the nucleus. Proton and neutron decay work in similar manners. Gamma decay is when nucleons leaving produces holes in lower-energy states, which higher-energy nucleons move into, releasing the energy in a high-energy photon. Spontaneous fission also works similarly to alpha decay: in fact, alpha decay is a version of spontaneous fission!

So, to answer your question simply, some decays are associated with the weak force, some aren't. Depends on which decay you're talking about.