2 Democritus ( BC)Goal of Greek philosophers was to explain the natural worldBelieved that all materials could be broken down smaller and smaller parts until you reach a pointThis point was what Democritus called “atomos” which in Greek meant “indivisble” or “uncuttable”This particulate view of nature was not too popular among many Greek thinkers due to its rejection by AristotleMatter is made up of 4 elements: fire, air, water, and earthMatter was continuous all of one pieceUnfortunately for mankind, the ideas of Democritus would not come back into the public domain for nearly 2,000 years!
3 John Dalton: Atomic Theory 1.) All matter is made up of atoms. Atoms are indivisible 2.) All atoms of an element are identical in every respect (have the same masses and same properties) 3.) Atoms of different elements are different (have different masses) 4.) Compounds are formed by a combination of 2 or more different kinds of atoms (same ratio of atoms) 5.) A chemical reaction is a rearrangement of atoms
4 Dalton’s Atomic Theory Dalton’s theory seemed to be quite similar to Democritus don’t you think?One part of Dalton’s atomic theory has been rejectedMake a prediction of which statement you think is incorrect and a hypothesis as to why you think thatWe will discuss this more later
5 J.J. Thomson: 3 experiments, 1 big idea Do atoms have parts?Thomson suggested that they doAdvanced the idea that cathode rays are really streams of very small pieces of atoms3 experiments led him to this
6 Thomson’s 1st experiment Had already been found that cathode rays deposited an electrical chargeWhat Thomson was interested in was whether or not he could separate the charge from rays by bending them with a magnetFound that when rays entered slit and into electrometer, it measured a large amount of negative chargeElectrometer did not register much charge if rays were bent so they would not enter slitThomson concluded that the negative charge and the cathode rays must somehow be stuck together (you cannot separate the charge from the raysElectrometer: device for measuring electrical charge
7 Thomson’s 2nd experiment Prior to Thomson’s 2nd experiment, all attempts had failed when trying to bend cathode rays with an electric fieldA charged particle will normally curve as it moves through an electric field, but not if it’s surrounded by a conductorThomson realized that others had failed probably because traces of gas remaining in the tube were being turned into electrical conductorsTo test this idea, he took great pains to extract nearly all the gas from a tube, and found that now the cathode rays did bend in an electric field after allNotice which way the cathode ray is being bent after passing through electric field. Why is this?
8 After 2 experiments….“I can see no escape from the conclusion that cathode rays are charges of negative electricity carried by particles of matter. But, what are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?”—J.J. Thomson
9 Thomson’s 3rd Experiment Sought to determine the basic properties of the particlesThomson calculated the ratio of the mass of a particle to its electric charge (m/e)What he concluded was astounding:The mass to charge ratio for cathode rays turned out to be far smaller than that of a charged hydrogen atom—more than 1,000 times smaller2 possibilities:1.) the cathode rays carried an enormous charge2.) they were amazingly light relative to their charge2nd possibility was eventually proven
10 Summary of Thomson’s Findings Discovered the electron (negatively charged subatomic particle) in 1897Developed the “plum pudding” model in 1904Probably easier to call it the “watermelon” modelSeeds: negative particlesFruit: positively charged low density material
11 Ernest Rutherford Student of J.J. Thomson (1894) Early studies based on radioactivityDiscovered that some materials emit radiationAlpha particle: Eventually was confirmed to be helium nuclei, which meant an alpha particle was simply 2 protons and 2 neutronsLike Thomson, Rutherford was interested in the deflection patterns of alpha particles when exposed to electric & magnetic fieldsFound the deflection patterns by measuring alpha particles position on photographic filmBy accident, he noticed that if the alpha particles passed through a thin sheet of mica, the images on the film were blurred
12 Rutherford and alpha particles The images were sharp if the mica was not presentSomething about the mica sheet was causing the alpha particles to scatter at seemingly random small angles, resulting in blurred images
13 Rutherford & GeigerWanted to study the effects of alpha particles with matterHad to develop a way to count individual alpha particles when they hit the screenFound that a screen coated with zinc sulfide emitted a flash of light each time it was hit by an alpha particleRutherford & Geiger had to sit in a dark room for hours and individually count the flashes of lightRutherford asked Geiger to measure the angles of deflection when the alpha particles were passed through a thin ( cm) sheet of gold foilWhen the gold sheet was bombarded with alpha particle, Geiger found that the scattering smallResults were consistent with Rutherford’s expectations. Because he knew that alpha particles had a considerable mass and moved quite rapidly, he anticipated that virtually all of the alpha particles would go through the metal foil without much disruption
14 Rutherford, Geiger, & Marsden Marsden was a graduate student working in Rutherford’s lab and Geiger had suggested that a research project should be given to MarsdenRutherford: “why not let him see whether any alpha particles can be scattered through a large angle?”Marsden had found that a small fraction (perhaps 1 in 20,000) of the alpha particles were scattered through angles larger than 90 degreesRutherford was in awe“It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!”
