Transcript Document
Where are we? We have measured mass of proton: 1.66 x 10-24 g We have measured mass of electron: 1836 times lighter than proton We have measured charge of proton: +1.602 x 10-19 Coulombs We have measured charge of electron: -1.602 x 10-19 Coulombs We know protons are at the center of atom. Neutrons were found—fixed the problem of “missing” atomic masses. We expect neutrons to be at the center of atom, too (why?). We imagine electrons are distributed around the atom somehow, like satellites in orbit. Because the electrons are so fast, we imagine them doing most of work of chemistry: the currency of chemistry is electrons! Pinpointing Electron Positions ….required 2 breakthroughs, one conjecture, and a lot of theoretical work. The Einstein/Planck breakthrough (early 1900s). 1) Light can have particle properties! 2) The particles are called photons 3) Each photon carries energy of: E hc h = Planck’s constant c= speed of light Graceful swans vs. hummingbirds Hey, what what are the units of energy? Energy is the capacity to do work. It has work units. Work is force times distance. Force is mass times acceleration. So…. E W = f d = m a d Units: kg (m/s2) m = kgm2/s2 This thing is called a Joule Joule, Calorie, Speed of Light 1 Joule = 1 ntm = kg m2/s2 4.184 Joule = 1 Calories = 0.001 Food Calories (more later about this). Planck’s constant: 6.63 x 10-34 Js c = 3 108 m/s (= 186,000 miles per second) Sample problem Remember Emission Spectra Bohr atom (the conjecture): electrons have to stay on specific paths. Dotted orbit: the “ground” state of hydrogen. Solid orbit: one of the excited states of hydrogen in a discharge tube. When electron in excited state drops to lower state, light given off. When an electron comes out of a high orbit, into a lower one, light is given off. (Think space shuttle landing). From the wavelength, you can calculate the difference in energies between the orbits. D E = hc / Also interested in periodicity, Bohr proposes that the number of electrons each orbit can support grows with the energy of that orbit, described by an “energy quantum number”, n: # of electrons = n = 1, 2, 3…. 2 2n = 2, 8, 18… 2 2n Hey, it works….for small atoms. 1st row of periodic table: 2 atoms 2nd row: 8 atoms 3rd row: ooops! That is a big “ooops” and, besides, Bohr was not able to explain WHY the electrons had to traverse these discrete, quantized orbits. The de Broglie Breakthrough (1924) Wave-particle duality = h/mv or... mv= h m = mass, v = velocity Problems Stop here? But now we need to understand waves. pronounced “lambda” pronounced “nu” Oldest physics joke: what’s n? Waves diffract. They stand. Wave addition is weird. Bohr’s circular orbits turn out to be wrong, but we can at least see why orbits have to be quantized. When you work the math… •the actual position of electrons can't really be specified •best we can do is say where they PROBABLY are •they are likely to be in cloud-like zones (called orbitals, not orbits) of varied shape •electrons with more energy can assume orbitals of increasingly bizzarre shape. These shapes sort of "fall out of" the mathematics, and it is not really easy to get a physical feel for them. •these shapes are described by "secondary quantum numbers" (the orbital number, n, proposed by Bohr is the primary quantum number). Shapes of atomic orbitals http://www.albany.net/~cprimus/orb/ n=1, l=0, m=0 n=2, l=1, m=0 n=3, l=2, m=0 n=2, l=0, m=0 n=3, l=1, m=0 n=4, l=3, m=1 s-type p-type d-type (top) f-type (bottom) Quantum Numbers Our purpose •Introduce the four quantum numbers •Explain how they relate to chemical periodicity •Explain how they relate to orbital shape & size •Give some more examples of physical periodicities •These are really the keys--keys as in open the door to molecular understanding. •So pay attention! The Energy quantum number determines how much diversity you can have. Energy-rich electrons can "buy" more “fancy” quantum states than energy-poor ones. Shapes of atomic orbitals http://www.albany.net/~cprimus/orb/ n=1, l=0, m=0 n=2, l=1, m=0 n=3, l=2, m=0 n=2, l=0, m=0 n=3, l=1, m=0 n=4, l=3, m=1 s-type p-type d-type (top) f-type (bottom) Example 1 (row 1) if n = 1, then you can have l = 0 only. You can only have ml = 0. ms can be +1/2 or - 1/2. Notice: there are only two elements, H and He, on top line of periodic table. Example 2 (row 2) if n = 2, you can have l = 0 or 1 For l = 0, you can have ml = 0 only. ms can be +1/2 or - 1/2. Check out Li and Be For l = 1, you can have ml = 0 or +1 or -1 For each of these three ml values, you can have ms = +1/2 or - 1/2 for a total of SIX states Check out the six elements: B,C,N,O,F,Ne Crayola Periodic Table 1s 2s 2p 3s 3p 4s 4p 3d Electron Configurations There are all kinds of mnemonic devices for this (see your textbook) but the easy solution is: Just follow the periodic table! Example: Calcium = 1s22s22p63s23p64s2 Why does Calcium behave rather like Barium? Because both end in s2 What's it mean? The ending electron is the one with highest-energy. This is the one that does the most work. Its “work habits” are the same in either element. Daily Evaluation/Amusement Q. Q. What is quantize? Ions vs. isotopes. Q. What are Joules? A. To make or be discrete! A. different neutrons vs. different electrons. A. A measure of energy; 4.184 of them can heat 1 mL of water by one degree centigrade. Scientific notation & using it: see me! Why everything can be a wave. Some in my pocket to read out loud. Less amusing, but important • Will we get some kind of study guide for the exam? • What info on the exam? • Why don’t we do more homework problems in class? • What to put on cheat sheet. • Why homework problems are assigned that do not have answers in the back of the book. Announcements Reminder: homework posted, due March 1 •Midterm #1, Thursday, Feb. 24 (next week!) •Help session: Tuesday, Feb. 22, 5 p.m., • some classroom on this floor—TBA. •Graded team homeworks will be outside of my office Friday ~ 10 a.m. Sorted by team. •I retain the “official” answers, but you can take your individual answers for study. •The graders worked pretty hard to make CORRECTIONS and SUGGESTIONS! •Answer key will be posted in display case near my office. (Maybe also on web). What was the single most important thing you learned today? What one thing do you wish you understood better? What is your action item?