# Tag Info

175

For organic matter, such as bread and human skin, cutting is a straightforward process because cells/tissues/proteins/etc can be broken apart with relatively little energy. This is because organic matter is much more flexible and the molecules bind through weak intermolecular interactions such as hydrogen bonding and van der Waals forces. For inorganic ...

159

You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is accelerating continuously and would thus radiate away its energy and fall into the nucleus. One of the reasons for "inventing" quantum mechanics was exactly this conundrum. The Bohr model was proposed to ...

121

Wow, this one has been over-answered already, I know... but it is such a fun question! So, here's an answer that hasn't been, um, "touched" on yet... :) You, sir, whatever your age may be (anyone with kids will know what I mean), have asked for an answer to one of the deepest questions of quantum mechanics. In the quantum physics dialect of High Nerdese, ...

109

The questions of whether you can detect light emitted from an (isolated) atom and whether you can resolve an atom from its neighbours are completely independent. The spacing between different atoms in a regular material remains impossible to resolve using visible light, whose wavelength is several thousand times larger. You can "see" individual atoms by ...

96

This entirely depends on what you mean by "see". Let me start of by noting: As per my knowledge, atoms are small beyond our imaginations No. Atoms are quite big compared to certain other things we play around with, like its constituents (protons, electrons) in particle accelerators. The size of atoms is of the order 0.1 nanometres (of course, there is a ...

83

In "back of the envelope" calculations like this, all you can really do is look at orders of magnitude. As others have pointed out, not all apples have the same size, and not all atoms have the same "size". All we can then work with is orders of magnitude, so $1\ \rm cm$ and $6\ \rm cm$ (although many people in the comments are saying $3\ \rm cm$) should ...

68

You are not correct in your latter part of the analysis; the chemical properties (which is mostly what matters in ordinary matter) almost only depend on the electron shell, and in particular the outermost electrons (called the valence electrons). So more protons mean more electrons and a different electron shell, meaning different chemical properties. Why ...

53

I think the other answers which mention electrostatics capture the physics behind things being rigid correctly. However, I wanted to specifically point to your question of "why are they rigid when they're mostly vacuum?" I'd like to draw your attention to Guyed Masts: A Guyed mast is a tower whose rigidity depends on several guy-wires surrounding it. If ...

47

If protons decay, then what you say is true: all atomic nuclei are indeed unstable, and a so-called "stable" nucleus simply has too long a half-life for its decay to be observed. The most tightly bound nucleus is $^{62}$Ni, with a binding energy per nucleon of 8.79 MeV [source], which is less than 1% of the mass of a nucleon. On the other hand, the decay of ...

46

I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus. Even if the electron actually orbits the nucleus, wouldn't that orbit eventually decay? Yes. What you've given is a proof that the classical, planetary model of the atom fails. I can't reconcile the rapidly moving electrons required by the planetary ...

44

Neither of those statements are true. It's an easy approximation to make: a neutron star has all of that 'space' removed from between nucleons --- so we just need to know how big a neutron star of mass equal to the solar system would be. Well, the only significant mass is the sun (jupiter is about 1% the mass of the sun---negligible). If the sun were ...

43

Groups in Seattle, Colorado, and perhaps others managed to measure and verify Newton's inverse-square law at submillimeter distances comparable to 0.1 millimeters, see e.g. Sub-millimeter tests of the gravitational inverse-square law: A search for "large" extra dimensions Motivated by higher-dimensional theories that predict new effects, we ...

41

I'll do that teacher thing and turn your question around back at you. Why isn't the spectrum of the lithium atom just the spectrum of the hydrogen atom plus the spectrum of the helium atom? And, for that matter, why is the helium spectrum not simply two copies, somehow, of the hydrogen spectrum? Why do atoms have unique spectra in the first place? The ...

40

This is a good example of how Science works. Geiger and Marsden observed that some of the alpha particles were being backscattered. This is inconceivable if the alpha particle is scattered by a lighter particle. If one considers a particle of mass $m$ and initial velocity $v_1$ striking a target of mass $m'$ at rest, without changing its direction, then ...

37

In liquids and solids the difference in energy between energy levels becomes very small, due to the electron clouds of several atoms bein in very close proximity of one another. These similar energy levels will form 'bands' of indistinguishable spectral lines. In gases however, atoms will be spaced loosely enough such that the interaction between atoms ...

