Lecture 3: The Discovery of the Electron and the Structure of the Atom

Lecture 3: The Discovery of the Electron and the Structure of the Atom Summary This lecture began with discussion of ground-breaking experiments that brought the scientific community closer to understanding the structure of the atom. These included J.J. Thomson’s experiments with the cathode ray tube (CRT), which allowed him to find the charge-to-mass ratio of electrons, which he realized were negatively charged particles found in all atoms. Robert Millikan was able to ascertain the mass and the charge of the electron using his oil drop experiment. The atom was not the most fundamental particle after all, rather it was made of smaller particles like electrons, and as we learn next those smaller things that make up atoms have a structure to them. The electronic structure. Building on the radiation studies of Marie and Pierre Curie, Ernest Rutherford was able to identify three types of particles found in radiation: alpha particles, which were large and positively charged, beta particles, which were small and negatively charged, and gamma rays, which didn’t deflect or interact with the photographic plate in Rutherford’s experiment at all. Rutherford then conducted the gold foil experiment, which showed that atoms have positive charge that is localized in the core of the atom rather than dispersed among the negative charge, as Thomson had proposed. This positively charged center of the atom is called the nucleus. The positively charged particles in the nucleus are called protons. For any neutral atom, the number of protons present must equal the number of electrons present. This number is called the element’s atomic number, which is denoted by the letter Z. Except for the case of a neutral hydrogen atom composed of only one proton in its nucleus and one electron, all atoms also contain neutrons in their nuclei, which are, as the name might suggest, neutrally charged particles. To obtain the mass number of a given atom, the number of protons and neutrons in that specific nucleus are added together. Atoms of the same element can have different numbers of neutrons. This makes them different isotopes of the same element. To obtain the mass given for each element in your periodic table, a weighted average is taken of the mass numbers of that element’s naturally occurring stable isotopes. The weight is assigned according to the isotopes’ percent abundance in nature. After Rutherford’s experiments, there remained the question of how an atom structured in this way could be stable. Niels Bohr was the one who postulated that the angular momentum of the electrons in atoms must be quantized, which means that it takes on discrete values. This meant that the electron’s energy and distance from the nucleus were quantized as well—the electron can only occupy certain orbits. Why this matters The discovery of the electron led to a profound new understanding of the atom, which in turn led to an entirely new theory (quantum mechanics), which in turn led to orbitals, bonding, and the building blocks for all of modern chemistry, as we’ll see soon. But the discovery of the electron also led to a new capability that revolutionized the world: the ability to paint with electricity. The phosphor screen that was on the end of Thomson’s cathode ray tube was the precursor to television, and the beginning of a technological revolution that changed almost every aspect of our lives. TV wasn’t invented by Thomson himself, but Thomson’s discovery of this new fundamental paint brush – the electron itself – was crucial for the concept to take off. Look at what the man on the left in this image is holding: it’s an early version of a TV, which you can now recognize as a giant cathode ray tube! John Logie Baird was one of the first to commercialize television sets, selling his first one in 1925. You can see the inside of an early TV in the middle picture: note that it was comprised of a CRT plus a coil to induce a magnetic field – essentially the same setup that Thomson used in his groundbreaking experiments. If electrons were scanned across the phosphor screen fast enough, an image could be painted so many times per second that it looked like motion to the human eye. This was easy to do, because changing a magnetic field quickly and precisely (to position electrons on the screen) can be achieved with simple electronic circuitry. Over the next generation, electrons became a dominant way to exchange visual information. The chemistry of the screen itself is also essential. A ‘screen’ is actually a phosphor coating that was often put on a piece of glass and was the target of the electron beam. Phosphor is a broad term for materials that emit light in response to a stimulus. This act of emitting light is called luminescence. It can get confusing since the Man with CRT & open TV images © sources unknown. term “phosphorescent” means that the material slowly emits light over time after being stimulated. A good example of a phosphorescent material would be glow-in-the-dark toys, where the stimulus is light, and the phosphor can keep glowing for many hours. On the other hand, a “fluorescent” material also emits light in response to a stimulus, but as a quick flash lasting nanoseconds. The term phosphor can refer to either of these, but the coating on the CRT screen and in those early televisions is the fluorescent kind. You can imagine why: if it didn’t emit light quickly,, the image would have afterglows that lasted for hours, which wouldn’t exactly make for a nice movie-watching experience. To make matters even more confusing, the term phosphor (which as we just established can refer to materials that are phosphorescent or materials that are fluorescent), is not to be confused with the element phosphorus. Phosphorous also emits light, but of a different nature entirely, since its glow comes from a chemical reaction (a process called “chemiluminescence”). So, to sum up: the screen we’re talking about uses a phosphor which is fluorescent, not phosphorescent, and doesn’t have any phosphorus in it. Great. Chemically speaking, what are these phosphors? There are so many different materials that are phosphors that it would take an entirely separate book to go through them all. They can be small molecules, complex solids, or liquids. They can emit light in response to many different stimuli, from light to heat to electric fields to the electrons we’re discussing here in our CRTs. The first television screens