Announcements More and more people contacting me for help. Yay! Principle of fairness: we can quiz you on stuff that’s on the schedule, but not if we are way far behind it. Also, questions will usually be simple when we haven’t covered it in lecture or recitation. I will post some new MC stuff very soon.
Today’s lecture is brought to you by the numbers … 3 & 4 Russo & Silver Chapters #3 and #4. Virtual Book Chapters #2 and #3
Chapter 3 title: Evolution of Atomic TheoryChapter 4: Modern Atomic TheoryIt is really about Discreteness, subatomic particles & beginning concepts of periodicity.
The CEF guides also lay out the facts, to wit: Atoms exist. Atom means un-cuttable or indestructible (which is a lie). Atoms are incredibly small! There are enormous numbers of them. Composed of electrons, protons and neutrons. Protons weigh 1836 times more than electrons. Neutrons weigh about the same as protons. Amazingly, all the mass is at the middle of the atom. The atom’s behavior is dictated by the number of protons. All atoms with the same number of protons behave the same, even though they might have different numbers of neutrons. If you lay out the elements according to mass (or number of protons) their chemical properties repeat.
This should all be supremely unsatisfying to you: 1. How do we know? 2. Why should you care?
Dr. Ivar Giaever…..1973 winner of the Nobel Prize in physics, announced his resignation Tuesday, Sept. 13, 2011, from the premier physics society in disgust over its officially stated policy that “global warming is occurring.” The official position of the American Physical Society (APS) supports the theory that man’s actions have inexorably led to the warming of the planet, through increased emissions of carbon dioxide. Giaever does not agree — and put it bluntly and succinctly in the subject line of his email (to APS) , “I resign from APS,” Giaever wrote. Giaever was cooled to the statement on warming theory by a line claiming that “the evidence is incontrovertible.” “In the APS it is ok to discuss whether the mass of the proton changes over time and how a multi-universe behaves, but the evidence of global warming is incontrovertible?” he wrote in an email to Kate Kirby, executive officer of the physics society. http://dailycaller.com/dc-trawler/#ixzz1Y2oB2T1L
Funny he should mention protons. Just recently there is news on protons: we have a new radius estimate! Old: rp = 0.8768(69) fmNew: rp = 0.8775(51) fmSo…how many sig digs is that? rp
Now we are going to dissect some of the historical observations that led us to believe atoms are as we depicted them in previous lecture, which was advertised as a “preview”. For example, how do we know mass of proton?
Chemistry is "discrete"; it obeys integer arithmetic. There is precisely ONE sodium atom per ONE chlorine atom in salt. There are TWO hydrogens per ONE oxygen in water. Exactly. WHY?
But if there are so many atoms, how can we count? What beats counting?
Chemists count by weighing. This example shows polyethylene pellets. 0.0144 gram/pellet 5.94 grams of pellets How many pellets? Now, do you suppose all polyethylene pellets have the mass of exactly 0.0144 grams? http://www.bp.com/genericarticle.do?categoryId=9013612&contentId=7021458
Formation of HCl from H2 and Cl2 2 grams of H2 plus 71 grams of Cl2 = 73 grams of HCl 4 grams of H2 plus 142 grams of Cl2= 146 grams of HCl etc. It is always 35.5 g of Cl2 to 1 g of H2 Any other ratio results in "leftovers"
Formation of HI from H2 and I2 2 grams of H2 plus 106 grams of I2 = 108 grams of HI 4 grams of H2 plus 212 grams of I2= 216 grams of HI etc. Any ratio mass other than 1:53 results in "leftovers"
Some elements produced more than one substance, like Carbon & Oxygen. • Substance 1: • 12 g C + 16 g O Ratio: 12/16 = 3/4 • Substance 2: • 12 g C + 32 g O Ratio: 12/32 = 3/8 • Substance 3: • 12 g C + 48 g O Ratio: 12/48 = 3/12 = 1/4
Probably? What?!!? Deduction, Hope and Guess No compound was ever found between C and O where the C/O ratio was more than 3/4. So this is probably CO.
If the 4/3 compound is CO, then…. The 3/8 compound is CO2 The 3/12 compound is CO3
Apparently, THERE ARE RULES…for how atoms join. Not only is the idea that just one atom could be put together in multiple ways to get all the materials wrong, so also is any notion about mixing atoms in random ratios.Matter is atomically discrete!
There were LOTS of "relative mass" experiments. Ones like these 3 helped decide the periodic table. No leftovers Oxygen Hydrogen Water* Leftovers 100 g 8.2 g 73.2 g 35.1 g Oxygen 1 g 4.1 g 1.13 g 3.98 g Hydrogen 65 g 8.2 g 73.2 g None!
…….…..which leads to Oxygen being 8 times heavier than Hydrogen…unless some new H-O compound is discovered!!!!
Hydrogen peroxide In the words of a Chem1001 S07 student, Dalton eventually had to learn that “water ain’t no ho”. One of many uses: turning hair blonde.