36

Wikipedia explains this rather well but I'll pick out the relevant stuff for you. Before the Geiger–Marsden experiment, the general idea was that atoms were built of some permeable positive substrate in which some negative particles were floating around; the so called plum-pudding model. If we shoot $\alpha$ particles on this setup they should all pass ...

33

To be fair, this is actually explained in your link. To put it simply, If you illuminate it with the right light, it starts shining so bright that a good camera can detect it. To make it work, the atom has to be as motionless as possible. This is achieved by "freezing it" and using magnets to hold it still. Close-up for completeness:

31

You have your "prove" in the wrong place. The way to prove that ground-state electrons in hydrogen atoms don't emit radiation is the following: Construct a sample of ground-state neutral hydrogen atoms. Place this sample near a detector which is sensitive to the sort of EM radiation you expect. Die of old age waiting for a signal, because ground-state ...

29

The cosmological estimation of the number of atoms in the observable universe works as follows: one of the Friedmann equations can be written as $$\dot{a}^2 -\frac{8\pi G}{3}\rho a^2= -kc^2,$$ where the scale factor $a(t)$ describes the expansion of the universe, $\rho$ is the total mass density (radiation, baryonic matter, dark matter, and dark energy) ...

29

Fundamental particles are identical. If you have two electrons, one from the big bang and the other freshly minted from the LHC, there is no experiment you can do to determine which one is which. And if there was an experiment (even in principle) that could distinguish the electrons then they would actually behave differently. Electrons tend to the lowest ...

28

The treatment of electrons as waves has combined with spherical harmonics (below image) to form the foundation for a modern understanding of how electrons "orbit." Tweaks to the spherical harmonic differential equations yields the Schrodinger equation, which yields the accepted models of electron orbital structures: The only element for which the ...

28

Einstein's mathematical model of brownian motion furnished strong support of the atomic model but did not furnish airtight proof of its uniqueness (that is, the nonexistence of alternative models) at the time it was proposed. It is worthwhile to note that it wasn't his objective to logically exclude the possibility of alternative models but rather to ...

27

If the question is interpreted as why don't atoms and other bound systems expand the answer is that the general expansion of space cannot do continuous work against the electromagnetic force that holds an atom together or any other force that holds a bound system together. However the accelerating expansion of the universe can exert a small "constant" ...

27

Yes, in the sense that you understand the "Why does this happen?", we really don't have an answer. That an electron emits a photon is an allowed interaction in the underlying quantum (field) theory. This process has a certain probability to occur. And that's all we can say about it. As far as we know, there is no "trigger" for the emission, it is truly a ...

27

I notice that online definitions of this experimental law always say, molecules or atoms. The problem with just calling them all "molecules" and being done with it is some are uncomfortable with using that term for unbound atoms. If you have a container of He, there are no "molecules" in it. So when it says "molecules or atoms", it means "molecules or ...

25

We are never 100% certain of anything. The scientific method falsifies wrong theories, but it does not verify those we colloquially call "correct" or "true" If we tomorrow detect a normal oxygen atom decaying, we'll have to devise new theories to explain it. But we don't expect the things we call stable to ever decay (that's why they're called stable). We ...

24

Your question starts out with questioning whether the numbers match up very well, and then you proceed to throw away all the accuracy in your numbers to demonstrate that they don't. While throwing away accuracy in-favor of considering just the orders of magnitude is useful for approximating (and is what the other answers are explaining is okay), it turns ...

23

I don't think that this is a physics restriction, but one of current engineering capability. As you link points out, using 12 atoms allowed the information to be retained without effecting the information stored next to it. You will also need enough data-mass to allow for the reading and writing of the information without affecting the data next to the one ...

23

There is a rigorous formal analysis which lets you do this. The true problem, of course allows both the proton and the electron to move. The corresponding Schrödinger equation thus has the coordinates of both as variables. To simplify things, one usually transforms those variables to the relative separation and the centre-of-mass position. It turns out that ...

22

Briefly, The Bohr--planetary model doesn't really address these issues. Bohr, a genius, just asserted that the phenomena at the atomic level were a combination of stationarity while being in an orbit, and discrete quantum jumps between the orbits. It was a postulate that yielded some agreement with experiment and was very helpful for the future ...

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