How much specific elements sink inside a planetary caldron with variations of heat and thermals and chemistry, depends not just on their density but also the element's chemistry. This question discusses Uranium in Earth's core and the top answer suggests (I believe correctly) there's essentially no Uranium in Earth's core, it's largely in the crust because Uranium readily oxidizes, so it forms lighter elements that don't sink but rise with the silicates and basalt, which is (roughly) similar density. Uranium floats in the Earth's mantle because it binds with Oxygen.
Free Oxygen (O2) should destroy any Germane, but there's likely so little free Oxygen in Jupiter's atmosphere that the Germane is able to be present (in a fraction of parts per billion - it's very rare in Jupiter, but it's detectable).
Also, given Jupiter's internal heat, thermodynamically unfavorable doesn't matter much. There's enough heat that chemistry tends to operate more in an equilibrium balance, and not always in the exothermic direction, like we usually see in most natural reactions at temperatures we see on Earth.
Finally, and at risk of stating the obvious, Germane is one of those "weird" heavy gases, due to the atomic electrical symmetry, and because it's a gas, not a solid and given Jupiter's significant thermal currents, it mixes in Jupiter's atmosphere. It likely freezes out of Jupiter's upper atmosphere when it gets too cold, but it's present in the lower atmosphere, present enough to be detected anyway. (I think it's less than 1 ppb). - That's a brief answer anyway. I've not read any of the recent articles on this. They're all pay articles. I invite anyone who has read them to give a more detailed answer.
So what the heck is it doing there, considering free germanium would
sink and free hydrogen would float?
I want to add that gases behave differently than solids or liquids. In chemistry, sometimes things mix and form solutions (like water and alcohol), sometimes they don't and for layers (like oil and water).
Gases are particularly good at staying mixed and not layering, especially where wind and updrafts are present.
why aren't scads of other simple components mentioned in discussions
of Jupiter's atmosphere, like methylamine
There is Methylamine in Jupiter's atmosphere. Saturn's too. This research study mentions "bands" of it. This one also discusses it.
urea, and oxamic acid, to name just a few?
Urea is a solid at room temperature. It decomposes when melted so it's not stable as a gas. It melts at 133-135 C. It might never be stable on Jupiter as a gas since it's not even stable as a liquid above 135 C.
Oxamic Acid, isn't even stable by itself, it's water soluble. It's also Oxygen abundant, and free Oxygen is rare on Jupiter.
So those last 2 aren't good examples, neither is a stable as a gas at all, Germane is stable in Jupiter's atmosphere, though it's present in very small quantities.
An important point to consider with trace gases in planetary atmospheres is formation and half life. Methane, for example isn't stable in Earth's atmosphere. It reacts with Oxygen, though at a few hundred parts per billion, it's rare enough that it doesn't react a lot. Methane is also constantly being produced in the stomachs of mammals (gas) but it also gets destroyed, reacting with oxygen whenever it gets near a flame or lightning, combining with Oxygen to form CO2 and Water. It has a half life of about 10-15 years, but because it's constantly being created by incomplete digestion and decomposition, it's always present in our atmosphere - in about 1 part per million.
CO2 has a longer half life in our atmosphere, mostly it isn't destroyed, it needs to be rained out. It's also constantly being resperated, destroyed in photosynthesis and re-created by respiration.
Point of all this is that any trace element in a planet's atmosphere, must have a source of creation and a half life of, destruction if you will.
A curious exception on Earth is Argon. Argon is a byproduct of radioactive decay and as a noble gas, it doesn't react with anything, so it gets created and it just stays in our atmosphere. It gets created slowly but because it's never destroyed, it's currently about 1% of our atmosphere.
GeH4 follows the same basic guideline. It gets created inside Jupiter and it's stable enough in Jupiter's atmosphere that it's equilibrium between creation and destruction leads to about 1 part per billion. I don't know nearly enough to say what it's atmospheric half life is.
As you said, there are "scads" of elements in Jupiter's atmosphere. There re countless more that are NOT in it's atmosphere. For an element to be present in an atmosphere it needs some degree of stability and some form of creation.
