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A Quick History of Chemistry With thanks to Isaac Asimov
Objectives • SOL • 1i—construction and defense of a scientific viewpoint • 2i—historical and quantum models • Chapter readings • 5, section 1 • 13, section 1
As easy as LMN • No one knows where the Latin word “elementum” comes from. We get our word ELEMENT from it. • Some think maybe the Romans had an expression that something was as simple as “L-M-N,” just as we say something is as easy as “A-B-C.” • We use the word element to refer to a substance which cannot be broken down into a simpler substance.
The Greeks • Aristotle suggested that everything was composed of 4 elements: • Water, Air, Earth and Fire • He later added a 5th element which he called “aether” which the stars and the heavens were made from. • Click on the next slide to see three of these “elements.”
Earth Wind and Fire(http://www.earthwindandfire.com/bio.html) Yes, I’m “old school.” But I love these guys. And, I even saw them in concert once, back in the 80’s. OK, back to chemistry history…on the NEXT slide…
Democritus—400 BC(http://www.gap-system.org/~history/Mathematicians/Democritus.html) Democritus was the first to suggest that matter was composed of atoms, which he called “atamos” meaning “indivisible.” Unfortunately, he came from a small “hick town” and people didn’t believe him. Aristotle, for example, ridiculed him. Because Aristotle was more respected, Democritus’ ideas faded into obscurity.
The Alchemists(http://www.crystalinks.com/bacon.html) Roger Bacon was an English alchemist who may or may not have been the first European to invent gunpowder. Some credit him; some say it was a German guy. I always thought the Chinese did thousands of years ago. Alchemists actually discovered some chemistry, but much of their work was to turn other thing, generally lead, into gold. This is called a “transmutation” reaction, and cannot be done by chemical reactions. However, in Chapter 28, we’ll learn about nuclear reactions, and turning lead into gold is now possible, but still isn’t profitable.
Alchemy • The ancient Greeks were thinkers. They talked the talk but didn’t walk the walk. • The ancient Egyptians, on the other hand, were a practical people and invented a lot of things. They made the first glass, and dyes and medicines, for example.
Alchemy • The Greeks called this “chemia,” after the Egyptian word “Chem,” which also meant black. • Chem was the Egyptians’ name for their own country. • Some people thought that chemia meant “black magic,” since some of the processes the Egyptians were using seemed like magic.
Alchemy • When the Arabs later conquered Egypt, they added the prefix “al” which means “the.” • So it became “al chemia” • And later in English, it was called alchemy. And that was appropriate because for a while, alchemy was as much phoney-baloney magic as it was real science.
An Alchemist at work…(http://en.wikipedia.org/wiki/Alchemy) Paracelsus was a famous alchemist who actually discovered salt in about 1530. Salt had been known for thousands of years, but he realized some of its “properties” and was probably the first person to realize that elements could come together to form compounds, which are totally different from the elements they are made from. Paracelsus also is credited with discovering zinc. The alchemist in the picture may or may not actually be Paracelsus, but it’s a famous painting of one.
Robert Boyle(http://en.wikipedia.org/wiki/Robert_Boyle) A 17th century British nobleman (the youngest of 14 children born to the Earl of Cork). He met Galileo and was an alchemist. Maybe the last of the alchemists and the first one to be a “real” scientist. Boyle invented a vacuum pump, did many experiments on gases, and is credited with “Boyle’s Law.” P1 x V1 = P2 x V2 This law states that pressure and volume are “inversely proportional” to each other; in other words, as pressure goes up, volume goes down, and vice versa.
More on Boyle • Boyle started calling himself a “chymist” because the term alchemist had gotten a bad name. The spelling was eventually changed to chemist. • He questioned the old Greek notions of the elements. He even did some experiments which proved that water, still thought to be an element, wasn’t an element at all, but actually a compound.
Elements • Even in Boyle’s time, a few substances were known, but they weren’t known to be elements yet. • For example, gold and silver and copper and lead were known since the ancient times, but they weren’t known to be elements. • It’s also thought that alchemists actually did discover 4 elements in the middle ages (As, Sb, Bi and Zn).
