Ch 21: Nuclear Chemistry. Section 21.1 - Radioactivity

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Radioactivity Wilhelm Roentgen made a big discovery in He found that invisible rays were emitted when electrons bombarded materials. He named these rays, X-rays. At the same time, Henri Becquerel was studying minerals that emitted light after being exposed to sunlight, a phenomenon called phosphorescence.

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Ch 21: Nuclear Chemistry Section Radioactivity Radioactivity Wilhelm Roentgen made a big discovery in He found that invisible rays were emitted when electrons bombarded materials. He named these rays, X-rays. At the same time, Henri Becquerel was studying minerals that emitted light after being exposed to sunlight, a phenomenon called phosphorescence. Radioactivity Marie Curie and her husband Pierre were working with Becquerel and took his mineral sample and were able to isolate the components emitting the rays. Marie named the process by which materials give off such rays Radioactivity. Radiation: the penetrating rays and particles emitted by a radioactive source. Radioactivity Marie Curie was the first woman to win the Nobel Prize and is the only person to receive Nobel Prizes in two different sciences--physics and chemistry! Nuclear Reactions vs. Normal Chemical Changes Marie Curie discovered that: Chemical Reactions were affected by Pressure and Temperature, while Nuclear Reactions are not. Nuclear Reactions involve the nucleus, which changes the type of element. Chemical reactions involve electrons, not protons and neutrons. When a substance emits radiation, it changes its identity. A radioactive element has an unstable nucleus. Isotopes are atoms of the same element that have different numbers of neutrons and mass number. Radioisotopes: isotopes of atoms with unstable nuclei Nuclear Stability and Decay Nuclear Force: the attractive force that acts between all nuclear particles that are extremely close together, such as neutrons and protons in a nucleus. Band of Stability: the location of stable nuclei on a neutron vs. proton plot. Types of Radiation The three types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation. There is also positron emission They can be separated by an electric field, as shown below. Alpha Radiation Alpha Particle: a positively charged helium isotope. Written with the symbol: It contains 2 protons and 2 neutrons and has a 2+ charge Po Pb He Beta Radiation 146 C N e Positron Emission 15 8 O N e Gamma Radiation Gamma Rays: high energy electromagnetic radiation. The emission of gamma rays does not change the atomic number or mass number of a nucleus. Used to destroy tumors Po Po Section 21.2 Penetrating Abilities and Half-Life Penetrating Abilities Alpha: stopped by piece of paper. Beta: stopped by thin metal Gamma: stopped by thick lead and concrete. Half-Life 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-Life Calculation Carbon-14 emits beta radiation and decays with a half-life of 5730 years. Assume you start with a mass of 2040 g of Carbon-14. a.How long is three half-lives? 3 x 5730 years = 17,190 years b.How many grams of the isotope remain at the end of three half-lives? 2040 2 2 2 = 240 grams Or 2040 2 3 = 240 grams Half-Life Calculation The half-life of Zn-71 is 2.4 minutes. If one had g at the beginning, how many grams would be left after 9.6 mins elapses? a.Figure out how many half-lives went by. 9.6 2.4 = 4 half-lives b.Then divide beginning amount by 2 to the number of half-lives. 100 2 4 = 6.25 grams *Dont worry about significant figures, just write whatever you get on your calculator. Half-Life Calculation Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 g sample would have decayed after exactly two half-lives? a.Half-life calculations tell us how much remains, not how much is gone. Solve for remains first, and subtract that from the initial amount 2 2 = 2.5 grams remains = 7.5 g decayed