Introduction to students and teachers. These materials may not be - PDF document

Slide 1 / 100 Slide 2 / 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 Introduction to students and teachers. These materials may not be used for any commercial purpose without the written Atomic Structure 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 www.njctl.org Slide 3 / 100 Slide 4 / 100 Matter Chemistry You will recall that we define matter as anything that has mass and takes up space. Chemistry is the study of matter and the changes it undergoes. Molecules of a Atoms of an element compound The type of matter that is changing and what types of changes it undergoes determines the field of chemistry that is being Mixture of studied. Molecules of elements a diatomic and a element compound Slide 5 / 100 Slide 6 / 100 What is Matter Made of? Existence of the Atom Ancient Greeks thought all matter was some combination of four The idea of the atom has been around for some 2500 "elements" 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. 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 Slide 8 / 100 An alternate ancient theory Transmutations According to this four element theory, if one were to just change the proportions of these four "elements", maybe you turn one A greek philospher named Democritus believed that matter consisted of tiny spheres which he called "atomos" moving substance into another - like tin into gold! through empty space, which he called the "void". For instance, they may have thought that tin just needs a little more of the "element" earth in order to be turned into gold. The word "atomos" means indivisible in Greek and this is where we get the word "atom" of course. tin + earth ---> gold Yay!! There were a number of holes in his theory, among them being that atoms are divisible, but he was on to something. 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 9 / 100 Slide 10 / 100 Dalton's Postulates Dalton's Atomic Theory Building on Democritus' idea, in Dalton had a few major components to his theory the early 1800s, English chemist John Dalton was the first scientist Matter is composed of atoms, to observe the physical world and which are indivisible. Each matter and via these observations, compound consists of a set ratio this draw some conclusions about of atoms. atoms. Atoms of same element are C C C C identical Atoms of different elements are Various atoms and molecules as depicted in Si C different John Dalton's 1808 book: A New System of Chemical Philosophy 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 Slide 12 / 100 1 Which one of the following is not one of the postulates of Dalton's atomic theory? Law of Conservation of Mass A Atoms are made of protons, neutrons, and electrons. Dalton knew that chemical reactions occur by B All atoms of a given element are identical rearrangement of atoms . The law of conservation of mass had already been discovered. The masses of chemicals C Atoms are neither created nor destroyed in chemical reactions. before and after a reaction remained the same, so the number of atoms before and after a reaction had to be the D Compounds are formed when atoms of more than one element combine same as well! E Each element is composed of extremely small particles called atoms. click here for an explanation of conservation of mass

Slide 13 / 100 Slide 14 / 100 . Dalton’s Postulates Digging into the Atom: Discovery of the Electron 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. In the late 1800's scientists were passing electricity through glass tubes containing a very small amount of a particular gas such as Today we know that there are some forms of reactions in which mass oxygen. 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 - POWER POWER OFF ON 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 When the power was turned on, the tube emitted light, ie. chemical reactions. it glowed. Note: The positive electrode was called the anode and the negative called the cathode . Slide 15 / 100 Slide 16 / 100 . . Digging into the Atom: Discovery of Digging into the Atom: Discovery of the Electron the Electron Scientists found that they could deflect this beam by subjecting it to There was much speculation about what these "cathode rays" were. an additional electrical field. Some thought they were a wave, others a stream of particles. To test this, they placed an object so as to interrupt the beam. + The deflection of the beam towards the positive electrode + - + - Since only particles would indicates that the beam carried fail to pass through an a negative charge and traveled object, they - from the cathode to the anode. object believed that these shadow Since they "originated" from the "cathode rays" were cathode, they were called POWER POWER particles. " cathode rays ". ON ON Slide 17 / 100 Slide 18 / 100 . . Digging into the Atom: Discovery of Digging into the Atom: Discovery of the Electron - DISCUSSION QUESTION the Electron - DISCUSSION ANSWER + deflection 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 POWER ON beam a great deal. What does this tell us about the ratio of the charge Since the particles were easily deflected, they must have either a to the mass of these particles? really small mass (small things are easier to move than big things) or a deflection + 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: POWER ON charge charge = large ratio = large ratio mass mass

Slide 19 / 100 Slide 20 / 100 . . Digging into the Atom: Discovery of Thompson's Major Conclusion the Electron A scientist named J.J. Thomson was able to determine this charge to mass ratio to be: These particles, now called "Electrons" must be 1.76 x10 11 Coulombs of charge/ kg of mass or C/kg a fundamental building block of all atoms. 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 Atoms were indeed divisible! found in all gases they experimented on and they all had the same charge to mass ratio. Slide 21 / 100 Slide 22 / 100 2 What characteristic about the cathode rays led them 3 What evidence indicated that these cathode rays to believe they were negatively charged? had particle like properties? A They were small A They were small B They were easily deflected B They were not able to pass through a thick object C They were deflected towards a positive electrode C They moved very quickly D They moved quickly D They were easily deflected towards a positive electrode E They were found in all atoms E They were not altered by a magnetic field Slide 23 / 100 Slide 24 / 100 . 4 Which one of the following is not true concerning Millikan Oil Drop Experiment: Finding cathode rays? the charge and mass of an electron A scientist named Millikan squirted oil drops into a box and then A They originate from the negative electrode. passed high energy x-rays at the box hoping to knock electrons off the air molecules and onto the oil drops. B They travel in straight lines in the absence of electric or magnetic fields. Oil drops C They move from the cathode to the anode. + X-rays D They are made up of electrons. - The characteristics of cathode rays depend on the material from E which they are emitted. 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.