Jupiter is nearly 90% hydrogen. It's not practical for it's hydrogen to float around the heavier elements. Think of it as a sea of hydrogen, with other stuff in the hydrogen, some mixes and floats, some sinks, some chemically reacts, but the hydrogen is always present.
Anyway, I only meant to touch on some of the basics and not give so long an answer.
I mean literally, how did you figure it out? What did you google? I
ask because I'm dealing with hundreds of compounds and I'm hoping you
found a nice centralized location for information like that.
We're getting a little outside the guidelines here, but briefly, I looked up the molecular structure of each of the compounds you suggested and they didn't look like gases to me. They looked too complex and crooked shaped, so I was suspicious from the get go.
But the answer to your question is simply that I checked Wikipedia, which can be long (Urea) or very short (Oxalic acid), and not always 100% reliable. Once there I did a word search for "boiling point" and "Melting point" when boiling point didn't show up. For any molecule to be a gas it needs a boiling point, so that's the first quick and easy thing to search.
You can also look for phase diagrams, which are great, but rare for anything but the most common molecules.
For Oxalic acid, I was wrong, it is a solid. Wikipedia only mentioned it in an aqueous solution. This site gives it a melting point but no boiling point, which suggests to me, it doesn't make it as a gas, or, at least, not easily. Maybe under very specialized circumstances, perhaps in a pressure tank filled with noble gases, but I'm just speculating.
Googling "Melting point" "boiling point", next to the name of the element isn't briliant, but it can provide a quick answer.
Gases tend to be smaller molecules, noble gases or when more massive, they need to be symmetric. A chemist probably wouldn't put it that way, but that's the gist of it.
Take line 2 of the periodic table, Lithium (number 3) to Florine (number 9) - ignoring noble gases for now. These atoms have 4 electron pair orbitals in their outer shell which want to form a tetrahedron. CO2 has 2 double bonds - making it a straight line, or non-polar is the chemistry term. I used the word symmetrical in my initial answer. Non-polar is, I think, more correct.
That non-polarity or symmetry is why CO2 doesn't bond easily with other things, because it's a straight line, with equal charges on each end. because it doesn't bond easily with other molecules, it's a gas at relatively low temperature. CH4, while a tetrahedral, not a line, is also non-polar and has an equal charge on all sides, so it also remains a gas at quite cold temperatures.
Water (H20) is different. Oxygen shares 2 single bonds or pairs of electrons with Hydrogen, it fills it's other 2 outer electron pairs itself, so it's bent shape and it's polar. This polarity gives H20 a side with a positive charge and a side with a negative charge. That polarity helps it bond with itself. That's why water stays wet or stays ice in much warmer temperature than similar mass molecules like CH4 and heavier molecules like CO2.
For a gas, non-polarity helps. A straight or tetrahedral, or flat triangle or six sided polyhedral (kind of a cube shape) and a few carbon chains are, as far as I know, the only options for non-polarity. Very light, polar molecules can be gas molecules too. All this varies somewhat with temperature and pressure, so there's no exact answers.
Common gases at room temperature. - note, this is not a complete list.
I'll resort them and add a few.
H2 (hydrogen) 2.02
N2 (nitrogen) 28.01
O2 (oxygen) 32.00
F2 (fluorine) 38.00
Cl2 (chlorine) 70.91
BR2 (bromine) - at about 60 degrees C.
I2 (Iodine) - at about 180 degrees C.
AT2 (Astatine) - at about 337 degrees C.
It's worth noting that all of these, with the exception of Hydrogen and Nitrogen, bond more readily with other elements and are unlikely to be present in a planetary atmosphere. (Photosynthesis creating O2 being an exception to this rule).
He (helium) 4.00
Ne (neon) 20.18
Ar (argon) 39.95
Kr (krypton) 83.80
Xe (xenon) 131.30
Radon (radioactive, short half life, but it is a gas)
These can be present in a planet's atmosphere as they are mostly chemically inactive. (Xenon is slightly reactive).
Gas molecules (non-polar)
CH4 (methane) 16.04
NH3 (ammonia) 17.03
C2H6 (ethane) 30.07
PH3 (phosphine) 34.00
CO2 (carbon dioxide) 44.01
C3H8 (propane) 44.10
C4H10 (butane) 58.12
BF3 (boron trifluoride) 67.80
SF6 (sulfur hexafluoride) 146.05
(I'll say more about these shortly).
