# Tag Info

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The first estimate of Avogadro's number was made by a monk named Chrysostomus Magnenus in 1646. He burned a grain of incense in an abandoned church and assumed that there was one 'atom' of incense in his nose at soon as he could faintly smell it; He then compared the volume of the cavity of his nose with the volume of the church. In modern language, the ...

16

Here's the key point: If [the nucleus] was still uranium what does that say about the internal clock of the nucleus which had been ticking for 99% of the half-life ? The answer is "nothing," because a single nucleus doesn't have an internal clock. Here's a better model for what's happening in an unstable nucleus. Think of it as an enclosed container ...

15

You are thinking of a lego or a jigsaw puzzle. You have to think of huge numbers and tiny dimensions . There are 10^23 molecules in a mole. The atomic dimensions are less than 10^-9 meters. The lattice that has been broken will not fit the puzzle if it has been moved further than this last small distance because the molecular forces will not match. In ...

15

No, the elements of the periodic table don't form any representation of a group or, more precisely, any irreducible representation. Even more precisely, the real insights by Mendeleev – that the reactivity etc. is a repeating function of the atomic number – doesn't follow from any property of a representation that could be derived by group theory. The ...

15

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 ...

11

I found two web pages that explain the phenomenon quite well, and even looks into the misconceptions people have. The candle flame heats the air in the vase, and this hot air expands. Some of the expanding air escapes out from under the vase — you might see some bubbles. When the flame goes out, the air in the vase cools down and the cooler air ...

10

Bonds - lots of bonds Chemical reactions take in energy to break bonds and give off energy when they make bonds. Big organic molecules like those in petrol have lots of weak carbon-carbon and carbon-hydrogen bonds which don't take a lot of energy to break. But when it burns the combustion products make lots of very strong Carbon-Oxygen and Hydrogen-Oxygen ...

10

The first undeniably reliable measurements of Avogadro's number came right at the turn of the twentieth century, with Millikan's measurement of the charge of the electron, Planck's blackbody radiation law, and Einstein's theory of Brownian motion. Earlier measurements of Avogadro's number were really only estimates, they depended on the detailed model for ...

10

There does seem to be a lot of mythology around about the "grape in a microwave" experiment. I have never see any publications on the subject in a respectable journal, however from chatting to other scientists there seems to be a consensus about what happens. It's all rather boring really. The grape is the right size (about a quarter wavelength) and shape ...

10

First of all you need to understand how the rubber is held together in the first place. Rubber, plastics, carbon fibre and pretty much all forms of living tissue are held together in the same manner. All, or a large part of the molecules are long strings, either repeating the same simple pattern, or a number of slightly different patterns organised in random ...

9

Avogadro's number was estimated at first only to order of magnitude precision, and then over the years by better and better techniques. Ben Franklin investigated thin layers of oil on water, but it was only realized later by Rayleigh that Franklin had made a monolayer: http://en.wikipedia.org/wiki/Langmuir%E2%80%93Blodgett_film If you know it's a monolayer, ...

8

This is actually a more complex question than you might think, because the distinction between mass and energy kind of disappears once you start talking about small particles. So what is mass exactly? There are two common definitions: The quantity that determines an object's resistance to a change in motion, the $m$ in $\sum F = ma$ The quantity that ...

8

Although there is no known group representation which encapsulates all the properties of the periodic table, there are, however, attempts to gain a representation theoretical understanding of the periodic table at least qualitatively and there are recent works mainly by M. Kibler in this direction, please see the following two articles ...

8

It is an interesting question, I will take a stab at it. Hopefully my answer will be generic enough to remain plausible. First of all the ISM (Interstellar Medium) which collapsed to form the sun would not likely have been well mixed. It would be some sort of turbulently mixed mixture of recent SN remnants, and the molecular cloud with which it presumably ...

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 ...

8

235U is a nonrenewable resource in that when we use it in weapons or power plants it is not regenerated (and eventually after a long series of decay steps, mostly ends up as lead). However, there is a not entirely negligible amount being extruded by mid-oceanic rifts and volcanos, so there will continue to be a small supply that becomes physically reachable ...

