Radioactive Decay

Announcements The first submissions for your lab experiments are due on Wednesday 25th. You must have a preliminary idea that I will check before you can continue. We will have our first quiz on subatomic particles on Wednesday 25th also.

Goal of the class To understand radiation and its characteristics Question of the day: What is emitted from the nucleus during radioactive decay? Previous answer: Energy from photons is needed in order to change orbitals Previous question: What is needed for an electron to change orbitals?

Radiation Radiation is energy moving through space Particle radiation is sometimes called ionizing radiation due to its ability to make ions. Ionizing radiation can cause cancers. Radiation is emitted when a nucleus undergoes some change Radiation is a type of heat transfer or light travelling from the sun.

Types of Radiation There are three main types of radioactive decay, each with different properties and penetration depths. α decay – penetrates a few centimetres of air β decay – penetrates a few millimetres of aluminium γ decay – penetrates about a metre of lead Gamma several m of concrete

α-decay Alpha decay is when two protons and two neutrons break free and shoot out of the nucleus Causes the atomic number to drop by 2 and mass by 4 It mostly occur in very large elements Less stable due to the size of their nucleus Speak about how it’s the same as a He nucleus Show that U-235 will decay to Th-231 + He nucleus A new element is formed!

β decay In Beta decay a neutron changes into a proton plus an electron. The proton stays in the nucleus and the electron leaves the atom with high energy, and we call it a beta particle. This means the atomic mass number remains unchanged and the atomic number increases by 1. An example of electron emission (β− decay) is the decay of carbon-14 into nitrogen-14: 14 6C → 14 7N + e− + ν e In this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation. This new element has an unchanged mass number A but an atomic number Z that is increased by one. As in all nuclear decays, the decaying element (in this case 14 6C) is known as the parent nuclide while the resulting element (in this case 14 7N) is known as the daughter nuclide. The emitted electron or positron is known as a beta particle. C 6 14

γ decay In Gamma decay the particles of the atom remain unchanged. It simply rearranges itself into a lower energy state releasing a high energy photon. Gamma decay often occurs with other radioactive decays. Nickel undergoes gamma decay. Ni 28 60 *

Half-Life Atoms decay spontaneously Occurs within a certain probability The half-life is the average time for half of the atoms in a specific sample will decay Example: If you have 20 grams of a radioactive substance with a half life 50 years. On average, how much will you have in 100 years? You would have 5 grams.

Practice If you were to have 256 atoms of carbon-14 (half-life of 5730 years), about how many would have after about 23000 years? You would have 5 grams.

Vocabulary Radiation – the process of energy is emitted as particles or waves . Ionizing – capable producing ions

Homework Read: Chemical Building Blocks page 140-144 Chemical Building Blocks workbook page 57-59 Experiment ideas by Wednesday. http://phet.colorado.edu/en/simulation/build-an-atom