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Revision as of 21:24, 11 August 2012
Welcome to the explain xkcd wiki! We already have 12 comic explanations!
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Explanation
This explanation may be incomplete or incorrect: Created by a COMPLAINING EQUATION. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon. |
This comic humorously describes the changing view of what an atom is.
- Small hard ball model
The first model shown, in 1810, is said to be a "small hard ball model." Around this time, John Dalton came up with the most famous maxim of chemistry: "All stuff is made of atoms." Dalton used the idea to explain what is today known as stoichiometry. Thus humans thought up the idea of atoms – but in lieu of any ideas of how they work, the scientific community likely thought of them as "hard round balls"; thus the name described here. Nowadays, people generally assume that atoms are small hard balls when first exposed to them, because they have seen diagrams of molecules where atoms are universally represented as small, hard, round balls connected by sticks.
- Plum pudding model
In the late 19th and early 20th centuries, the study of these "atom" things faced a crisis: where would the newly discovered "electrons" go? In 1904, physicist J. J. Thomson, who discovered electrons, had an idea: maybe the electrons were small point charges moving around in a big mass of positive charge. This was the "plum pudding model", the second model on the comic, called this because people imagined the positively charged mass as a "plum pudding". (The title text references Thomson as well, along with the humorous observation that plum puddings themselves are made of atoms.) The problem with this approach is that opposite charges generally repel, resulting in the more mobile or unbalanced charges forming a surface shell around the others, attempting to escape, rather than being content to being randomly distributed among them.
- Tiny bird model, Rutherford model
This was one of many competing ideas in the formative years of what-are-atoms-made-of-ology, where Randall claims a 1907 "tiny bird model" (the third model shown) would fit in well. But ultimately, the tentative winner in the battle was the model of Thomson's student Ernest Rutherford, who discovered that the positive charge seemed to be in the center of the atom, and put down his Rutherford model, or "planetary model", in 1911, where electrons orbit a positive charge. This model has often been compared to the orbit of the planets around the sun. This is the fourth model put down.
- Bohr model
But there were a few problems; Maxwell's equations complained, for instance, saying that accelerated (here: flying on the circle instead of a straight line) charges like the electrons would lose energy emitted as electromagnetic waves and would quickly orbit into the nucleus. Niels Bohr patched the model up with the newfangled idea of quantum mechanics, creating his "Bohr model", the fifth model shown here, in 1913. Bohr proposed that electrons could only exist in distinct "energy levels" at discrete distances from the nucleus: proposing that physics behaves differently at the small scale of atoms than the large scales we are comfortable with. This turns out to be true.
- Nunchuck model, Chadwick model
If this sounds like today's model, you didn't pay enough attention; note that at this time, nobody thought of splitting up the nucleus into protons and neutrons. But pretty soon people noticed that protons and neutrons existed; Randall facetiously suggests a "nunchuck model", the sixth model shown, of a packet of protons swinging a packet of electrons around. But more seriously, James Chadwick, who discovered the neutron, figured that the atom had a nucleus of neutrons and protons, along with a bunch of electrons orbiting around it in a Bohrish manner. This is what the layman today often thinks of as an atom, and is thus the seventh model shown here. One can imagine a handle filled with electrons bonded by the strong nucleur force to a chain made of neutrons, bonded again by the strong nuclear force to a handle made of protons. The heavier protonic handle acts loosely as an orbital center as the electron-filled opposite handle swings wildly around it, attempting to resolve its electrostatic attraction within the restraints of its chain.
- 538 Model
The eighth model shown is a "538 model" in 2008. 538 is a statistical analysis website that gained fame in 2008 for predicting every race but 2 correctly in the US presidential election. It has since been known for making mathematical models for everything; the model jokingly suggests that 538 has modeled and presumably made predictions about the atom. The pie chart shows the statistical composition of neutrons, protons and electrons, 38%, 31%, and 31% respectively. This could either be the average of a massive body with several isotopes or represent gallium-69, the most abundant isotope of gallium, with 31 protons, 31 electrons and 38 neutrons. FiveThirtyEight has previously been mentioned in several xkcd comics, including in 477: Typewriter, 500: Election, 635: Locke and Demosthenes, 1130: Poll Watching, 1779: 2017, and 2002: LeBron James and Stephen Curry. It's notable that after the websites wild success in predicting one electrion, it failed poorly on the next; a testament to how meaning can be lost when everything is lumped into similar categories based on limited observations, without noticing changes in the details.
- Quantum model
But is the Chadwick model what scientists endorse today? No! Today physicists subscribe to a quantum model, which is the ninth model shown here. Instead of electrons, there are quantum clouds, or more simply, the parts of the atom aren't in any particular point, but rather a probability field of possible locations. This is very abstract, and in the last model, the model is postulated to get so abstract that it is just a "small hard ball surrounded by math" model, the last model shown. This then is remarkably similar to the model we started out from, the "small hard ball model" (without the math).
- “Small hard ball surrounded by math” model
The picture for the "small ball surrounded by math" depicts a circle with several numbers around it. While the numbers seem to symbolize the "surrounding math" in a general sense, some of them suggest constants used in actual mathematical equations or other numbers related to the quantum model. The shapes and densities of the atomic orbitals are calculated with the Schrödinger_equation, which is complex and difficult to solve. For this reason atoms are generally precisely considered in only very simple simulations, and the details of interactions of many atoms at large scales that form our daily lives are incredibly hard to precisely understand and predict on an atomic level. It comes down to "these roundish things we call atoms are moving around in these approximate ways obeying this complex equation with too many numbers involved in most situations to accurately model, so let's use a different, empirically derived formula that describes the behavior of the system in general."
Number | Explanation |
---|---|
18 | Maximum number of electrons in the third (M) electron shell |
0.1 | 1/10th, a simple decimal |
π | The number pi present in many physics equations, often as its double value (2π); also in the definition of the reduced Planck constant present in quantum-mechanical equations. |
173 | Possibly a typo (could be 137) referring to the fine structure constant which value is approximately 1/137 |
√2 | An irrational constant, the square root of two, which comes up frequently |
4i | A simple complex number; i is considered the square root of -1 and using it can provide for more complex mathematics |
Transcript
This transcript is incomplete. Please help editing it! Thanks. |
- [One large panel with a caption centered on top and ten small drawings in two rows. Each drawing has a description below it.]
- Models of the Atom
- over time
- [A somewhat imperfectly drawn circle.]
- 1810
Small hard ball model
- [A rounded-corners trapezoid inside which there are four small plus signs and four small circles with minus signs inside them.]
- 1904
Plum pudding model
- [A bigger circle, with four birds on the surface and music notes above.]
- 1907
Tiny bird model
- [A small circle with dots circling around it, drawn with paths.]
- 1911
Rutherford model
- [A circle with a plus sign with three circles around it, each with a dot.]
- 1913
Bohr model
- [A nunchuck swinging, with the left stick filled with circles with plus signs and the right stick filled with circles with minus signs.]
- 1928
Nunchuck model
- [A nucleus with three circles around it, each with a dot.]
- 1932
Chadwick model
- [A pie chart, where a part of it has a circle, a part of it has a circle with a minus sign and a part of it has a circle with a plus sign.]
- 2008
538 model
- [A circle, with (...)]
- Today
Quantum model
- [A circle with surrounded with numbers.]
- Numbers: 18, 0.1, π, 173, √2, 4i
- Future
"Small hard ball surrounded by math" model
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