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Introduction to the Atomic Model Ms. Grobsky Exploring the Black Box Imagine you’ve received a present that you are not allowed to open. You REALLY want to know what’s inside! What might you do to figure out the contents? Exploring the Black Box In your group, try to figure out the contents and internal structure your present WITHOUT opening it Note: Do NOT shake the box too violently as the contents may fall out! As a group, be sure to answer the questions on the hand-out! So What is a Black Box? A black box refers to any system that has inner workings that cannot be directly observed because it is not possible or feasible Can you think of any other black box examples? An atom is an example of a black box! The Atomic Model and the Nature of Science When scientists wanted to determine the structure of an atom, they were not able to actually see inside of the atom They had to rely on empirical evidence, indirect evidence, observation, and inference when developing their ideas-just as you did when determining the contents of your present! The Atomic Model and the Nature of Science Scientists developed a model to express their new ideas The understanding of the structure of the atom has changed many times over the course of centuries As new evidence emerged, the ideas of the atomic model changed Lecturing about the scientists of the atomic model isn’t quite as fun and interesting as understanding: HOW was the atomic model developed? WHY has the atomic model changed over the years? The Evolution of the Atomic Model Democritus to Rutherford Mind Catalyst When asked the question, “What is matter?”, Democritus responded with the following: By convention there is color, By convention sweetness, By convention bitterness, But in reality there are atoms and space • What do you think he meant by this quote? • Nothing exists except atoms and empty space; everything else is opinion Democritus The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? 400 BC Atomos His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained This piece would be indivisible He named the smallest piece of matter “atomos,” meaning “not to be cut” Atomos To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes Atoms were infinite in number, always moving and capable of joining together Democritus vs. The Philosophers The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter Their ideas held sway because of their eminence as philosophers The atomos idea was buried for approximately 2000 years Then Along Came Dalton’s Model In the early 1800s, the English chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms Dalton’s atomic theory is comprised of 4 postulates which were based on chemical observations made in the lab This theory is commonly referred to as the billiard ball model Dalton’s Postulate #1 Each element is composed of extremely small particles called atoms Dalton’s Postulate #2 All atoms of a given element are identical H H Dalton’s Postulate #2 But, the atoms of one element are different from the atoms of all other elements O H Dalton’s Postulate #3 Atoms of one element cannot be changed into atoms of a different element by chemical reactions In other words, atoms are neither created nor destroyed in chemical reactions They are indestructible! Dalton’s Postulate #4 Compounds are formed when atoms of more than one element combine A given compound always has the same relative number and kinds of atoms Dalton’s Atomic Model This theory became one of the foundations of modern chemistry even though Dalton did not have any direct evidence for the existence of atoms! BUT, it needed work Which of Dalton’s principles do you know to not hold true? Thomson’s Atomic Model In 1897, the work of an English scientist J. J. Thomson provided the first hint that atoms were made up of even smaller particles Thomson’s Experiment Thomson studied electrical discharge through a glass tube pumped almost empty of neutral air When a high voltage was applied to the electrodes, negatively-charged radiation, called cathode rays, would travel from the negative electrode to the positive electrode The path of the cathode rays could be altered by the presence of a magnetic field Cathode Ray Tube Thomson Model Thomson observed that cathode rays behaved identically regardless of the identity of the metal used as cathode material He described cathode rays as streams of negatively-charged particles which came from within the atom • As a result of his work, a particle smaller than an atom had to exist • The atom was divisible! • Thomson called the negatively charged “corpuscles,” today known as electrons And... • Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom • Thomson thought the positive (+) and negative (-) particles were spread out in atoms like: ppuldudmisn (plums in puddin’) Charge and Mass of the Electron Thomson also determined the charge to mass ratio of an electron to be 1.76 108 C/g Once the charge-to-mass ratio of the electron and charge of the electron was known, the mass of the electron could be deduced! In 1909, Robert Millikan succeeded in doing just that with his oil-drop experiment! Millikan’s Oil Drop Experiment Oil drops are sprayed above a positively charged plate containing a small hole As the oil drops fall through the hole, they are given a negative charge Gravity forces the drops downward The applied electric field forces the drops upward When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate Millikan’s Oil Drop Apparatus Charge and Mass of Electrons Using this experiment, Millikan determined the charge on the electron to be 1.60 10-19 C Knowing the charge to mass ratio, 1.76 108 C/g, Millikan calculated the mass of the electron as 9.10 10-28 g With more accurate numbers, we get the mass of the electron to be 9.10939 10-28 g! And Then There was Rutherford… The work of Thomson and Millikan provided growing evidence that the atom is composed of smaller particles In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure Time for Rutherford’s Discovery Activity!