Dates to Remember States of Matter Next test runs Tues. through Thurs. Vocab quiz available now. (a) Gas (b) Liquid (c) Solid Which state of matter is associated with the most electrical potential? 1 animation 2 Do you understand this graph? What is happening What here? are you plotting? A. The material is heating up B. The material is cooling down C. The material is melting D. Both A and C temperature Added Why does it get warmer when it begins to snow? Energy released when tiny droplets freeze to form snow - warms the air. What kind? KE (thermal) at yellow arrows Electrical potential -- T is not changing. Temperature Temperature of WHAT isn t changing? Time (Removing Internal Energy) freezing point 3 4 Temperature decreasing Temperature Hand Warmers liquid Time (Removing Internal Energy) freezing point solid Evaporation Why does a wet finger held up in a breeze feel cold? Where does water go? The water molecules are moving. Some of them break free. (Which ones?) What happens to the average speed of those that remain? Can the wind increase the rate of evaporation? 5 6
Chapter 15-The Nuclear Atom A continuing saga in the development of models to explain the structure of matter. Where we ve been: Continuous model Advantage simple, describes states of matter Disadvantage Can t turn lead into gold Molecular / atomic model Advantage explains temperature, phase changes, Brownian motion, etc Disadvantage too complex, can t explain light emission. 7 8 Then along came Thompson... Here s where he started. 9 Molecular Model of Matter 1. Matter is made up of small, unseen particles called molecules. 2. Each kind of matter is made up of a different kind of molecule. 3. The molecules are in constant motion -- they jiggle, collide and rebound due to electromagnetic forces. 4. The motion and interaction of molecules can be analyzed, understood and predicted by using the laws of force and motion and the laws of conservation. 10 Start with point #1 J.J. Thomson discovers the electron Is matter really made up of atoms and molecules, or is there something smaller? Can you break atoms and molecules into fragments? 11 12
Gas Discharge Tube - - - - - - Showed matter was made up of charged pieces negative particles are identical small mass; electrons positive particles differ depending on gas large mass even small differences with same gas What were the positive particles? A. Protons B. Nuclei C. Atoms missing a few electrons 13 14 Thomson Model of Atom (Plum Pudding Model) Atoms consist of a thin positive fluid, which contains most of the mass, with embedded point-like negative electrons to balance the charge Positive goop Negative particles 15 Two more experiments The Milikan oil drop experiment Showed that charge is quantized (a big word meaning it comes in certain sized lumps) Mass spectrometers Show that mass is quantized Time-of-Flight Mass Spectrometer pulse accelerator plates positive particles same kinetic - Length of flight path detector kinetic = 1/2 (mass)(speed) 2 Heaviest particles move slowest 17 18
RESULT: the masses of different atoms!!! Testing the Thomson model The Rutherford experiment time mass comes in lumps The art of figuring out what s inside by shooting at it. 19 20 Rutherford experiment Results: Rutherford s Experiment 1911 Video clip It was like shooting at ghosts! As expected, most went right on through. But, unexpectedly, a few bounced back! 21 22 So, what is inside? Either plum pudding can ricochet bullets, or there is something else in the box. What would repel charged bullets? A dense charged nucleus If all of the charge is in the middle, where do you put the electrons? Make them orbit like planets around the sun. What do you call a model of the atom that looks like the solar system? What would happen if you shot electrons? A. They d do the same thing that the alpha particles did. B. They d all bounce off. C. They d all go through. D. They d all get absorbed by the gold. 23 24
The solar system model of the atom 1. there are positive and negative parts *Thomson model 2. the positive part is a tiny, dense nucleus 3. the negative parts are electrons around the nucleus, orbiting wherever they want. 4. the atom is mostly empty space At nuclear density, a pin would weigh as much as a battleship. Problem: Still can t explain why atoms emit unique, discrete spectra Not only that, but accelerating (orbiting) electrons should continually radiate, loose, and spiral into the nucleus However, if electrons are stationary, they d fall into the nucleus too. 25 26 Bohr s Answer (helpful but still imperfect) Niels Bohr: Nobel Prize 1922 The Rutherford model a patch. Electrons move in certain allowed orbits only -- quantized. Don t know why! Electron has a characteristic depending on orbit; smaller radius, less. Electrons radiate (emit) only when they jump to an allowed orbit of lower. Electrons absorb only when they jump to a higher orbit. Atoms only give off and absorb certain specific frequencies of light because their electrons can only have certain specific energies. 27 Bohr Spectra Discrete orbits explain discrete spectra Blue photon Red photon emission possible energies purple photon cyan photon absorption 28 Imagine an atom with only 4 Bohr orbits, i.e. 4 electron levels. How many possible discrete colors can there be in its emission spectrum? (Assume only 1 electron in this atom) Hint: count the transitions, and don t double count. A. 1 B. 2 C. 4 D. 6 E. > 10 Which photon has the highest Quizlet: frequency? Answer 3 2 1 0 6 colors are possible don t t count any twice! 29 30
How does the Bohr atom account for the discrete spectrum of an atom? Think about walking up steps, or up a ramp. What energies can you have on steps? What energies can you have on a ramp? Models of Matter so far: Model Continuous Molecular Plum Pudding Nuclear / solar system Bohr Explains appearances Brownian motion charged parts Rutherford s experiment discrete spectra Does NOT explain Brownian motion charged parts Rutherford s experiment discrete spectra see next slide 31 32 Problems with the Bohr Model 1. Why are only certain orbits possible (not like a solar system!) 2. Why doesn t the undisturbed atom radiate? (Why don t the electrons fall into the nucleus?) Because Bohr says so is not a good answer 3. Works really well for atoms with 1 electron, not well at all for other atoms. 33