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Atomic Theory

Atomic Theory. Mr. Montero Chemistry Dr. Michael M. Krop High School. Early Atomic Theory. Material World is made up of tiny indivisible particles. A-tomos (Not divisible) There was one unique atom for every substance No proof. Democritus (460 – 370 BC). Early Atomic Theory.

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Atomic Theory

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  1. Atomic Theory Mr. Montero Chemistry Dr. Michael M. Krop High School

  2. Early Atomic Theory • Material World is made up of tiny indivisible particles. • A-tomos (Not divisible) • There was one unique atom for every substance • No proof Democritus (460 – 370 BC)

  3. Early Atomic Theory Democritus is WRONG!!! Matter is continuous. The essence of matter is called hyle. However, I offer no proof. Everyone believed me but it turns out I WAS WRONG. Aristotle (384 – 322 BC)

  4. The Law of Conservation of Mass Total mass of reactants = Total mass of products Mass is neither created nor destroyed during chemical or physical reactions. Antoine Lavoisier (1734-1794)

  5. The Law of Definite Proportions In a pure compound, the elements combine in definite proportions to each other 9 g H2O = 1 g H + 8 g O 18 g H2O = 2 g H + 16 g O Water is Oxygen and Hydrogen combined in a 8:1 mass ratio Joseph Louis Proust (1754 -1826)

  6. Dalton’s Atomic Theory • Elements are composed of very small indivisible particles called Atoms • Atoms of a given element are identical to each other. Atoms of different elements are different from each other. John Dalton (1766-1844)

  7. Dalton’s Atomic Theory • Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. • Atoms of an element are not changed during a chemical reaction. Reactions involve rearranging of atoms. John Dalton (1766-1844)

  8. Rearrangement of Atoms in a Chemical Reaction Combustion of methane

  9. STM image of the surface of gallium arsenide (GaAs). Gallium atoms (blue spheres), Arsenic atoms (red spheres).

  10. DEFINITION OF THE ATOM Basic building block of matter. Smallest particle of an element that retains the chemical identity of the element.

  11. Is the atom truly indivisible? Is the atom indivisible like a solid ball? NO!!! Atoms are composed of electrically charged particles called (Subatomic Particles)

  12. Law of Electrostatic attraction Like charges repel each other. Unlike charges attract each other.

  13. Cathode Rays What is traveling from the Cathode (-) to the Anode (+)? Why is the ray bent by a magnet?

  14. Discovery of the Electron Cathode Rays are actually particles (since light does not bend) with a “negative” charge. ELECTRONS! J.J. Thomson (1856-1940)

  15. Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. (Think of a Chocolate Chip Cookie)

  16. Mass of the Electron • Millikan finds the mass of the electron in 1909 • The mass of the electron is 9.11 x 10-31 kg • That is 2000 times lighter than Hydrogen Robert Millikan (1868-1953)

  17. Millikan’s Oil Drop Experiment The Oil Drop Apparatus

  18. Millikan’s Oil Drop Experiment

  19. Conclusions from the study of the Electron • Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. • Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons • Electrons have so little mass that atoms must contain other particles that account for most of the mass

  20. Radioactivity I discovered that Uranium gives of Radioactivity. It spontaneously emits radiation. I suggested to Marie and Pierre Curie to work on this phenomenon Henri Becquerel (1852-1908)

  21. Radioactivity We are the most famous couple in Science We worked on Radioactivity. An element was named after us (Curium Cm) Pierre Curie (1859-1906) Marie Curie (1867-1934)

  22. Radioactive Particles α, β, γ • Alpha particles (α)are the nucleus of Helium (2 protons, 2 neutrons) +2 charge • Beta particles (β)are high-speed electrons. -1 charge. • Gamma Rays (γ)are high energy radiation, not particles. Zero Charge

  23. Rutherford’s Gold Foil Experiment

  24. Rutherford’s Gold Foil Experiment • Most Particles pass right through • Some particles are slightly deflected • Very seldom a particle will bounce right back • This experiment demolishes J.J. Thomson’s Plum-Pudding Model

  25. Rutherford’s Findings • The nucleus is Dense • The nucleus is Small • The nucleus is positively charged “Like Howitzer shells bouncing off of tissue paper!” Ernest Rutherford (1871-1937)

  26. How dense is the Nucleus? • A pea with the mass of 250 million tons • The mass of all cars in the US in an object that would easily fit in a teaspoon

  27. How small is the nucleus? • If the diameter of the atom is the size of a football field how big is the nucleus? The size of a Cherry!

  28. Other Subatomic Particles • Protons were discovered by Rutherford in 1919 • Neutrons were discovered by Chadwick in 1932

  29. Subatomic Particles

  30. How big is an atom? The diameter of a US Penny is 19 mm. The diameter of a copper atom is 2.6 Å. How many atoms of copper would fit side by side in a straight line across the diameter of a penny? 1 Å = 10-10 m Answer: 7.3 x 107 Cu atoms

  31. Atomic Definitions Atomic Number (Z): The number of protons in an atom Mass Number (A): The number of nucleons (protons and neutrons) Atomic Mass: Mass of the atom (expressed in atomic mass units or “u”)

  32. Atomic Symbols

  33. Isotopes • Dalton predicted that all atoms of the same element are identical. This is not true. • Isotopes are atoms that have the same number of protons but different number of neutrons (different mass number).

  34. Isotopes of Carbon

  35. Heavy Ice D2O is water that has Deuterium (2H) instead of Protium (1H ) Deuterium is heavier than hydrogen since it contains an extra neutron.

  36. Atomic Mass Unit (amu) • We need a more convenient unit to measure the mass of atoms. • Atomic mass of Carbon-12 is assigned an exact value of 12 amu • 1 amu = 1.66054 x 10-24g

  37. Adding the mass of protons and neutrons The mass of one 12C atom is EXACLTY 12 amu. 12C has 6 protons and 6 neutrons. 6 x (1.0073) + 6 x (1.0087) = 12.096 u Where is the mistake????

  38. Measuring the mass of isotopes Mass Spectrometer

  39. Calculating Atomic Mass • Same atoms have different masses (Isotopes) • In nature some isotopes are more abundant than others. • It is possible to take an average of the atomic masses of all existing isotopes. • The average atomic mass is the number reported in the periodic table

  40. Calculating Atomic Mass • The atomic mass depends on the relative abundance of the isotope. Therefore, it is a weighted average Finding the atomic mass of Carbon. Naturally occurring Carbon is Composed of 98.892% 12C and 1.108% 13C. Their masses are 12 u (exact) and 13.00335 u respectively. What is the average atomic mass? (0.98892)(12 u) + (0.01108)(13.00335 u) = 12.011 u

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