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Chapter 4 History and Atomic Structure IDEAS: • He believed that matter was not divisible • He believed that matter was made up of tiny particles – atomos. • Properties of matter were due to the size and shape of the atoms. • Changes in matter were due to the grouping of atoms (not from changes within the atom) PROBLEMS: • He could not experiment, therefore he could not validate his ideas. • (For example, he could not explain what held the atoms together) IDEAS • Questioned the idea that atoms moved through empty space. • He rejected the idea of atoms. PROBLEMS • Many people respected his ideas, therefore the atomic theory that was proposed 100 years earlier was rejected for the next 2000 years. Obj. 1…Dalton’s Atomic Theory • Four postulates (1808)... * * proton, neutron and electron 1. All elements are composed of tiny, indivisible particles called ‘atoms’. isotopes exist 2. Atoms of the same element are identical... each element is unique 3. Atoms of different elements can physically mix or chemically combine (compounds). 4. Chemical rxns. occur when atoms are separated, joined, or rearranged. • Much of this theory is still accepted, with 2 exceptions Obj. 2…Atomic Scientists • J. J. Thomson discovered electrons (e-) in 1897. • He passed an electric current through a glass tube filled with gas. • He discovered that a beam of negative charges traveled from the cathode (+) to the anode (-). Obj. 2 cont... • E. Goldstein discovered positive particles in atoms • He observed rays traveling in the opposite direction of cathode rays. • These particles were termed ‘ protons’ (p+) by Ernest Rutherford in 1920. • James Chaddwick discovered neutrons (n0) in 1932. • His discovery was based on the fact that different atoms’ atomic mass and atomic # (# of protons) were not adding up. in 1886. Obj. 2 cont... • Robert Millikan furthered Thomson’s work by describing the electron more in depth. • all e- carry exactly one unit of negative charge. • mass of an e- is 1/1840 the mass of a hydrogen atom (p+). Obj. 3 and 4…The Bohr-Rutherford Model • Ernest Rutherford conducted the famous ‘gold foil experiment’ (1911) which concluded that: 1. atoms are mostly space. (football arena) 2. atoms have a solid nucleus at the center which contains most of the mass. • This overturned the accepted ‘plum pudding model’ of the time. Obj. 3 and 4 cont... • The Gold Foil Experiment... (BBs and cream cheese!) • Stats... • 98% of particles went straight through (expected) • 2% of particles deflected at large angles • 0.01% of particles deflected straight back (canon balls & tissue!) Obj. 3 and 4 cont... • After Rutherford’s findings, Niels Bohr further explained the atom by concentrating on the e- (1913). • Why don’t the (-) e- fall into the (+) nucleus? • Bohr proposed that the e- travel on concentric orbits around the nucleus. • Each orbit has a fixed energy (energy level) and e- do not lose energy. • The progression of the atom... Plum Pudding Rutherford Rutherford-Bohr Quantum Obj. 5…Subatomic Particles PROPERTIES OF SUBATOMIC PARTICLES SYMBOL ELECTRICAL CHARGE RELATIVE MASS ACTUAL MASS (g) Electron e- -1 1/1840 9.11 x 10-28 Proton p+ +1 1 1.67 x 10-24 Neutron n 0 1 1.67 x 10-24 PARTICLE 0 Obj. 6…Characteristics of Elements 19 K atomic # element symbol Potassium element name 39.098 atomic mass Obj. 6 cont... • Atomic #: # of p+ in an atom identifies element (change atomic # = change of element). # p+ = # e- in neutral atom ** (+) charge = less e- than p+ ** (-) charge = more e- than p+ • Atomic mass: (a.k.a. mass #) mass of the nucleus p+ + n0 units are a.m.u. (atomic mass unit) Obj. 7…Atomic Symbols • There are two ways to represent elements: • Symbol Form: atomic mass atomic # (# of p+) # # X element symbol OR atomic mass # • Shorthand Form: name of element followed by atomic mass. • Ex... Aluminum - 27 Nitrogen - 14 Carbon - 14 • You can find mass #, atomic #, # of n0, and # of e- with either notation! X element symbol Obj. 8…Atomic Calculations • All mass of the atom is in the nucleus. - only p+ and n0 are in the nucleus. - if you know the mass of any atom, you can find the # of n0. - if you know the # of n0 and the # of p+, you can find the mass. • Ex... electrical charge on atom 31 15 +1 P Manganese - 55 element = _______________ Phosphorus atomic mass = ___________ atomic # = ______________ # of p+ = _______________ # of n0 = _______________ # of e- = _______________ 31 a.m.u.s 15 15 16 14 symbol = ______________ atomic mass = __________ atomic # = ____________ # of p+ = ______________ # of n0 = ______________ # of e- = ______________ Mn 55 a.m.u.s 25 25 30 25 Obj. 9…Isotopes • Isotopes: atoms of the same element with different atomic masses. - different # of n0 !!! • Three isotopes of Carbon: element of life (6p+, 6n0) Carbon - 12 extremely rare (6p+, 7n0) Carbon - 13 radioactive…carbon dating (6p+, 8n0) Carbon - 14 • Note: atomic # will NEVER change in isotopes… only mass and # of n0 do! • Which of the following are isotopes of the same element? 22 12 +3 X 22 10 X 25 10 -1 X Neon - 20 Fluorine - 20 Neon - 22 Obj. 10…Average Mass of Isotopes • Isotopes are naturally occurring. • The mass # of an element (periodic table) is the weighted avg. of all isotopes that exist in nature. - abundance of isotope is just as important as mass! • Ex... Natural copper (Cu) consists of 2 isotopes ... Copper - 63 (mass = 62 .930 g/mole) 69% Copper - 65 (mass = 64 .930 g/mole) 31% • To calculate avg. mass... mass x abundance for each isotope Step 1 : add the two values from step 1 together Step 2 : 43.42 62 .93 x .69 = 43.42 + 20.13 64 .93 x .31 = 20.13 63.55 g/mole Obj. 10 cont... • The average mass of an element is closest to the isotope that is most plentiful in nature. • Ex... Three isotopes of Oxygen: Oxygen - 16 Oxygen - 17 Oxygen - 18 99 . 759% 0.037% 0.204% • The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature.