9/22/09. Electrons. Radioactivity. Electrons. Electrons. Atomic Structure and Subatomic Particles. Chapter 2: Atoms and Elements

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1 Atomic Structure and Subatomic Particles Atoms are composed of subatomic particles: electron (e - ), proton (p + ) and neutron (n 0 ). Key discoveries: Chapter 2: Atoms and Elements 2 Radioactivity Electrical behavior: + attracts - but like charges repel Radioactive material Electrons Thomson (897) discovered the e - : fluorescent screen Beam of α, β, and γ + Electrically charged plates α-particle ( + ) Heavier, deflected less than β Atoms must contain smaller sub-units. β-particles ( ) Gamma ray (γ) No charge, no deflection high voltage + cathode ray Cathode rays Travel from cathode (-) to anode (+). Negative charge (e ). Emitted by cathode metal atoms. Electric and magnetic fields deflect the beam. Gives mass/charge of e - = 5.60 x 0-9 g/c Coulomb (C) = SI unit of charge 3 4 Electrons Millikan (9) studied electrically-charged oil drops. Electrons Charge on each droplet was: n (.60 x 0-9 C) with n =, 2, 3, n (e - charge) Modern value = x 0-9 C. = atomic units. These experiments give: m e = charge x mass charge = (-.60 x 0-9 C)(-5.60 x 0-9 g/c) = 8.96 x 0-28 g Modern value = x 0-28 g 5 6

2 Protons Atoms gain a positive charge when e - are lost. Implies a positive fundamental particle. The Nuclear Atom How were p + and e - arranged? Thompson: Ball of uniform positive charge, with small negative dots (e - ) stuck in it. The plum-pudding model. Modern science: m p = x 0-24 g m p 800 x m e. Charge = - x (e - charge). = x 0-9 C = + atomic units 7 8 The Nuclear Atom The Nucleus 0,000 times smaller diameter than the entire atom. α particles e - occupy the remaining space. Rutherford It was about as credible as if you had fired a 5-inch shell at a piece of paper and it came back and hit you. α particles 9 0 Neutrons The Nuclear Atom Atomic mass > mass of all p + and e - in an atom. Rutherford proposed a neutral particle. m n m p (0.% larger). m n = x 0-24 g. Present in all atoms (except normal H). Small light particles surrounding the nucleus. Occupy most of the volume. Charge = -. Atoms are neutral. Number of e = Number of p + 2 2

3 Scanning Tunneling Microscopy Scanning Tunneling Microscopy Fe atoms arranged on Cu. Atom (Chinese characters) 3 4 Sizes of Atoms and Units Atoms are very small. tsp of water contains 3x as many atoms as there are tsp of water in the Atlantic Ocean! Impractical to use pounds and inches... Need a universal unit system The metric system. The SI system (Systeme International) - derived from the metric system. Metric Units A decimal system. Prefixes multiply or divide a unit by multiples of ten. Prefix Factor Example mega M 0 6 megaton = x 0 6 tons kilo k 0 3 kilometer (km) = x 0 3 meter (m) deci d 0 - deciliter (dl) = x 0 - liter (L) centi c 0-2 centimeter (cm) = x 0-2 m milli m 0-3 milligram (mg) = x 0-3 gram (g) micro µ 0-6 micrometer (µm) = x 0-6 m nano n 0-9 nanogram (ng) = x 0-9 g pico p 0-2 picometer (pm) = x 0-2 m femto f 0-5 femtogram (fg) = x 0-5 g 5 6 Metric Units How many copper atoms lie across the diameter of a penny? A penny has a diameter of.90 cm, and a copper atom has a diameter of 256 pm. Some Common Unit Equalities pm = x 0-2 m ; cm = x 0-2 m.90 cm x x 0-2 m pm x =.90 x 0 0 pm cm x 0-2 m Number of atoms across the diameter:.90 x 0 0 pm x Cu atom 256 pm = 7.42 x 0 7 Cu atoms 7 8 3

4 Some Common Unit Equalities Report the mass of a 5.0 lb bag of sugar in kilograms. lb = 453. g Some Common Unit Equalities A patient s blood cholesterol level measured 65 mg/ dl. Express this value in g/l mg = x 0-3 g ; dl = x 0 - L 5.0 lb x 453. g lb = 2265 g = 2.3 x 0 3 g 65 mg dl x x0-3 g mg x dl x0 - L =.65 g/l = 2.3 kg 9 20 Uncertainty and Significant Figures Uncertainty and Significant Figures placeholders significant significant 2 22 Uncertainty and Significant Figures Significant Figures in Calculations dp = 3 dp =

5 Significant Figures in Calculations Significant Figures in Calculations dp = 2 dp = 4 sig. fig. = 5 sig. fig. = 4 = 4, Rules for Rounding > 5, round up. < 5, round down. = 5, check the 2 nd non-significant digit. round up if absent or odd; round down if even. Rules for Rounding Round the following numbers to 3 sig. figs. Round to 3 significant figures. last retained digit st nonsignificant digit 2 nd nonsignificant digit Rounds up to Rules for Rounding Rules for Rounding dp = 5 dp = 3 Answer dp = is the significant result. (5 sig. figs) = sig. figs. = Significant figures? = (5 sig. figs.)

