Chapter 19 Radioactivity Radioactivity – process by which certain elements emit particular forms of radiation.

Chapter 19 Radioactivity Radioactive Particles: alpha, beta, and gamma Alpha particle two protons and two neutrons +2 charge equal to the nucleus of a helium atom

Chapter 19 Radioactivity Beta particle an electron ejected by an atomic nucleus -1 charge No mass

Chapter 19 Radioactivity Gamma radiation High energy electromagnetic radiation. Far left of the EM spectrum with high energy and short wavelength. Gamma radiation has no electric charge and has no mass.

Chapter 19 Radioactivity

Chapter 19 Radioactivity

Chapter 19 Radioactivity Most radiation we encounter is natural background radiation that originates in the Earth and in space. Radiation can alter genetic information in the cell causing mutations. If the damaged DNA is in the reproductive cells, this can be transferred to the offspring.

Chapter 19 Radioactivity The leading source of natural occurring radiation is Radon-222 gas. It comes from Uranium deposits. It usually occurs in basements and leaks in through cracks in the floor. It also naturally occurs in tobacco and increases the risk of lung cancer The human body is itself a source of radiation About 20 mg (or 5000 Potassium-40 atoms) of Potassium are emitting gamma rays in the time it takes your heart to beat once.

Chapter 19 Radioactivity

Chapter 19 Radioactivity Strong nuclear force - acts between all nucleons over very short distances. Between two protons in the nucleus, the strong nuclear force is greater than the repulsive force of their like charges. Repulsive electrical forces are stronger over larger distances like the opposite ends of a large nucleus. Because the strong nuclear force decreases with distance, a large nucleus is less stable than a smaller one. This is why we have radioactive decay.

Chapter 19 Radioactivity

Chapter 19 Radioactivity Neutrons act as “nuclear cement” helping to offset the protons repulsive forces. Neutrons only have strong nuclear force and no repulsive forces because their lack of charge. This is why large elements have so many more neutrons. The downside of the neutron is that it is not stable by itself. So in very big nuclei, they will spontaneously transform into a proton and an electron. This destabilizes the nucleus. Why? See Figure 19.10 page 329 (Also in Carbon-14?)

Chapter 19 Radioactivity In an alpha decay, the resulting atom is two spaces back in the periodic table In a beta decay, the resulting atom actually jumps up one space on the table.

Chapter 19 Radioactivity Transmutation – The changing of one element to another Alpha Decay

Chapter 19 Radioactivity Thorium-234 -----> Protactinium-234 + electron This is a beta decay

Chapter 19 Radioactivity Radioactive isotopes decay at different rates Half-life – The time needed for half of the radioactive atoms to decay into something else. The shorter the half-life, the greater the radioactivity. Uranium-238 has a half-life of 4.5 billion years That means that in 4.5 billion years (if the Earth is still here) half the uranium in the Earth will be lead. We can measure the half-life with a radiation detector called a Geiger counter

Chapter 19 Notes

Chapter 19 Radioactivity

Chapter 19 Radioactivity When plants and animals die, the percentage of carbon-14 decreases at a constant rate according to its half-life. The half-life for carbon-14 is about 5730 years Scientists use carbon-14 dating to probe as much as 50,000 years into the past Uranium-238 decays to lead-206 while uranium-235 decays to lead-207 The amount of lead 206 & 207 in rock samples tells us how old the rocks are.

Chapter 20 Notes Nuclear Fission: The splitting of a nucleus (by a neutron in some cases) into fragments producing huge amounts of energy. Energy is mostly kinetic with some gamma radiation Energy released by splitting one Uranium-235 nucleus is 7 million times more than an explosion of a TNT molecule

Chapter 20 Notes

Chapter 20 Notes Chain Reaction: A self-sustaining reaction in which products, like neutrons, from one fission event stimulate further events.

Chapter 20 Notes Nuclear Fusion: The joining together of light nuclei to form a heavier nucleus, accompanied by the release of much energy. For fusion to occur, the nuclei must be traveling at high speeds when they collide to overcome the electrical repulsion between them. This speed translates to high temps like in our sun. This kind of fusion brought about by high temps is called thermonuclear fusion.

Chapter 20 Notes

Chapter 20 Notes Nuclear fusion releases more energy per unit mass than nuclear fission. Since the fusion elements are usually small (hydrogen and helium) they don’t create as much total energy as fission However, when enough hydrogen and helium atoms are fusing, the energy released is massive --- our sun! Fission has the capability to be much more stable than fusion.