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Chapter 25 – nuclear chemistry 1. What causes a transmutation of the nucleus to occur? 2. How are nuclear decay reaction equations balanced? 3. Do all radionuclides decay at the same rate? Radioisotopes Becquerel’s accidental discovery of transmutation and radioactivity. – Transmutation – the conversion of an atom of one element to an atom of another element Marie Curie coined the term radioactivity. – The process by which nuclei emit particles and rays. Unstable nuclei decay at a constant rate – cannot be affected. – In comparison to ____________ ________________. Decay releases energy, and transforms the element into a more stable and different element. Unstable isotopes Important – not all nuclei are radioactive! Many are quite stable and do not decay. Band of Stability Many nuclei are stable – their data is found on the band of stability. Unstable nuclei change their nuclear structure to move toward the n0/p+ ratio of stable nuclei on the band of stability. Transmutation processes* Alpha (α) Beta (β) Gamma (γ) Positron (β+) Proton or neutron capture * Gamma emission alone does not produce a new element. Alpha decay Nucleus releases a particle made up of 2 p+ and 2 n0. Large nuclei do this. Rutherford used this particle in his experiment! Which element produced it? 222 88𝑅𝑎 → 42𝐻𝑒 + 218 86𝑅𝑛 Beta decay Nucleus converts a neutron into a proton and an electron. n0 → p+ and eProton stays behind, electron (β particle) is ejected from the nucleus 14 6𝐶 → 14 7𝑁 + −10𝑒 Gamma decay High energy radiation emitted from the nucleus Nuclear structure is unchanged. 230 90𝑇ℎ → 226 88𝑅𝑎 + 42𝐻𝑒 + 𝛾 Positron decay Nucleus converts a proton into a neutron and an positively-charged electron. p+ → n0 and β+ 23 12𝑀𝑔 → 23 11𝑁𝑎 + +10𝑒 Electron Capture An electron from the closest energy level falls into the nucleus, which causes a proton to become a neutron. The atomic # goes down by one and the mass number remains unchanged. 0 𝑒−1 + 𝑝11 → 𝑛10 81 36𝐾𝑟 + 0 −1𝑒 → 81 35𝐵𝑟 Isotopic notation symbols Remember 𝐴 𝑍 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 To write equations for decay reactions, we use isotopic symbols and apply the law of matter conservation. Let’s write the symbols for these particles. Balancing Nuclear Reaction Equations Remember 𝐴 𝑍 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 The reaction arrow separates reactants from products. The mass numbers, and the atomic numbers, must add up equally on both sides. How long does decay take? Half life: the time required for ½ of a radioactive sample to decay. Nuclear reaction rate changes over time – first fast, then more slowly. What doesn’t change, however, is the half life. Examples: Am-242: 16 hours At-218: 2 seconds Gd-152: 1 x1014 years Half life graphs Amount remaining, or “counts” is plotted on the y axis. When amount goes to ½, one half-life has passed. Carbon dating