8

The photons of light, especially ultra violet photons from sunlight, carry energy that can break chemical bonds. The bonds in chemicals that make up colours, ie. dye molecules, are most affected because they create the colour effect by absorbing some photons (of one colour) and not others. Since they have chemical bonds of the correct energy to ...

8

The key fact you don't seem to realize is that the radioactive nuclei have no memory. Each second is a new second, and the probability the nucleus decays in the new second does not depend on how long it has been around. The only reason the decay rate of a material decreases with time is that after time a fraction of it has already decayed. Thus when the ...

7

A simple material will not undergo a liquid to solid transition as the temperature is raised. When you see this it means somthing more complicated than a simple phase transition is going on. In the example of egg white, what you are seeing is denaturation of the protein albumin. The heat causes the protein to lose its tertiary structure then form cross ...

7

Due to the crystal structure of the solid phase of water, the molecules arrange themselves in a rigid, ordered fashion and end up being, on average, farther apart from each other (than they are in the liquid phase), and thus less dense. Less dense things float because of buoyancy.

7

For the first rows in the periodic table, this can easily be explained by the fact that electrons possess four quantum numbers (usually $n$, $l$, $m_l$ and $m_s$). These numbers are restricted as such: $$n = 1, 2, 3, ...$$ $$l = 0, 1, 2, ..., n - 1$$ $$m_l = -l, -l + 1, ..., 0, ..., l - 1, l$$ $$m_s = -\frac{1}{2}, \frac{1}{2}$$ By Pauli's ...

7

The sodium and chloride ions actually separate in water, turning solid NaCl into Na+ and Cl- ions that can move freely through the solution. Electrons are one form of charge carriers and the most common, being that they have a net negative charge and are mobile inside of metals but free ions moving around in a solution also constitutes a current. EDIT in ...

6

A battery is basically just a chemical reaction. At the negative (cathode) end of the battery the reaction releases electrons while at the positive (anode) end of the battery the reaction consumes electrons. As long as the external circuit allows electrons to flow from the cathode to the anode the reaction goes and the battery generates power. If you break ...

6

The covalent bonds form when electrons attached to nearby nuclei can exist in a superposition state where they can partly be on another nucleus. This happens when the electron state they are mixing with is unfilled. For example, for H2, two hydrogen nuclei are close, there is no electrostatic energy for this in the first approximation because the electron ...

6

The divide is actually not between covalent and ionic, but rather a spectrum between localised and delocalised electrons. The history of all this is actually quite fascinating, and Phil Anderson in his book "More and Different" has a nice chapter on this. Essentially, around the time that people started doing quantum mechanics on molecules seriously, there ...

6

Dear Wade, your good question is easily answered if you consider "pressure" to be a derived quantity, and let us derive it. An average molecule (or atom) of an ideal gas - and your proposition only holds for an ideal gas - has kinetic energy equal to $$mv^2/2=3kT/2$$ It's because every degree of freedom carries $kT/2$ and there are three degrees of fredom ...

6

Decrease of the melting temperature with pressure increase is not enough to explain skating (I conducted the calculation myself, but please see http://scitation.aip.org/content/aapt/journal/ajp/63/10/10.1119/1.18028). AFAIK, skating can be explained by ice melting due to heating caused by friction. EDIT(10/22/2013): OK, so let us use the Clausius-Clapeiron ...

6

There is an earlier question on this site that addresses essentially the same issue: Conversion of mass to energy in chemical/nuclear reactions. As written in the answers there, the amount of energy that is lost or gained in a chemical reaction is roughly 10 (or more) orders of magnitude smaller than the mass of the participating molecules. I took a look ...

5

It's not often that dmckee and I differ (mainly because he's usually right :-) but we differ on this on. Or at least we differ if I've correctly understood what you're asking. In a hydrogen atom the 1s, 2s, etc wavefunctions are (subject to various approximations) good descriptions of the single electron and have well defined angular momentums. In ...

5

Yes http://www.nuffieldfoundation.org/practical-physics/volume-change-dissolving-salt-water This definitely is not material science but chemistry.

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