6 Atomic Numbers & Mass Numbers Same element - same number of p + Atomic Numbers & Mass Numbers Atomic mass unit (amu) = contains 6 p + and 6 n 0. 2 amu = x 0-24 g (mass of C atom) that A mass (in amu) of an atom For element X, write: X e.g. C A Z 2 6 Particle Mass Mass Charge (g) (amu) (atomic units) e x p x n x A 2 or X e.g. C or X-A e.g. carbon-2 (Z is constant for a given element) 3 32 Atomic Numbers & Mass Numbers How many p +, n 0 and e - are in the following elements: Mass Spectrometer Cu 25 Mg 29 p + = 29 e - (neutral atom: e - = p + ) = 34 n 0 2 p + = 2 e - (periodic table; neutral) 25 2 = 3 n p Al + = 3 e = 4 n Isotopes and Atomic Weight Isotopes and Atomic Weight Most elements occur as a mixture of isotopes. Magnesium is a mixture of: Hydrogen isotopes: H p +, 0 n 0 deuterium (D) tritium (T) 2 H p+, n 0 3 H p+, 2 n 0 24 Mg 25 Mg 26 Mg number of p number of n mass / amu

7 Isotopes and Atomic Weight Isotopes and Atomic Weights Boron occurs as a mixture of 2 isotopes, 0 B and B. The abundance of 0 B is 9.9%. Calculate the atomic weight of boron. Atomic mass = Σ(fractional abundance)(isotope mass) 0 B B (0.029 amu) =.994 amu % abundance of B = 00% - 9.9% = 80.09% (.0093 amu) = 8.87 amu Atomic weight for B = amu = 0.8 amu Isotopes and Atomic Weight Periodic table: 5 Atomic number (Z) B Symbol Boron Name Amounts of Substances: The Mole A counting unit a familiar counting unit is a dozen : dozen eggs = 2 eggs dozen donuts = 2 donuts dozen apples = 2 apples Amounts of Substances: The Mole A green pea has a ¼-inch diameter. 48 peas/foot. (48) 3 / ft 3 x 0 5 peas/ft 3. V of mol (6.0 x 0 23 peas)/(x 0 5 peas/ft 3 ) 6.0 x 0 8 ft 3 Amounts of Substances: The Mole mole of an atom = atomic weight in grams. Xe atom has mass = 3.29 amu mol of Xe atoms has mass = 3.29 g U.S. surface area = 3.0 x 0 6 mi 2 = 8.4 x 0 3 ft 2 He atom has mass = amu mol of He has mass = g height = V / area, mol would cover the U.S. to: 6.0 x 0 8 ft x 0 3 ft 2 =7. x 0 4 ft = 4 miles! dozen eggs is much heavier than dozen peas!

8 Molar Mass and Problem Solving Molar Mass and Problem Solving Calculate the number of atoms in a.000 g sample of boron. Cu-atom mass = g/mol (periodic table) Conversion factor: n Cu = g x mol Cu g = mol Cu g = mol Cu n B = (.000 g) mol B 0.8 g = mol B B atoms = ( mol B)( atoms/mol) = B atoms n = number of moles Summarizes Atomic numbers. Atomic weights. Physical state (solid/liquid/gas). Type (metal/non-metal/metalloid). In the USA, A denotes a main group element B indicates a transition element. International system uses 8. Periodicity Elements with similar properties are arranged in vertical groups Main group metal Transition metal Metalloid Nonmetal A period is a horizontal row Period number

9 Group A Alkali metals (not H) A group is a vertical column Group 2A Alkaline earth metals Group 7A Halogens Group 8A Noble gases The Alkali Metals and Alkaline Earth Metals Alkali metals (group A; ) Alkaline earth metals (group 2A; 2) Grey silvery white colored. Highly reactive. Never found as native metals. Form alkaline solutions Transition Elements, Lanthanides & Actinides Groups 3A to 8A Transition Elements (groups B 8B) Also called transition metals. Middle of table, periods 4 7. Includes the lanthanides & actinides. Lanthanides and Actinides Listed separately at the bottom. Chemically very similar. Relatively rare on earth. (old name: rare earth elements)