Physical Chemistry AMS 1 1.1 Atomic structure a) Historical facts of our modern atomic theory: i) Dalton's Atomic Theory (1808) ii) Discovery of Cathode rays (Crokes - 1895) iii) Cathode ray tube experiment (Thomson -1898) Discovery of electrons - negatively charged particles. Measurement of the e/m ratio of an electron. Thomson's model of an atom - "plum pudding - a sphere of positive charges in which negative electrons are embedded. iv) Millikan oil drop experiment (1909) Measurement of the electric charge of an electron. v) Rutherford gold foil experiment (1909) Discovery of the minute, massive and positively charged nucleus at the centre of the atom. Rutherford put forward the existence of proton which is positively charged and about 1800 times heavier than an electron. Rutherford' s model of an atom : Electrons move around the minute, massive and positively charged nucleus in circular orbit. vi) Bohr's model of an atom (1913) Electrons move around the positive nucleus in stable circular orbits. Electrons can lose or gain energy by jumping from one orbit to another. vii) Discovery of neutrons - (Chadwick 1934) viii) Development of quantum theory - discussed in later chapters Existence of atomic orbital - s, p, d and f orbital. b) The fundamental "sub-atomic" particles Particle Charge / C Electron Neutron Electron Relative charge +1 0 -1 Mass / kg Mass / u 1.0073 1.0087 5.4858 x 10-4 Key : C : coulomb u : atomic mass unit NB. Relative size of the atom and nucleus (ratio of diameter) : 10 -10 -15 m to 10 m.: c) Atomic number, Z : __________________________________________________ d) Mass number, A : ___________________________________________________ e) Isotopes : _________________________________________________________ _________________________________________________________________ Physical Chemistry f) AMS 2 Nuclides : any atomic species of which the atomic number and the mass number are specified. mass numbe r atomic number Notation for a nuclide : Atomic symbol eg., 14 6 C 1.2 Radioactivity (Discovered by Antonie Henri Becquerel and Marie Curie - 1896 & 1898) a) Nuclear reaction (different from a chemical reaction) In a nuclear reaction, there is a rearrangement of ________ and _________ in the nucleus of the atom. New elements are formed. b) Radioactivity Some nuclides (with _____ ____________ratio) are unstable & radioactive, and split into smaller ones. The nucleus divides into two. The electrons share amongst these two. Charged particles and Radiant energy (Radioactivity) are emitted when radioactive nuclide undergoes radioactive decay. Curie identified 3 different types of ionizing radiation from the decay of radioactive nuclides : Electrically charges plates Sheet of aluminium thin paper β + radioactive sources γ α - Thick lead sheet i) or α 2 He or He (alpha particle), fast moving helium nuclei, positively charged particles and highly ionizing, deflected towards a negative electrode, less penetrating than β radiation and γ radiation. Loss of an alpha particle : loss of 2 neutrons and loss of 2 protons from a radioactive nuclide. 226 222 4 88 Ra → 86 Rn + 2 He (α decay) α radiation As 4 2+ α particles are highly ionizing, they attract electrons He He2+ + 2e- → 4 2 ii) β radiation (beta particles) less penetrating than β − γ radiation; radiation : stream of fast moving electron travelling with speed of light. The electron is emitted by splitting a neutron in radioactive decay. Ionize gas through which they pass. 1 0 n → 11p + -10e Physical Chemistry eg., radioactive decay of NB . iii) γ Th → 234 90 AMS3 234 91 0 -1 Pa + e Deflected towards a positive electric field. symbol for β particle : −10 e or β radiation (gamma rays) uncharged rays, most penetrating High energy electromagnetic radiation; comparable to X-rays but with shorter wavelength; less ionizing than α and β radiation; not deflected by electric and magnetic field. Emitted when a nuclide emits .α or β particles. c) Balancing nuclear reactions Balancing the sum of mass numbers on both sides & the sum of atomic numbers on both sides. 16 a Find a & b. 7 N → bO + β - Ex . New Way Chemistry P.19 Checkpoint 1.1 d) Uses of radioactive isotopes i) Radiotherapy Cancerous tissues are treated by the use of γ rays obtained from a cobalt-60 source. The radiation is directed at the tumour site for a few minutes each day for between two to six weeks. The dosage must be carefully controlled. The other part of the patient body must be well-shielded from the radiation. ii) Sterilization of surgical instruments Sterilization of surgical instruments with radioactivity. iii) Detection of leakage of underground water or fuel pipes By introducing a short- lived radioisotope into the pipes. The level of radioactivity on the ground surface can then be monitored. A sudden increase of surface radioactivity indicates that there is a leakage of underground pipes. iv) Efficiency of lubricating oils Engine wear can be measured by using radioactive piston ring. As the ring wears away, the lubricating oil becomes radioactive. Thus, the efficiency of different lubricating oils can be tested. v) Carbon-14 dating Used to find the age of plant and animal remains. When living organism dies, it takes in no more radioactive carbon-14. Those carbon-14 already in the body of the dead can be used to find the age of the remains of organisms.