Gas molecules, polar
HCN (hydrogen cyanide) 27.03
CO (carbon monoxide) 28.01
NO (nitrogen oxide) 30.01
H2S (hydrogen sulfide) 34.08
HCl (hydrogen chloride) 36.46
N2O (dinitrogen oxide) 44.01
NO2 (nitrogen dioxide) 46.01
O3 (ozone) 48.00
SO2 (sulfur dioxide) 64.06
CF2Cl2 (dichlorodifluoromethane) 120.91 (only a little polar)
H20 (a gas under right temperature and pressure)
There are more polar molecules than non-polar, but outside of the somewhat odd CF2CL2, all polar gases are relatively light, SO2 being the most massive, molecular weight of 64.
You mentioned Methylamine which is basically ammonia (NH3) where one of the hydrogens is replaced by a methyl (CH3) group. NH2CH3.
DiMethylamine (CH3)2NH is also a gas at about 7 degrees C and up (boiling point).
Playing around with temperature and variations on the gas molecules (replacing H with CH3, replacing H with NH2, replacing H with OH, but remember, Oxygen tends to be spoken for, like a perfect 10 at a dance, so that's not a good one, unless there's life and a source of oxygen (photosynthesis).
Similarly the "Ane" series, more accurately called the Group 14 hydrides. Group 14: carbon, silicon, germanium, tin, and lead, and the hydrides, Methane, Silane, Germane, Stanane, Plumbane. All of these are polar and all are gas molecules. Most are very reactive with Oxygen. Methane requires a flame, but the other 4 react with Oxygen quite easily.
And as temperature goes up, you add new gases, but heat tends to destroy complex chemistry, so there's a bit of a trade-off. There's no easy answer as to what could be a gas and what couldn't, but starting with the building blocks and swaping might be a place to start. That doesn't always work though. CO2 is non polar and a gas. SO2, even though Silicon is in the carbon group, is polar and bent. It's not a gas (it's closer to sand) with a very high melting point.
So, the disappointing answer is, sometimes you can go down the column in the periodic table and find another and another gas simply by replacement and sometimes you can't. In the case of SIO2, the bonds are very different than CO2, and the melting point is over 3,000 degrees. Explained in detail in this question here.
Some of the Hexaflouride series is interesting. 4 of them are stable gases at Earth's temperature and generally non reactive enough to breath and sound like James Earl Jones when you talk, but they're not likely to be found in a planet's atmosphere cause they're vulnerable to photodisintigration and not likely to be reformed in significant numbers Other molecues are more likely to form.
And, ofcourse You could have a planet with a 3,000 degree surface temperature and all sorts of elemental gases, see cool periodic table with temperature slide) and at 6,000 degrees, all the elements are basically gases, but temperature that high, destroys any complex chemistry so you don't get complex molecules. Also, at temperatures that high, the exo/endo thermic direction no longer applicable. Molecules tend to form back and forth in an equilibrium ratio. That bit I remember from School.
On the Germane in Jupiter question, it does raise some other questions, what about the other Group 14 hydrides? What about Saturn? It's possible that Saturn has too little metallic hydrogen to create much Germane.
The trick in general is both formation and stability. If a gas has too short a half life (like if it's vulnerable to photodisintigration like a lot of carbon chains are), then it's not likely to last.
Jupiter also has storms and wind powerful enough to bring up some elements that aren't necessarily gas - like dust in a sense, up high into it's atmosphere, some carbon chains, sulfur and phosphorus and ammonium hydro-sulfide, (which, despite having a boiling point, it's more accurate to say it separates into Ammonia and Hydrogen Sulfide at 56.6 degrees C, in the cold upper clouds of Jupiter the two elements could combine, into something like a dust. Technically it's a salt. Source.
Apologies if I got carried away. I love thinking about planetary atmospheres. Can't wait till the J.W.S.T. gets a picture of some in other solar systems.
My kitten is trying to delete what I wrote, so I'll post it now - will tidy up later.