15 What could the results from this gold foil experiment mean? The + charge in an atom of gold was known, but if this charge were collected on a sphere the size of an atom (like Thomson suggested), the repulsion would be far too weakTo explain the force experienced by the alpha particle, the charge and much of the mass would have to be collected in a much smaller spherePublished results in 1911 and proposed a model for the atom that is still accepted todayAll of the positive charge and essentially of the mass of the atom is concentrated in an incredibly small fraction of the total volume of the atom, which he called the nucleus (Latin for “little nut”)
16 Further findings from Rutherford’s experiment Most of the alpha particles were able to pass right throughA small fraction came close to the nucleus of a gold atom as they passed through and were slightly deflected from the positive-positive repulsion of the alpha particle and nucleusBut occasionally, an alpha particle would run directly into the nucleus and would result in a great repulsion that deflected the alpha particle through an angle of 90 degrees or moreBy carefully measuring the fraction of the alpha particles deflected through large angles, Rutherford was able to estimate the size of the nucleus (JUST LIKE WE DID!)Found that the radius of the nucleus is at least 10,000 times smaller than the radius of the atomThe vast majority of the atom is therefore empty space!!
17 Someone throw out the plum pudding!! Rutherford revised Thomson’s plum pudding model, showing how electrons could orbit a positively charged nucleus, like planets orbiting a sunBecause the majority of the “plum-pudding” atom would be electrically neutral (no charge), the alpha particles would have no problem shooting throughRutherford’s atom
18 Along comes Niels BohrAlso student of Thomson and worked with RutherfordBohr, and many other, knew that Rutherford’s model made no sense based on one specific reason….We knew that any charged body (electron) that was in a state of motion other than at rest or in uniform motion in a straight line, will emit energyThus the electrons in this “solar system” model would be constantly emitting energyIF that were the case, the electrons would eventually run out of energy and spiral down into the nucleus and the entire atom would collapse!
19 BohrHad trouble making sense of line spectra data with Rutherford’s atom
20 Bohr’s ModelSimilar nucleus to Rutherford’s with both protons and neutrons insideNegative particles (electrons) are in specific orbits around the nucleusMain problem with Bohr model:Only accounted for hydrogen atomCouldn’t explain multi-electron atoms (anything other than hydrogen)
21 Subatomic ParticlesBased on evidence from Rutherford and Thomson
24 What is atomic mass? Also referred to as: Atomic weightAverage atomic massRelative atomic massImportant characteristic for elements:Each element has its own atomic massAtomic mass is an averageAverage of the masses of a number of different atomsSpecial kind of average called a weighted averageDifferent than the usual average you’re probably familiar with in mathAtomic mass
25 Understanding Weighted Averages Even though these are different models and have different features, they are both lemonas due to their distinct lemon-like shapeUsing this analogy, the models of the lemona are similar to the isotopes of an element29 protons makes both atoms copper—even though they differ in their numbers of neutronsJust like the “lemon-like” shape of a car makes it a lemona—even though they differ in their features
26 Average vs. Weighted Average What is the average weight of the 2 cars?4,000+5,000 2 =4,500Regular AverageWhat would happen if we added in extra information?
27 Weighted AverageWhat is the average weight of lemonas, taking into account the amount of each model? 4,000 𝑥 .95 +(5,000 𝑥 .05)=4,050 𝑙𝑏𝑠Because there are so many more GXs than GXLs, the weighted average is much closer to the actual weight of a lemona GX
28 Using Weighted Average with Different Atoms Atomic mass: a weighted average of the masses for all the isotopes of a certain elementMass = 63 amuMass = 65 amuIf we pulled out a random sample of 100 copper atoms, we would find that 69% of them would be Cu-63 and 31% of them would be Cu-6569% : Cu-6331% : Cu-65
29 Atomic Mass of Copper 63 𝑥 .69 + 65 𝑥 .31 =63.62 𝑎𝑚𝑢 69% : Cu-63 Why are the 2 atomic masses different then?
30 Difference between mass number and atomic mass Mass number: protons + neutrons1 proton or 1 neutron = 1 amuTherefore, if you have 6 protons and 6 neutrons, your atom is going to weigh 12 amuIf you have 6 protons and 7 neutrons, your atom is going to weight 13 amu
31 PracticeGallium has 2 stable isotopes, and the masses of Gallium-69 (60.11% abundant) and Gallium-71 (39.89% abundant) are amu and amu, respectively. Calculate the average atomic mass of Gallium
32 PracticeRubidium has 2 isotopes: Rubidium-85 (atomic mass of amu) and Rubidium-87 (atomic mass of amu). The atomic mass of Rubidium reported on the periodic table is amu. Based on this information, which of the isotopes of Rubidium is more abundant? How do you know?This is more of a thought problem. No real calculations are really necessary
33 PracticeMagnesium has 3 stable isotopes. Calculate its average atomic mass, using the information in the chart below.IsotopeMassAbundanceMg-24amu78.99 %Mg-25amu10.00 %Mg-26amu11.01 %