Elements • By 1700, about 14 elements were known. By the end of the 1700’s, around 1783, another 11 were known. • Chemistry was evolving during this time, but few chemists paid attention to the quantitative aspects of chemistry. They observed, but they didn’t measure. • In the late 1700’s a French chemist changed all that. He was Antoine Lavoisier, but first…
Phlogiston and Priestly(http://en.wikipedia.org/wiki/Joseph_Priestley) Phlogiston was a theory that explained how things burned and what happened when they did. Things that burned would release phlogiston to the air (so-called phlogisticated air). When a substance had used up all of its phlogiston, it would stop burning. And if you could remove the phlogiston from the air (dephlogisticated air) things would burn again. Joseph Priestley was a main supporter of this theory. He also was the inventor of something much more interesting: carbonated beverages, specifically soda-water.
Antione Lavoisier(http://www.answers.com/topic/antoine-lavoisier) Father of Modern Chemistry Proved that air was composed of 1/5 oxygen and 4/5 nitrogen Demonstrated experimentally the principle later renamed “The Law of Conservation of Mass.” Proved that hydrogen and oxygen combine to form water, proving at last that water was a compound. Beheaded on 5/2/1794 by guillotine during the French Revolution at age of 50.
More on Lavoisier • By insisting on careful measurement and thoughtful experimentation, Lavoisier turned chemistry from a series of interesting observations into a real science. • He explained the results that others had gotten. They knew what they had done. Lavoisier helped to explain why these things had happened. • He studied combustion reactions and discovered the importance of oxygen in both combustion and respiration.
More on Lavoisier • Lavoisier figured out that Priestley’s dephlogisticated air made things burn, and he renamed this as oxygen. • He also figured out that phlogisticated air was nitrogen (sometimes, carbon dioxide was also identifed as phlogiston). • He also replaced the phlogiston theory with a new theory of combustion. He said that when something burned it reacted with oxygen or was “oxidized.”
More on Lavoisier • He also invented the system of naming chemicals that we use today. • Prior to Lavoisier, people who discovered things named them whatever they wished. • He also published the first modern chemistry text (Traité élémentaire de chimie) thus spreading his knowledge literally around the world.
John Dalton(http://www.intute.ac.uk/sciences/blog/wp-content/uploads/2007/09/johndalton.jpg) A Quaker schoolmaster (became a teacher at the age of 12) who studied all sciences, but made his greatest contributions in chemistry. Developed Atomic Theory and Law of Multiple Proportions. Atomic Theory helped to explain many of the observations that scientists were making. Law of Multiple Proportions helped to explain that 2 elements could combine to form more than 1 compound; for example CO and CO2.
Dalton’s Atomic Theory • 1. All elements are composed of tiny indivisible particles called atoms. • 2. Atoms of the same element are identical. The atoms of any one element are different from those of other elements. • 3. Atoms of different elements can chemically combine with one another in small whole-number ratios to form compounds. • 4. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element cannot be changed into atoms of another element by chemical rxns.
Indivisible? • Well, Dalton did this work in the early 1800’s. • We know now that atoms are composed of protons, neutrons and electrons. Dalton didn’t know about them—they hadn’t been discovered yet! • HOWEVER, the atom is “the smallest part of an element that retains the properties of that element.” • So an atom of gold is still gold and is different from an atom of carbon.
E. Goldstein • German physicist Eugen Goldstein discovered the proton in 1886. • The proton is positively charged and determines the identity of an element. • The number of protons is a property called “atomic number.” Each element has a unique atomic number.
JJ Thompson(http://www.manep.ch/img/photo/challenges/nanotubes/thompson.jpg) In 1897, Thompson discovered the electron. Electrons are negatively charged and have almost no mass at all, compared to a proton. Thompson revised Dalton’s model of the atom with one of his own, called the “Plum Pudding Model.”
Plum Pudding Model(http://en.wikipedia.org/wiki/Plum_pudding_model) Plum Pudding is a British dessert in which plums are scattered more or less randomly throughout a cake (the pudding). Thompson knew atoms contained electrons, and knew they were negative. He also knew that the atoms overall were neutral. So, he proposed that the electrons were randomly distributed throughout. A little-known fact is that they weren’t just sitting there. In fact, they were moving, and Thompson proposed they were moving more or less in a circular fashion within the positively charged “rest of the atom.”
