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Chapter 4

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.

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Chapter 4

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  1. Chapter 4 History and Atomic Structure

  2. 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)

  3. 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.

  4. proton, neutron and electron called ‘atoms’. 1. All elements are composed of tiny, indivisible particles isotopes exist each element is unique 2. Atoms of the same element are identical... 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. 1…Dalton’s Atomic Theory • Four postulates (1808)... * *

  5. 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…Atomic Scientists • J. J. Thomson discovered electrons (e-) in 1897.

  6. E. Goldstein discovered positive particles in atoms in 1886. • 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. Obj. 2 cont...

  7. Robert Millikan furthered Thomson’s work by describing the electron more in depth. Obj. 2 cont... • all e- carry exactly one unit of negative charge. • mass of an e- is 1/1840 the mass of a hydrogen atom (p+).

  8. Ernest Rutherford conducted the famous ‘gold foil experiment’ (1911) which concluded that: 2. atoms have a solid nucleus at the centerwhich contains most of the mass. • This overturned the accepted ‘plum pudding model’ of the time. Obj. 3 and 4…The Bohr-Rutherford Model 1. atoms are mostly space. (football arena)

  9. 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!)

  10. After Rutherford’s findings, Niels Bohr further explained the atom by concentrating on the e- (1913). • 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. Plum Pudding Quantum Rutherford-Bohr Rutherford Obj. 3 and 4 cont... • Why don’t the (-) e- fall into the (+) nucleus? • The progression of the atom...

  11. ACTUAL MASS (g) RELATIVE MASS ELECTRICAL CHARGE SYMBOL PARTICLE Electron Proton Neutron Obj. 5…Subatomic Particles PROPERTIES OF SUBATOMIC PARTICLES e- -1 9.11 x 10-28 1/1840 p+ +1 1.67 x 10-24 1 1 n0 0 1.67 x 10-24

  12. 19 K Potassium 39.098 Obj. 6…Characteristics of Elements atomic # element symbol element name atomic mass

  13. 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)

  14. atomic mass X # element symbol atomic # # (# of p+) X atomic mass # element symbol • You can find mass #, atomic #, # of n0, and # of e- with either notation! Obj. 7…Atomic Symbols • There are two ways to represent elements: • Symbol Form: OR • Shorthand Form: name of element followed by atomic mass. • Ex... Aluminum - 27 Nitrogen - 14 Carbon - 14

  15. - if you know the # of n0 and the # of p+, you can find the mass. electrical charge on atom P +1 31 15 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. • Ex... Manganese - 55 Phosphorus element = _______________ Mn symbol = ______________ 31 a.m.u.s atomic mass = ___________ 55 a.m.u.s atomic mass = __________ 15 atomic # = ______________ 25 atomic # = ____________ 15 # of p+ = _______________ 25 # of p+ = ______________ 16 30 # of n0 = _______________ # of n0 = ______________ 25 14 # of e- = _______________ # of e- = ______________

  16. Isotopes: atoms of the same element with different atomic masses. element of life extremely rare radioactive…carbon dating • Note: atomic # will NEVERchange in isotopes… only mass and # of n0 do! Neon - 20 X +3 X X -1 22 22 25 Fluorine - 20 12 10 10 Neon - 22 Obj. 9…Isotopes - different # of n0 !!! • Three isotopes of Carbon: (6p+, 6n0) Carbon - 12 (6p+, 7n0) Carbon - 13 (6p+, 8n0) Carbon - 14 • Which of the following are isotopes of the same element?

  17. The mass # of an element (periodic table) is the weighted avg. of allisotopes that exist in nature. 63.55 g/mole Obj. 10…Average Mass of Isotopes • Isotopes are naturally occurring. - 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

  18. The average mass of an element is closest to the isotope that is mostplentiful in nature. • The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature. Obj. 10 cont... • Ex... Three isotopes of Oxygen: Oxygen - 16 99 . 759% Oxygen - 17 0.037% Oxygen - 18 0.204%

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