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CHEM 1711 & 1811 Lecture # 4 1 ATOMIC STRUCTURE • Dalton (1808) gave modern def'n. of atoms by hypothesizing that - each element is made of tiny particles called atoms - compounds form when atoms combine in definite proportions - reactions reorganize how atoms are connected together - reactions do not create or destroy atoms • law of conservation of mass • microscopes invented since 1980's allows us to "see" atoms SUB-ATOMIC PARTICLES nucleus (~ 0.01 pm) has protons and neutrons electrons in a delocalized "cloud" (~ 100 pm) proton : positively charged nuclear particle; number is atom's identity ; charge = + 1.602 x 10-19 C (C is coulomb, SI charge unit) e.g. neutron: lead, Pb proton number Z = 82 nuclear particle having no electrical charge; some elements have atoms with different numbers of neutrons CHEM 1711 & 1811 Lecture # 4 2 mass number: # of (p + n) isotopes: varieties of an element having different mass numbers carbon-12 carbon-13 carbon-14 electron: 6p 6p 6p 6n 7n 8n fast moving, negatively charged particle; much less massive than p or n ; much more easily transferred between atoms ; charge = - 1.602 x 10-19 C DEMO • always an integer Tesla coil excites a fluorescent light tube atoms are net electrically neutral check: # of e- = # of p How many electrons are in one silicon atom? cation: a positive ion ; an atom or molecule that has lost one or more e- anion: a negative ion ; gained one or more e- ATOMIC MASS MEASUREMENTS • electrons are the least massive sub-atomic particle mp = mn = 1.67 x 10-27 kg - me = 9.11 x 10-31 kg ignore net charge (to 3. s.f.) when considering mass i.e. atom mass = ion mass CHEM 1711 & 1811 Lecture # 4 atomic mass unit (amu or u) : 3 exactly 1/12 the mass of a carbon-12 atom 1 amu or 1 u = 1.6605 x 10-27 kg conversion to SI atomic mass: measured mass of a given isotope ; many sig. figs. ; value close to mass number e.g. • oxygen-16 mass = 15.995 u the relative abundance of isotopes is nearly constant for samples taken at different locations on Earth e.g. chlorine has 2 isotopes 35 Cl 75.77 % 37 Cl 24.23 % mass spectrum atom count 35 • 37 mass/u - samples may be isotopically enriched by special purification - unusual isotopes may serve as labels the average atomic mass for an element is found using the experimental masses and abundances of all its isotopes CHEM 1711 & 1811 Lecture # 4 e.g. 4 chlorine average mass = (34.97 u) (75.77 %) + (36.97 u) (24.23 %) = 35.45 u (check P.T.) SYMBOLS mass # net charge (default = 0) 16 8 proton # (not often used) - O 2 how many atoms in chemical formula How many p, n and e- are in this ion? (its name is superoxide ion) EXAMPLE How many p and e- are in one phosphate ion (PO43-) ?