Example: Building the Mass Table The very maximum of oxygen that will react with 1 g of hydrogen, producing zero leftovers, is 16 g. The very minimum of sulfur that will react with 1 g of hydrogen, producing zero leftovers, is 32 g. What does this say about the relative masses of oxygen and sulfur?
The Lightest In looking at all the elements that could react with 1 g of hydrogen, none was ever found that took less than 1 gram. Not only that, working through all other combinations of elements, none lighter than hydrogen was ever found. So, hydrogen becomes a convenient basis for all our masses.
Dmitri Mendeleev noticed that, if you put all the elements in order of their masses, relative to hydrogen, periodic trends appear…in how they react, how easy it is to electrify them, etc. Periodic table of the days Mendeleev at 63 years of age in 1897 http://en.wikipedia.org/wiki/Dmitri_Mendeleev
Periodic Table of Elements http://periodic.lanl.gov/default.htm
Spectroscopy (Emission spectroscopy shown) provided further evidence for discreteness…but not originally understood. Discrete Lines!
Cathode Ray Tubes were (still are!) one way to deconstruct “indestructible” atoms. All sorts of things once thought to be indestructible have turned out to be quite delicate. RMS_Titanic http://en.wikipedia.org/wiki/RMS_Titanic
What CRT experiments say: • You can see where the electrons are going, • how affected by magnet, etc. • The negative stuff of this tube is similar • to the negative stuff of the gas discharge tube. • This negative stuff behaves, in this experiment, • like a particle.
Canal tubes were used to zero in on the positive charges. Canal Ray Tube Accentuate the Positive!
The point is: beating up on atoms. Knock the • stuffing out of them to see what happens, • just like a kid with a toy! • And what happens is: • Some stuff goes to right collector (positively charged stuff). This stuff depends on the element is being deconstructed. • Some stuff goes to left collector (negatively charged stuff). This stuff does not depend on the element being beat up.
We don’t need no stinking canal ray tube. Try a pickle. http://www.youtube.com/watch?v=nNtvyI5BVEg
Radioactive decay studies provided a mixture of positive, negative & something new. • View Slide 3.6 • CRT-like device for determining charge of radioactive decay products
Here’s one of those gen ed lessons! So the main point is that we are beating up on atoms because we do not know what else to do with them. This is a perfectly legitimate take-home message for the whole course: when you don’t know what to do, start anyway, either boldly or carefully as the situation requires. On the other hand, if you don’t know what to do about your boyfriend, that doesn’t mean you should start beating him!
Because some of the laws of electricity & magnetism were known by the time of these experiments, scientists could determine that…. charge/mass = e/m = 1.76 x 1011 coulombs per kilogram Not very satisfying….what’s the electron mass, dammit? And what is its charge? Knowing the quotient is not enough!
Robert Millikan’s oil drop experiment was America’s first big success in physics. http://www.trinity.edu/fwalmsle/Millikan/oildrop.gif
Millikan oil drop demos. http://www.youtube.com/watch?v=XMfYHag7Liw http://www.youtube.com/watch?v=6ggP5W2xAP0 http://www.youtube.com/watch?v=3fNjWWew0k4
What goes into Millikan’s calculations • Size of oil drops mass of droplet • (from density = mass/volume, D=M/V) • Gravitational constant of Earth. • Electric field between top & bottom • plates of his device. What comes out of Millikan’s calculation The charge on oil droplets, one at a time!
Suppose we instead gathered information on how much money is in your wallet or purse. Alicia: $25.05 Kevin: $17.02 Javoris: $31.53 Rachel: $5.84 Clay: $128.98 Lakiya: $64.02 What is the smallest unit of money?
Suppose we instead listed scores on an exam. Alicia: 39 Kevin: 69 Javoris: 75 Rachel: 54 Clay: 18 Lakiya: 93 How much is each problem worth?
Millikan was really looking for the least common denominator, that’s all. His LCD was the charge on a single electron.
With the charge determined, he could go on to determine that electrons don’t weigh much! 1.76 x 1011 Coul/kg (from charge/mass study) and 1.602 x 10-19 Coul (from Millikan) So… 9.1 x 10-31 kg = 9.1 x 10-28 g
Oh, my goodness! That’s lighter than my offensive playbook!
Now go after the protons… Mass spectroscopy—used in drug design today.
Proton is massive compared to electron! • 1836 times heavier than the electron. • 1.673 x 10-27 kg/proton or 1.673 x 10-24 g/proton. • Suppose we add the proton and electron mass: • 1.673 x 10-24 g + 9.1 x 10-28 g = 1.674 x 10-24 g. • This is APPROXIMATELY the mass of most hydrogen atoms. • We define the "atomic mass unit" or a.m.u. as 1.66 x 10-24 g. • Compute the inverse of this number: 6.022 x 1023 hydrogen atoms/g. • Hmm......keep that number in mind. It is called Avogadro's number http://en.wikipedia.org/wiki/Proton-to-electron_mass_ratio