Wikipedia:Reference desk/Archives/Science/2019 November 5

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November 5

Noble metals (Ru–Pd; Os–Au)

Why do the noble metals have such high electronegativity values (2.2 to 2.54)? — Preceding unsigned comment added by Sandbh (talk • contribs) 01:50, 5 November 2019 (UTC)

I don't know but if they were very low or high they couldn't be noble. They're also in the middle of the table where you would expect middling values. Sagittarian Milky Way (talk) 02:19, 5 November 2019 (UTC)

{{ec}} Looking at the table at Electronegativity#Electronegativities of the elements reveals an even more (by eye:) anomaly in Group 6 (Cr, more dramatically Mo, and even more dramatically W, as compared to the adjacent Group 5 and Group 7;. And most of the transition-metal block has electronegativity increasing down each group, which is again not the usual trend. DMacks (talk) 02:21, 5 November 2019 (UTC)

Maybe it has something to do with metallophilic interactions? Droog Andrey already explained this for Mo and W here: "Mo/W electronegativity is higher only on Pauling scale because of strong multiple homoatomic bonds between their atoms. In chemical sense, their electronegativity is not very high." Given aurophilicity the same thing may be at work for Au. Though I notice that Smokefoot also mentions there that apart from Au, electronegativity of metals is not really mentioned much in inorganic chemistry because ligands are much more important for determining the properties of complexes. Double sharp (talk) 07:59, 5 November 2019 (UTC)

BTW, note that Droog Andrey's electronegativity scale (which is on the table he and one of his colleagues publish) seems to be accounting for this and trying to show electronegativity in his "chemical sense". On that scale (with Li through Ne fixed at 1.00 through 4.50 at intervals of 0.50), the noble metals are not so high, only 1.62 to 1.93. (Including Hs through Rg brings the upper limit only to 1.99.) Though note that even here, Au (1.93) still shows up as more electronegative than Si (1.90) and almost on par with Rn (1.94); Rg (1.99) even shows up between Ge (1.96) and B (2.00). So I guess gold is still somewhat special, even if the rest aren't really, corroborating Smokefoot's observation. Ds, Rg, and Cn should also be special in that way if you could get any significant amount (which, at the moment, you really can't). (Pt appears at 1.84, between Pb and Sn at 1.82 and 1.86 respectively; the rest appear further away, behind Ga, Cu, and sometimes even the rather active Ni and Zn.) Double sharp (talk) 08:07, 5 November 2019 (UTC)

• The answer for the OP is that electronegativity is a model, and all models are wrong (though some are useful). In the case of electronegativity, the scale is quite useful for determining bonding polarity among main group elements, but the model starts to break down when you get to the transition elements. As noted above, there have been some attempts to correct the scale so that there are not the anomalies you find with transition metals. --Jayron32 12:03, 5 November 2019 (UTC)

There are some other correlations with the high EN values of the noble metals. They occur free in nature. They collectively have the highest standard reduction potentials of the metals. They have among the highest ionization energies of the metals. Ditto electron affinity.

In period 5, I see that Ru Rh and Pd are in a race to complete their 4d subshell, so much so that Ru and Rh each have only one 5s electron, and Pd has none. Contrast their lighter 3d congeners Fe, Co and Ni each of which have full 4s² subshells. I guess that the energy levels of the period 5 4d and 5s subshells are closer together, which facilitates movement by the 5s electrons (as influenced by the nuclear charge?).

I'm not that puzzled by the period 6 noble metals Os Ir Pt and Au, since they "suffer" from poor shielding by their 4f¹⁴ electrons. And I understand relativistic effects cause a contraction of their s orbitals, which presumably increases exposure to the nuclear charge. Sandbh (talk) 00:24, 7 November 2019 (UTC)

Horizon

Horizon#Examples says that for an observer standing on the ground with h = 1.70 metres, the horizon is at a distance of 4.7 kilometres. Unless I'm missing something, this looks too large. On a city street in a clear day I, standing 1.80 m tall, without elevation, see to a distance of around 1 km at most with unobstructed view and certainly not 4.7 km. To see or spot objects that far from me I need to walk or ride further. Do I misunderstand something? 212.180.235.46 (talk) 22:39, 5 November 2019 (UTC)

Very slight slopes of 1 part per hundreds would be enough to make the horizon a kilometer away. Sagittarian Milky Way (talk) 00:46, 6 November 2019 (UTC)

They aren't considering visibility. In many places fog or smog makes 4.7 km visibility quite rare. You said a city street, so some pollution is to be expected. It isn't all that apparent when pollution limits visibility from 4.7 km to 1 km. SinisterLefty (talk) 04:31, 6 November 2019 (UTC)

I was always told that a man of 2 meters could see 5 km if he was standing on the beach on a clear day. Hence without any obstruction or without any hills. I would suspect that this should be a known calculation and could quite easily be calculated by using the circumference of the earth. This may be best referred to the Mathematics desk... Thanks. Anton 81.131.40.58 (talk) 09:06, 6 November 2019 (UTC)

The math is literally given at the article linked by the OP. Someguy1221 (talk) 09:13, 6 November 2019 (UTC)

If the the temperature graph by altitude is unusual enough it can also refract the horizon closer. Sagittarian Milky Way (talk) 13:20, 6 November 2019 (UTC)

And if there's a dark surface in the direction viewed, rising and falling air currents of different temperatures and densities can also cause visual distortions. SinisterLefty (talk) 16:16, 6 November 2019 (UTC)

In a city you're really unlikely to find ground flat enough that you could see it curve away. Even roads that look totally flat to the naked eye can have a rise or a drop of a meter per kilometer, which is only felt by cyclists and probably creates the appearance that the road is flat but disappears behind the horizon too soon (when the rise breaks). Even if you're standing on a truly flat and straight road, but there's a two-meter-tall invisible hillock a kilometer away, it will still end your horizon. 93.142.93.32 (talk) 22:15, 6 November 2019 (UTC)

That's true. To find a "flat" (meaning spherical) surface on Earth, your best bet would be a small lake on a windless day, so no waves get in the way. SinisterLefty (talk) 05:28, 7 November 2019 (UTC)

But beware of optical trickery. Tigraan^{Click here to contact me} 10:16, 7 November 2019 (UTC)