were black and white, so the goal was simply to use phosphors that glowed white when they were struck by electrons. Powders made from zinc, cadmium, and sulfur and a dab of silver were 15 early ingredients. In order to achieve color TV, blue, green, and red phosphors were needed. The electron beam could be pointed at whichever ones were needed to make a given color, again using the magnetic field to guide the electrons with fantastic precision. It turned out the green and blue phosphors were pretty straightforward to make, zinc-sulfide with a touch of copper for green, and silver for blue. But red was a different story: for red phosphors it took additional decades’ worth of research to find anything that worked. This is the reason why color television didn’t come on the scene until the mid-1960’s, a full 40 years after black white television was introduced! The red phosphor challenge was finally solved by using a complex mixture of yttrium, oxygen, sulfur, and a bit of europium for good measure. Since we’ve listed yttrium, I have to mention that it is one of four – yes four! – elements that were discovered in the same cave in the same town, Ytterby, Sweden. Yttrium, terbium, erbium, and ytterbium are all named after this town. We’re still waiting for cambrium or bostonium. Now, it’s only your parents or grandparents who will remember what watching TV on a CRT was like. That’s because TVs nowadays work on a different principle, namely light emitting diodes (LEDs). But in the end, it’s still painting with electrons! An LED emits light using the cascade of electrons from high energy to low energy in a solid material. We’ll learn all about that material (it’s called a semiconductor) and why electrons can make light this way a little later in the semester. The light that comes out of an LED in your TV (or phone or tablet) still often passes through a phosphor, since the phosphor can be used to fine-tune the color of light we see. In this case the phosphor is stimulated by light instead of electrons, but the outcome is the same: light and color are produced using electrons and chemistry. Why this employs How can we get a job based on the discovery of the electron? That’s kind of like asking how we can get a job related to the knowledge of atoms – in other words, electrons and atoms make up everything so anything would qualify, at least if it has any reference to matter in the employment description. But how about if we go back, specifically, to that first beam of electrons that Thompson made to discover their very existence. . . what jobs might involve beams of electrons. We already discussed painting with electrons in the topic of displays in Why This Matters, but it turns out there’s a whole lot more you can do with a beam of electrons than just make pictures. To do so, though, we’ll need more power. A lot of power. If you crank up the power of that electron beam enough, and you keep it in a vacuum (just like Thompson did) to make sure they don’t scatter off of anything, then you can use the electron beam to create very localized heat, enough to melt any material. If it’s done to join two materials together in a precise manner we call it welding. Electrons have a lot of advantages in welding because they can be focused to very small areas using magnetic fields, and the power can be dialed in to whatever the job requires (and there’s almost no limit to how powerful you can make a beam of electrons). On the higher end, for industrial welding, electron beams can get up to much as 10,000,000 Watts per mm3 which can heat up a metal at a rate of 1,000,000,000 Kelvin per second! Electron beam welding is a big industry, and there are many jobs associated with it. You could weld metal parts together at GE for its aviation business, and I mention that company specifically because it was James Russel who invented electron beam welding while he was at GE, back in the 1950’s (as a fun side note, he also invented the first CD). If you do a search for electron beam welding you’ll find huge numbers of listing for companies that provide it as a service to other companies. Bodycote, for example, states that they are, “the world’s leading provider of heat treatments and specialist thermal processing services,” and with 5,700 employees in 26 countries they have a lot of jobs. But there are many smaller scale operations that hire people who like to shoot massively high energy electrons at metal parts, like the family-owned Roark in Brownsburg, IN, which boasts one of the largest electron beam welding chambers in the U.S., or Precision Technologies, Inc., which has a cool-sounding 5-axis electron beam welder that goes up to 150 kV, “capable of welding parts as small as a few grams up to ones weighing several tons,” or how about Fraunhofer FEP which develops customized electron beam technologies and states, “Using electrons we improve your materials and products!” Cool. Extra practice 1. Your friend Wonder Woman tells you that her bracelets are made of a metal, Feminum (this is legit, it was in the ‘70s TV show), only found on her home island. Feminum (symbol Fm) has atomic number 120 and atomic mass 285.47. The element has 2 isotopes, 285 Fm and X Fm. The natural abundance of 285 Fm on the island is 77%. a. What is the other isotope, and what is its natural abundance? b. How many protons and neutrons does each isotope have? 2. The number above the Li symbol on the periodic table (on the right) is the number of protons in a lithium nucleus. What is the number below the symbol? How do you calculate it? 3. Assuming all the protons (red) and neutrons (blue) are visible, use proper notation to write out the atom that corresponds to each of the following nuclei: Image courtesy of CK-12 Foundation, License: CC BY-NC. Solutions 1. 77 % abundance of one isotope implies 23 percent abundance of the other, since we know there are only two isotopes. The atomic mass is the weighted avg. of the two isotopes. Solve the weighted average equation to obtain the missing mass number: (0.77)(285) + (0.23)(x) = 285.47 x = 287.04, rounds to 287 2. The number below the atomic symbol is the atomic mass. It’s not specific to the atom—it’s a weighted average of the element’s stable (non-radioactive) isotopes, with weights given according to the isotopes’ abundance in nature. For example, lithium’s atomic mass is 6.94, and its two stable isotopes are 7 (92.4% abundance) and 6 (7.6% abundance) 3. a) mass : 13, atomic : 6, atomic symbol: C b) mass : 7, atomic : 3, atomic symbol: Li