Ernest Rutherford(https://reich-chemistry.wikispaces.com/file/view/Ernest_Rutherford.JPG) The Plum Pudding Model wouldn’t last long, because one of JJ’s former students did some experiments that forced the model to be revised again. Rutherford was from New Zealand, and like his mentor, Thompson, also won a Nobel Prize for his work. His “work” was the famous “gold foil” experiments, where he was researching alpha particles (see Chapter 28 stuff again). As sometimes happened, Rutherford didn’t set out to discover what he actually did.
The Gold Foil Experiment(http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf) Check out the link!(http://www.rsc.org/chemsoc/timeline//pages/1911.html)Reference for below… Rutherford created a device to “shoot” αparticles at a thin piece of gold foil, literally only a few atoms thick. He expected them to go through with little or no deflection. But that’s NOT what happened. Some bounced straight back as if they had hit a brick wall!
Shocked, SHOCKED!The Nuclear Model • Rutherford was completely surprised by this result. • He had accidentally discovered the nucleus. • Rutherford said that most of the mass of the atom was contained in a small, dense center which was positively charged. • The electrons still rotated around the nucleus, but most of the atom was composed of “empty space.” • We usually call Rutherford’s model the “nuclear model.”
Neils Bohr(http://www.usd.edu/phys/courses/phys300/gallery/clark/bohr.html) Rutherford’s nuclear model only really lasted for about 3 years, before Neils Bohr (who, oh by the way, also won a Nobel Prize for this) revised it again. Bohr asked a question: if the electrons are rotating around the nucleus, why don’t they “run out of energy.” As they did, they would come closer and closer, attracted by the opposite charge of the nucleus, and eventually collapse onto the nucleus, destroying the atom in the process. This doesn’t happen, and Bohr answered why. His model is usually called “the Planetary model,” because in his model, electrons “orbit” the nucleus much as our planets orbit the Sun. Soccer goalie on Denmark’s 1908 Olympic team AND a Nobel Prize winner!!
Bohr’s Planetary Model(http://www.rsc.org/chemsoc/timeline//pages/1913.html) But the electrons don’t just orbit anywhere. They actually exist in orbits that Bohr called “energy levels.” Each energy level has a certain amount of energy. Electrons can move to a higher energy level by gaining energy. Or they can drop to a lower energy level by losing (or emitting) energy.
Energy Levelshttp://library.thinkquest.org/C006669/media/Chem/img/bohr.gif • An energy level is a “region around the nucleus where an electron is likely to be moving.” • The first energy level (n = 1) has the lowest energy. It is called “the ground state.” • Things in nature prefer to be in the lowest possible energy state.
Spectral Lines for Hhttp://www.daviddarling.info/images/hydrogen_spectrum.gif • Electrons can ABSORB energy and move to a higher energy level. • This is called “an excited state.” If an electron moves from n=1 (ground state) to n=3, it is in an excited state. • When an electron loses energy, it drops to a lower energy level. When an electron loses energy, we say it EMITS energy. If that energy is in the visible part of the spectrum, we can see those transitions. The lines are characteristic for hydrogen. They are like a fingerprint to identify H. The Ballmer series is the only ones you can see, but the others can be detected.
More Bohrhttp://www.daviddarling.info/images/hydrogen_spectrum.gif • In Bohr’s model, the energy levels get closer together as you get further away from the nucleus. • If the electron gets far enough away from the nucleus, it can escape (n = ∞). • We no longer have an atom. We have an ion, since the atom has lost an electron. The electrons can jump from one level to another. They can jump more than one level at a time by absorbing or emitting enough energy. An electron cannot jump to a spot midway between levels (n ≠ 2.5)
Need for a Better Model • Bohr’s model has some limitations. • It worked very well for hydrogen (the simplest atom with only 1 electron). It allowed scientists to make detailed calculations that explains the behavior of H. • It didn’t work for other elements, mostly because the caluclations were so detailed and complex they couldn’t be done. • It also violated the Heisenberg Uncertainty Principle (but that hadn’t been discovered yet). We’ll get to that.
The Modern Model of the Atom • Many scientists (Louis DeBroglie, Max Planck, Albert Einstein, Erwin Schroedinger, and many others) worked on the model of the atom. • Actually, they weren’t working on the model of the atom. They were just working on interesting scientific problems. But they all made contributions to our current understanding of the atom. • Quantum mechanics is the “modern” model of the atom. By the early 1930s, it had been “born.” It’s the model we still use today.
