Radioactive Decay When elements have unstable nuclei, they decay, forming more stable nuclei and giving off energy. In this lesson, you will learn what changes happen with each kind of decay.

Radioactive Decay What makes unstable nuclei unstable? They are unstable because the electrostatic repulsion between the protons is near to or greater than the strong nuclear force holding the nucleus together. This happens when there is an unfavorable balance of protons and neutrons OR when the nucleus is just too large. Unstable nuclei decay in order to become stable.

Radioactive Decay The largest stable nucleus is Bismuth-209. Every atom whose mass is more than 209 a.m.u. is unstable because it is too massive to be stable. Some smaller atoms are unstable because the ratio of protons to neutrons is not ideal – there may be too many or too few neutrons. Technetium and promethium are the only smaller atoms with no stable isotopes. Does samarium have a stable isotope? Does berkelium have a stable isotope?

Radioactive Decay This graph shows the ratio of neutrons (going across the x axis) to protons (going up the y axis). The red line shows where protons and neutrons are equal in number. The green region has stable isotopes. Isotopes whose protons and neutron ratio put them in the yellow region are unstable.

Radioactive Decay Now for the mechanics of radioactive decay (YAY!) Radioactive decay comes in three varieties: alpha, beta, and gamma.

Alpha Decay Alpha decay, symbolized α (Greek letter alpha) is the least penetrating form of radiation. It consists of a relatively large particle consisting of two protons and two neutrons – the same as a helium nucleus. Because the particle is large, it can be stopped by simple means – a sheet of paper stops it.

Alpha Decay In alpha decay, the nucleus gives off this particle, called an alpha particle. In the diagram below, a uranium atom emits an alpha particle, consisting of two protons and two neutrons. Since it now has two fewer protons, the atom is no longer uranium. What has two fewer protons than uranium? Thorium does! Notice also that the mass number has also gone down by 4; two protons and two neutrons have left. t1.gstatic.com/images?q=tbn:2a496FevcUXDrM:

Alpha Decay This is the equation representing the alpha decay of astatine-211. Notice that in this example, the alpha particle is represented differently than in the previous example. What new daughter atom was created by the emission of the alpha particle from the At-211? Why did the atomic number go down by 2? Why did the mass go down by 4? 8583

Alpha Decay In alpha decay, the atomic number always goes down by two, because two protons have been emitted. In alpha decay, the mass number always goes down by four, because four particles in total have left the nucleus.

Alpha Decay What is the equation for the alpha decay of curium-247? Start by writing the symbol of curium. Put the mass number to the left, up high. Put the atomic number to the left, down low. Draw the arrow to the right of the symbol. Write the symbol for an alpha symbol. Add a plus sign after that. Write the atomic number of the new daughter atom. Write the mass of that new number right above it. Use your PT of E to find what the new element is and write its symbol next to the mass and atomic numbers.

Alpha Decay

Beta Decay Beta decay, symbolized  (Greek letter beta) involves a much smaller particle. The beta particle is an electron. As such, it has much more energy, speed, and penetrating power than an alpha particle. It takes a sheet of aluminum to stop a beta particle. What’s the difference between the before and after atoms? One neutron has turned into a proton! It does this by emitting an electron, a beta particle.

Beta Decay In beta decay, a neutron turns into a proton and an electron. The electron is emitted as beta radiation, while the proton stays in the nucleus. That means that the nucleus has one more proton than before, changing the identity of the atom. It has one fewer neutrons, but that does not matter much, except to make it more stable. In beta radiation, the atomic number goes up by one, but the mass does not change.

Beta Radiation To write the equation for beta radiation, we start with the complete symbol for our starting atom, which in this case is cobalt-61. We then add the arrow and the symbol for the beta particle. What will the atomic number for the new daughter atom be?

Beta Radiation The atomic number has gone from 27 to 28, because a new proton was created! What will the new mass be? Hmm… A neutron became a proton, but that does not change the mass.

Beta Radiation The new mass is the same as the old mass. Often, a radioactive atom will do both alpha and beta radiation, one right after the other. That makes the atomic number go down two, then up one.

Alpha and Beta Radiation Here’s an example of both happening sequentially. In truth, gamma radiation would also be emitted.

Gamma Radiation Gamma radiation does not have a particle at all; it’s a high-energy wave. Gamma radiation is very penetrating. To stop it requires a thick concrete barrier.

Gamma Radiation Gamma radiation, symbolized γ (Greek letter gamma) seldom happens on its own. It is a by-product of alpha and beta radiation. Because gamma radiation is a wave and not a particle, it does not change the composition of the nucleus.