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Chapter 2 Skeleton

Chapter 2 Skeleton - Chapter 2 Atoms Molecules and Ions...

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Chapter 2. Atoms, Molecules, and Ions Lecture Outline 2.1 The Atomic Theory of Matter • Greek Philosophers: Can matter be subdivided into fundamental particles? Let’s cut applies! • Democritus (460–370 BC): All matter can be divided into indivisible atomos. • Dalton: proposed atomic theory with the following postulates: elements composed of atoms All atoms of same element are identical In chemical rxn, atoms are not changed Compounds are combinations of elements • Atoms are the building blocks of matter. These laws provide basic understanding of atoms. • Law of constant composition: Relative kinds and numbers of atoms are constant for a given compound • Law of conservation of mass (matter): Total mass before rxn = total mass after rxn • Law of multiple proportions: If 2 elements (A & B), combine to form new compound. The mass of B, which combines with A, is ratio of small whole numbers Example: Water and hydrogen peroxide are both composed of Hydrogen and Oxygen. In forming water, 8 g of O combines with 1 g of Hydrogen In forming hydrogen peroxide, 16 g of O combines with 1 g of Hydrogen; hence, 2.2 The Discovery of Atomic Structure • By 1850 scientists knew that atoms consisted of charged particles. • Subatomic particles are those particles that make up the atom. • Recall the law of electrostatic attraction: like charges repel and opposite charges attract. Cathode Rays and Electrons • Cathode rays were first discovered in the mid-1800s from studies of electrical discharge through partially evacuated tubes (cathode-ray tubes or CRTs). See figures 2.3 and 2.4 in your text.
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• Cathode rays = radiation produced when high voltage is applied across the tube; ELECTRONS!. • In 1897 J.J. Thomson determined the charge-to-mass ratio of an electron. How? • Charge-to-mass ratio: 1.76 X 10 8 C/g • C is a symbol for coulomb (It is the SI unit for electric charge) This means: Charge of e- / mass e- = constant What is the limitation of these experimental results? Change/mass ratio Charge e - Mass e - • Millikan Oil-Drop Experiment • Goal: find the charge on the electron to determine its mass. See figure 2.5 in your text • Oil drops are sprayed above a positively charged plate containing a small hole. • As the oil drops fall through the hole they acquire a negative charge. • Gravity forces the drops downward. The applied electric field forces the drops upward. • When a drop is perfectly balanced, then the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate. • He concluded the charge on the electron must be 1.60 x 10–19 C. • Knowing the charge-to-mass ratio of the electron, we can calculate the mass of the electron: Charge of e- / mass e- = 1.76 x 108 C/g g109.10C/g101.76C101.60Mass28819 THIS MASS IS 2000 TIMES SMALLER THAN HYDROGEN!!!!
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