Heisenberg Uncertainty Principlehttp://www.ostheimer.at/mambo/images/stories/Werner_Heisenberg_Tafel.jpg • The Heisenberg Uncertainty Principle states that for a very small particle, such as an electron, you cannot know both its exact momentum and its exact position at the same time. Since momentum = mass x velocity and since the mass of the electron is known, for all practical purposes, the Heisenberg Uncertainty Principle says that you can’t know both the position of the electron and the speed of the electron, at the same time.
More Heisenbergcheck out Heisenberg on YouTube link on website • You can know where it is, but you won’t know how fast it is going. You can know how fast it is going but you won’t know exactly where it is. • Is that true for any “particle” or just for electrons? • It is true for any particle, but for large particles (and compared to an electron, even a grain of sand is infinitely huge) the uncertainties are so incredible small that it seems as if you can know it’s exact position and it’s exact speed.
Photoelectric Effecthttp://www.guidetothecosmos.com/images/slide12_plus.jpg Gee, bet this guy never amounted to anything • The photoelectric effect was discovered by Albert Einstein. • He found that light of a certain energy could “knock electrons loose” from certain metals. • Oh BTW, Einstein published “Theory of Relativity” 6 years later.
Light Knocks Electrons Off of Atoms, if it has Enough Energy • Alkali metals (lithium, sodium, potassium, etc) seem to be very prone to this, if the light is of a sufficient energy. • Einstein called this the photoelectric effect. In this way, light is behaving not as a wave but as a particle.
Photoelectric Effect, So What? • Anyway, you might not be terribly impressed with Einstein’s discovery. • However, if electrons can be pried loose from the metal, they can move around. • If they can move around, the movement of electrons can generate a small amount of electricity. • If you can capture this electricity, you can do useful work.
Solar Power • Solar power is based off of this principle. A photoelectric cell is constructed which has a certain type of metal in it. • When sunlight shines on it, some of the electrons are pried loose. • The cell generates an amount of electricity. • With hundreds or thousands of these in series, you can take a small amount of power generated in each cell, and multiply that by the total number of cells, and use that generated power to do work in your house.
OK, well so what? • This was one of the assumptions that helped lead scientists to quantum mechanics. • While in graduate school in France, a young scientist named Louis de Broglie asked himself this question • If light can act as a particle, can a moving particle also act as a wave?
De Broglie Equationhttp://jkphysics.in/images/De-broglie.jpg • The answer was yes. • λ = h / m x v • λ = wavelength • h = Planck’s constant • m = mass • v = velocity • In the study guide, we calculated the wavelength for a baseball pitched at 90 miles per hour, using de Broglie’s equation. • And the wavelength for the baseball is 8.2 x 10-38 meter. Of course, we have no instrument capable of detecting such an incredibly small distance.
It’s all Starting to Come Together Now… • But, electrons have masses which are much, much less, and they have wavelengths which can be measured much more easily. • So if particles could act as a wave, and electrons are particles, would it help our understanding of the atom to think of electrons as “waves?” • The answer was yes and quantum mechanics was the result. Previously, scientists had treated electrons just as particles, and tried to use all the normal math techniques that they used on particles they could see. Those techniques worked well with large particles, but with electrons, not so much.
Quantum Mechanicshttp://www.hmi.de/bereiche/SF/SF7/PANS/english/nobel/Schroedinger/Schroedinger_01.jpg • When scientists started to use the math techniques that they used with waves, everything started to come together and make total sense. • Erwin Schroedinger finally made the “connection” between deBroglie’s work and Bohr’s work. • He said that electrons weren’t orbiting in certain orbits around the nucleus, but instead described them as being found in “certain geometric forms around the nucleus.” • We’re going to call these areas where we find electrons “atomic orbitals.” His deceptively simple but really complex equation.
A Magic Carpet Ride… • Our little history tour took us from Democritus in 2,400 BC through a series of important discoveries (many of which were Nobel Prize worthy) to Quantum Mechanics, which was developed and finalized in the 1930s. • Quantum mechanics describes the behavior of atoms very well, and so we think it’s a good model.