# Tag Info

61

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, ...

39

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 ...

38

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 ...

28

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 ...

22

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 ...

19

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 ...

18

Let's talk about the balloon first because it provides a pretty good model for the expanding universe. It's true that if you draw a big circle then it will quickly expand as you blow into the balloon. Actually, the apparent speed with which two of the points on the circle in a distance $D$ of each other would move relative to each other will be $v = H_0 D$ ...

17

No, there aren't any holes like that in the EM spectrum. There are other ways of creating photons than by having electrons bound in atoms transition from one level to another. (For example, you can create pretty much any frequency of photon you want by accelerating a free electron.)

14

Yes, quantum mechanics – even non-relativistic quantum mechanics for several electrons orbiting nuclei – fully, quantitatively, and comprehensively explains all of chemistry (including biochemistry and, in fact, biology). This fact has been known since the late 1920s. To understand the periodic character of the properties of the elements, one must realize ...

14

This is quite far from a silly thought although this is not apparent at first sight. Apart from a couple of details which are well understood and have firm physics behind them - such as the fact that deuterium and tritium exist in some proportion and the hyperfine-structure distinction between ortho- and parahydrogen, as far as we can tell all hydrogen atoms ...

13

Common sense of touching can be expressed in "scientific means" as an event when exchange-repulsion interaction between 2 objects (you and the geek) extends some arbitrary value, say 1meV. I leave finding an agreeable threshold which is easy to measure to later discussion. :)

12

Do not confuse mass with charge. Although the proton is more massive, the magnitude of its positive charge is equal to the magnitude of the electron's negative charge. Hence, neutral atoms!

12

I assume you're talking of the hydrogen atom; the hamiltonian of the nucleus + electron system is $$H = \frac{p_e^2}{2 m _e} + \frac{p_n^2}{2 m _n} - \frac{e^2}{|r_e - r_n|}.$$ You can do a change of coordinates (center of mass coordinates) $$\vec{R} = \frac{m_e \vec{r}_e + m_n \vec{r}_n}{m_e+m_n} \\ \vec{r} = r_e -r_n$$ and find the conjugate momenta to ...

11

Short answer: The space between the nucleus and the electron is not empty space, it is filled with an electron cloud. (You will understand this answer better if you read the long answer) Long answer: Firstly, physics is a description of what we can observe. Depending on the scale of which you are describing, physicists, over the years, have different ...

11

For any kind of magnetic data storage you need a magnetic state that is stable over time. The magnetic moment of an isolated single atom does not have any preferred direction, therefore the energy states are degenerated. The 12 atoms used in this experiment is not a lower limit, in principle it can also work with 2 atoms given the right magnetic ...

10

In addition to dmckee's answer this link summarizes the experiments of antiprotons catching protons and neutrons, creating temporary nuclei. It is the symmetric state to the one in the question : an anti-proton-neutron nucleus that lasts for a bit (fig 5.7). Anti-protons can be made in abundance and controlled experimentally because they are charged, ...

10

In particle physics, you have to be specific about what you mean by breaking down a particle: A neutron in an atom can decay into a proton, an electron, and an electron-antineutrino. But this does not mean that a neutron is made of a proton, an electron and an electron-antineutrino. What the neutron is made of are three quarks (one up and two down). If by ...

10

Carbon-14 makes up about 1 part per trillion of the carbon atoms around us, and this proportion remains roughly constant due to continual production of carbon-14 from cosmic rays. The half life of carbon-14 is about 5,700 years, so if we measure the proportion of C-14 in a sample and discover it's half a part per trillion, i.e. half the original level, we ...

10

First you say It's easy to visualise and comprehend the excited states of electrons, because they exist on discrete energy levels that orbit the nucleus By way of preparation, I'll note that in introductory course work you never attempt to handle the multi-electron atom in detail. The reason is the complexity of the problem: the inter-electron effects ...

10

If you could compress the mass into that small a space, it would collapse into a black hole, at which point the notion of "size" becomes harder to define, with space-time being so warped. The "event horizon" radius would be about 3 km, if I get the formula correctly. The idea of "there's a lot of space in atoms" comes from computations which state that the ...

10

In any proper quantum mechanical understanding of the atom, a bound electron does not have a position and follow a path (i.e. have a time-varying position) in the sense that it would have in a classical or semi-classical theory. Instead the electron "has a state" or "occupies an orbital" (an orbital not a orbit!), and because there is not a path there is ...

10

Does position count as a characteristic? If so, then yes, atoms do have unique characteristics. Edit: Perhaps I should rephrase this. If position counts as a characteristic, then yes, atoms do have unique characteristics.

9

Its due to the WP-Rayleigh–Taylor instability: it is an instability of an interface between two fluids of different densities, which occurs when the lighter fluid is pushing the heavier fluid. Hot hair raises and colder goes down. A Mushroom cloud formed by hot wet air : google for images or videos of 'Rayleigh–Taylor'

9

Okay, so I did some poking around and the 66th-75th editions of the CRC Handbook of Chemistry and Physics all have the incorrect atomic mass of Cu-63 [62.939598], and from 76th edition on they seem to have figured it out. Those isotope mass tables are put together from a number of sources, so it's hard (time consuming) to tell exactly where the error came ...

9

The atoms in the enamel of the teeth are not exchanged with the environment. Indeed, long after your death their isotopic composition can be used to tell where you lived while your teeth were growing.

9

If you mean to ask "do the orbital radial probability distributions overlap?", the answer is yes: Image Credit making the electrons in each orbital "meet" at some point As you can see from the image, the electron orbitals are not position eigentstates. If you're imagining two point-like electrons in different orbitals colliding, you're not thinking ...

9

The other answers are correct, but I think they miss one important point. The energy levels are not really discrete, because: Frequencies are not exactly defined. Due to the Heisenberg uncertainty principle, the location and frequency of a photon cannot both be exactly determined. Therefore, emission (and absorption) does not happen at an exact ...

9

The speed of sound in air determines how fast wave phenomena propagate in the air through collisions between molecules, it does not determine how fast other objects immersed in the air travel. If an object in a fluid like air begins at rest, then the speed of that object after some time is determined by the net external force on that object. Say, for ...

8

Long answer: Any Chemistry textbook. Short answer: The number of electrons of an atom is the same as the number of protons in the nucleus. This number of electrons (Identical to the position number in PSE!) defines all the chemistry of that atom.

8

While quantum mechanics explains the gross features of the periodic system, many fine details of the periodic table of elements are computable numerically from various approximations to QED, but are conceptually ill understood. See, e.g., Eric R. Scerri, How Good Is the Quantum Mechanical Explanation of the Periodic System? Journal of Chemical Education ...

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