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 12 (Answer) / 100 Slide 13 / 100 1 Which one of the following is not one of the Dalton’s Postulates postulates of Dalton's atomic theory? Dalton had the right idea with his postulates but he was not A Atoms are made of protons, neutrons, and electrons. completely correct. He was limited by the equipment he had to observe reactions. Answer B All atoms of a given element are identical A Today we know that there are some forms of reactions in which mass C Atoms are neither created nor destroyed in chemical reactions. does change, and atoms are changed from one type to another. You learned about these last year in Physics. They are called Nuclear Reactions. D Compounds are formed when atoms of more than one element combine Also remember that today we know atoms can be broken down into [This object is a pull tab] smaller bits. We also know all atoms of an element are not identical - E Each element is composed of extremely small particles called atoms. 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 Slide 15 / 100 . . Digging into the Atom: Discovery of Digging into the Atom: Discovery of the Electron the Electron In the late 1800's scientists were passing electricity through glass Scientists found that they could deflect this beam by subjecting it to tubes containing a very small amount of a particular gas such as an additional electrical field. oxygen. + The deflection of the beam - towards the positive electrode - + + + - indicates that the beam carried a negative charge and traveled POWER POWER - OFF from the cathode to the anode. ON Since they "originated" from the cathode, they were called When the power was turned on, the tube emitted light, ie. POWER " cathode rays ". ON it glowed. Note: The positive electrode was called the anode and the negative called the cathode . Slide 16 / 100 Slide 17 / 100 . . Digging into the Atom: Discovery of Digging into the Atom: Discovery of the Electron the Electron - DISCUSSION QUESTION There was much speculation about what these "cathode rays" were. Once these rays were understood to be particles, they were in search Some thought they were a wave, others a stream of particles. To test of their properties - like their mass and the size of their charge. this, they placed an object so as to interrupt the beam. 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? Since only particles would fail to pass through an deflection + object, they + - object believed that these shadow "cathode rays" were - POWER particles. ON POWER ON

Slide 18 / 100 Slide 19 / 100 . . Digging into the Atom: Discovery of Digging into the Atom: Discovery of the Electron the Electron - DISCUSSION ANSWER + deflection 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 POWER ON Since the particles were easily deflected, they must have either a Keep in mind, at this point they knew neither the charge nor the mass, really small mass (small things are easier to move than big things) or a just that the ratio was large indicating EITHER a large charge or a really large charge (opposites attract). Therefore, the ratio should be 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 huge number charge to mass ratio. If the charge was really big: If the mass was really big: charge charge = large ratio = large ratio mass mass Slide 20 / 100 Slide 21 / 100 . 2 What characteristic about the cathode rays led them Thompson's Major Conclusion to believe they were negatively charged? A They were small B They were easily deflected These particles, now called "Electrons" must be a fundamental building block of all atoms. C They were deflected towards a positive electrode D They moved quickly E They were found in all atoms Atoms were indeed divisible! Slide 21 (Answer) / 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 Answer B They were easily deflected B They were not able to pass through a thick object C 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 [This object is a pull tab] E They were not altered by a magnetic field