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Chapter 2

Chapter 2. Atoms, Molecules, and Ions. Scientific Method. Introduction. The general process of advancing scientific knowledge through observation, laws, hypotheses, or theories is called the scientific method. Experiment and Explanation. What Is an Experiment?.

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Chapter 2

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  1. Chapter 2 Atoms, Molecules, and Ions

  2. Scientific Method Introduction • The general process of advancing scientific knowledge through observation, laws, hypotheses, or theories is called the scientificmethod

  3. Experiment and Explanation What Is an Experiment? Experimentand explanation are the heart of chemical research. It is an observation of phenomena carried out in a controlled manner so that the results can be analyzed.

  4. Law Hypothesis Theory A tentative explanation A tested hypothesis A concise statement or equation What Is an Explanation? After a series of experiments, a researcher may see some relationship or regularity in the results

  5. A question Observations (Experiments) Hypothesis Further experiment devised based on hypothesis Negative results lead to Modification or rejection of hypothesis and formulation of new hypothesis Positive results support hypothesis A theory follows after results consistently support hypothesis Further experiments

  6. Development of a Simple Theory by the Scientific Method Observation: Every swan I've ever seen is white. Hypothesis: All swans must be white More Observations: Every swan any other scientist has ever observed in any country has always been white. Theory: All swans are white. The next swan I see will be white. Prediction: Note, however, that although the prediction is useful, the theory does not absolutely prove that the next swan I see will be white

  7. A Postulate Something assumed without proof and is accepted as true

  8. Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton.

  9. Dalton’s Postulates Each element is composed of extremely small particles called atoms.

  10. Dalton’s Postulates All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

  11. Dalton’s Postulates Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions. A relatively modern statement of the law of “conservation of mass”

  12. Law of “conservation of mass, the atomic scale For an ordinary chemical reaction

  13. Law of “conservation of mass”, macroscopic scale For an ordinary chemical reaction:The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

  14. Dalton’s Postulates Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. A statement of the law of “definite compositions” or “constant composition”

  15. Law of Constant CompositionJoseph Proust (1754–1826) The elemental composition of a pure substance never varies regardless of the method of preparation or the source. The structure of water is always H2O, whether it is a rain or a river or a tap water

  16. Relation between ratios 16.00/12.01 = 1.332 = 2.664:1.332 = 2:1 32.00/12.01 = 2.664 Law of “Multiple Proportions” If two elements combine to form more than one compound, the mass ratio of the elements in one compound is related to the mass ratio in the other compounds by a small whole number Compound m(O)/ m(C) CO CO2

  17. Law of “Multiple Proportions” No. of O atoms/ No. of C atoms 1/1 2/1

  18. Relation between ratios m(H)/ m(C) Compound 1 C2H2 CH4 4 C2H6 3 1 C6H6 A Question What is the relation between the ratios of the mass of hydrogen to the mass of carbon in each of the hydrocarbons: C2H2, CH4, C2H6, and C6H6?

  19. Atomic Structure

  20. Cathode Ray a: In presence of an electric field b: In presence of a magnetic field c: In presence of an electric field and a magnetic field, where both fields cancel each other

  21. C: Coulomb The Electron • Streams of negatively charged particles were found to emanate from cathode tubes. • J. J. Thompson is credited with their discovery (1897).

  22. Charge of the Electron Millikan’s Oil Drop Experiment

  23. Discovery of the Nucleus 1. Radioactivity 2. Rutherford Scattering Experiment

  24. 1. Radioactivity • The spontaneous emission of radiation by an atom. • First observed by Henri Becquerel. • Also studied by Marie and Pierre Curie.

  25. 1. Radioactivity • Three types of radiation were discovered by Ernest Rutherford: •  particles •  particles •  rays Charge: +2 Charge: -1 Charge: 0

  26. 2. Discovery of the Nucleus Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

  27. Animation: Scattering of Particles by Gold Foil Back to slide 25

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

  29. Subatomic Particles SI Units: Atomic Units

  30. Atomic Symbols X: symbol of atom of an element Atoms of elements are symbolized by one or two letters. A: Mass number (number of protons plus number of neutrons) Z: Atomic number (number of protons)

  31. Atomic Number, Mass Number, and Isotopes Atomic number (Z) = number of protons Mass number (A) = number of protons + number of neutrons

  32. Isotopes 12 6 C Different 13 6 C A representation of two isotopes of carbon Same number of protons Same number of electrons Different number of neutrons Different mass number

  33. A modern form of the periodic table

  34. For carbon Ignoring the very small contribution of 14C isotope, we get Average Mass In general

  35. An Example: Chlorine has two naturally occurring isotopes, 35Cl (isotopic mass 34.9689 amu) and 37Cl (isotopic mass 36.9659 amu). If chlorine has an atomic mass of 35.4527 amu, what is the percent abundance of each isotope?

  36. Mass Spectrometry is used to measure masses of atoms The mass spectrum of neon

  37. Molecular Compounds Molecular compounds are composed of molecules and almost always contain only nonmetals.

  38. Molecular elements • H2, • O2, N2, • F2, Cl2, Br2, I2

  39. An Example: benzene Empirical formula: CH Molecular formula: C6H6 Types of Formulas • Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound. • Molecular formulas give the exact number of atoms of each element in a compound.

  40. Molecular and structural formulas and molecular models

  41. Ions • When atoms lose or gain electrons, they become ions. • Cations are positive and are formed by elements on the left side of the periodic chart (Metals). • Anions are negative and are formed by elements on the right side of the periodic chart (Nonmetals).

  42. Common Cations of Transition Metals

  43. Ions Are Not Electrically Neutral What is the number of protons in Cr3+? What Is the number of electros in Cr3+? What is the number of neutrons in Cr3+? What is the number of protons in S2-? What Is the number of electrons in S2-? What is the number of neutrons in S2-?

  44. Ionic Bonds Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.

  45. A model of a portion of crystal

  46. Effect of a magnetic field on charged particles.

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