1. Nuclear Stability and Decay 1500 different nuclei are known. Only 264 are stable and do not decay. The stability of a nucleus depends on its neutron-to- proton ratio. When neutron # vs. proton # is graphed, the region of stable nuclei is called the band of stability. 25.2 Nuclear Transformations

2. Beta Emission Some nuclei have too many neutrons relative to the number of protons. These nuclei decay by turning a neutron into a proton to emit an electron from the nucleus. (Beta Decay) 25.2 Nuclear Transformations Nuclear Equation: 6 C  7 N + -1 e 14 14 0

3. Positron Emission & Electron Capture Some nuclei are unstable because they have too few neutrons relative to protons. These nuclei increase their stability by converting a proton to a neutron by capturing an electron. (Electron Capture) Another possibility is positron emission. During positron emission, a proton changes to a neutron. 25.2 Nuclear Transformations Nuclear Equation: 6 C + -1 e  5 B 11 0 11 Nuclear Equation: 6 C  5 B + +1 e 10 10 0

4. Alpha Emission All elements beyond 83 are radioactive. These nuclei have both too many protons and too many neutrons. These nuclei emit alpha particles to increase their neutron-to-proton ratio. 25.2 Nuclear Transformations

5. Gamma Radiation Why is gamma radiation emitted during radioactive decay? Answer: During radioactive decay mass (subatomic particles) is converted to energy. e = mc 2 25.2 Nuclear Transformations

6. Half –Life The time required for one-half of the nuclei of a radioisotope sample to decay to products. After each half-life, half of the existing radioactive atoms have decayed into atoms of a new element. Half-lives can be as short as a fraction of a second or as long as billions of years. 25.2 Nuclear Transformations

7. Half –Life One isotope that has a long half-life is uranium-238. Uranium-238 decays through a complex series of radioactive isotopes to the stable isotope lead-206 as shown in the figure. 25.2 Nuclear Transformations

8. Transmutation Reactions The conversion of an atom of one element to an atom of another element is called transmutation. Elements above atomic #92 are called transuranium elements. Transuranium elements are considered synthetic (do not occur in nature). All transuranium elements are radioactive. They are created in nuclear reactors and nuclear accelerators. 25.2 Nuclear Transformations