Slide 1 / 100 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org Slide 2 / 100 Introduction to Atomic Structure www.njctl.org Slide 3 / 100 Chemistry Chemistry is the study of matter and the changes it undergoes. The type of matter that is changing and what types of changes it undergoes determines the field of chemistry that is being studied.
Slide 4 / 100 Matter You will recall that we define matter as anything that has mass and takes up space. Molecules of a Atoms of an element compound Mixture of Molecules of elements a diatomic and a element compound Slide 5 / 100 Existence of the Atom The idea of the atom has been around for some 2500 years in one form or another. In 1909 the existence of atoms was proved by Ernest Rutherford. Let's go through a historical account of how scientists arrived at Rutherford's proven existence of the atom. Slide 6 / 100 What is Matter Made of? Ancient Greeks thought all matter was some combination of four "elements" Earth Air Fire Water So, according to this theory, what makes copper - or any other substance unique was it's unique blend of these four elements.
Slide 7 / 100 Transmutations According to this four element theory, if one were to just change the proportions of these four "elements", maybe you turn one substance into another - like tin into gold! For instance, they may have thought that tin just needs a little more of the "element" earth in order to be turned into gold. tin + earth ---> gold Yay!! Except, it never worked for them. Transmutation does occur in the natural world but to do it artificially requires modern technology and not according to this theory! Slide 8 / 100 An alternate ancient theory A greek philospher named Democritus believed that matter consisted of tiny spheres which he called "atomos" moving through empty space, which he called the "void". The word "atomos" means indivisible in Greek and this is where we get the word "atom" of course. There were a number of holes in his theory, among them being that atoms are divisible, but he was on to something. Slide 9 / 100 Dalton's Postulates Building on Democritus' idea, in the early 1800s, English chemist John Dalton was the first scientist to observe the physical world and matter and via these observations, this draw some conclusions about atoms. Various atoms and molecules as depicted in John Dalton's 1808 book: A New System of Chemical Philosophy
Slide 10 / 100 Dalton's Atomic Theory Dalton had a few major components to his theory Matter is composed of atoms, which are indivisible. Each compound consists of a set ratio of atoms. Atoms of same element are C C C C identical Atoms of different elements are Si C different Atoms are not changed, created, Cl Cl H H or destroyed in a reaction, they H Cl are simply rearranged Cl H Slide 11 / 100 Law of Conservation of Mass Dalton knew that chemical reactions occur by rearrangement of atoms . The law of conservation of mass had already been discovered. The masses of chemicals before and after a reaction remained the same, so the number of atoms before and after a reaction had to be the same as well! click here for an explanation of conservation of mass Slide 12 / 100 1 Which one of the following is not one of the postulates of Dalton's atomic theory? A Atoms are made of protons, neutrons, and electrons. B All atoms of a given element are identical C Atoms are neither created nor destroyed in chemical reactions. D Compounds are formed when atoms of more than one element combine E Each element is composed of extremely small particles called atoms.
Slide 13 / 100 Dalton’s Postulates Dalton had the right idea with his postulates but he was not completely correct. He was limited by the equipment he had to observe reactions. Today we know that there are some forms of reactions in which mass does change, and atoms are changed from one type to another. You learned about these last year in Physics. They are called Nuclear Reactions. Also remember that today we know atoms can be broken down into smaller bits. We also know all atoms of an element are not identical - elements found in nature can vary in number of neutrons. However, for the purposes of general Chemistry, Dalton's Postulates are still a pretty reasonable approximation of what is actually happening in chemical reactions. Slide 14 / 100 . Digging into the Atom: Discovery of the Electron In the late 1800's scientists were passing electricity through glass tubes containing a very small amount of a particular gas such as oxygen. - + + - POWER POWER OFF ON When the power was turned on, the tube emitted light, ie. it glowed. Note: The positive electrode was called the anode and the negative called the cathode . Slide 15 / 100 . Digging into the Atom: Discovery of the Electron Scientists found that they could deflect this beam by subjecting it to an additional electrical field. + The deflection of the beam towards the positive electrode + - indicates that the beam carried a negative charge and traveled - from the cathode to the anode. Since they "originated" from the cathode, they were called POWER " cathode rays ". ON
Slide 16 / 100 . Digging into the Atom: Discovery of the Electron There was much speculation about what these "cathode rays" were. Some thought they were a wave, others a stream of particles. To test this, they placed an object so as to interrupt the beam. + - Since only particles would fail to pass through an object, they object believed that these shadow "cathode rays" were particles. POWER ON Slide 17 / 100 . Digging into the Atom: Discovery of the Electron - DISCUSSION QUESTION Once these rays were understood to be particles, they were in search of their properties - like their mass and the size of their charge. Scientists determined that a very weak electrical field could deflect the beam a great deal. What does this tell us about the ratio of the charge to the mass of these particles? deflection + + - - POWER ON Slide 18 / 100 . Digging into the Atom: Discovery of the Electron - DISCUSSION ANSWER deflection + + - - POWER ON Since the particles were easily deflected, they must have either a really small mass (small things are easier to move than big things) or a really large charge (opposites attract). Therefore, the ratio should be a huge number If the charge was really big: If the mass was really big: charge charge = large ratio = large ratio mass mass
Slide 19 / 100 . Digging into the Atom: Discovery of the Electron A scientist named J.J. Thomson was able to determine this charge to mass ratio to be: 1.76 x10 11 Coulombs of charge/ kg of mass or C/kg Keep in mind, at this point they knew neither the charge nor the mass, just that the ratio was large indicating EITHER a large charge or a small mass. What was VERY interesting was that these particles were found in all gases they experimented on and they all had the same charge to mass ratio. Slide 20 / 100 . Thompson's Major Conclusion These particles, now called "Electrons" must be a fundamental building block of all atoms. Atoms were indeed divisible! Slide 21 / 100 2 What characteristic about the cathode rays led them to believe they were negatively charged? A They were small B They were easily deflected C They were deflected towards a positive electrode D They moved quickly E They were found in all atoms
Slide 22 / 100 3 What evidence indicated that these cathode rays had particle like properties? A They were small B They were not able to pass through a thick object C They moved very quickly D They were easily deflected towards a positive electrode E They were not altered by a magnetic field Slide 23 / 100 4 Which one of the following is not true concerning cathode rays? A They originate from the negative electrode. B They travel in straight lines in the absence of electric or magnetic fields. C They move from the cathode to the anode. D They are made up of electrons. The characteristics of cathode rays depend on the material from E which they are emitted. Slide 24 / 100 . Millikan Oil Drop Experiment: Finding the charge and mass of an electron A scientist named Millikan squirted oil drops into a box and then passed high energy x-rays at the box hoping to knock electrons off the air molecules and onto the oil drops. Oil drops + X-rays - Click here to see an animation of the experiment By measuring the energy necessary to stop the drops from descending, he was able to determine the charge per drop. The more energy needed to prevent the drop from falling, the smaller the charge of the drop.
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