Nonmetal Hydrides

Copyright © Karl Dahlke, 2023

The previous chapter explored gases lighter than air, and heavier than air. This chapter presents some exotic compounds, usually gases, formed by combining nonmetals with hydrogen. These substances are usually flammable and toxic. The exception in every way is water, H2O, the saturation of oxygen, which is inflammable (in fact it puts out fires), necessary for life, and liquid.

Start at the top, in column IV, with carbon. Sate each bond with hydrogen to get methane, CH4. This is a colorless odorless gas, and it is harmless, unless you light a match. An earlier chapter described the burning of methane and other hydrocarbons for heat and energy.

Below carbon is silicon, leading to silane, SiH4. Unlike methane, silane has a sharp smell similar to concentrated vinegar, and is toxic. 1% concentration can be lethal. Silane is also flammable, but the products are SiO2 and water, instead of CO2 and water. Burn a room full of silane gas and you might find a layer of sand (silicon dioxid) on the floor. Flammable is an understatement - almost any impurity, even moisture in the air, can act as a catalyst and cause silane to spontaneously combust. Of course the gas can be smelled well before it reaches flashover concentrations.

Moving down column IV we find germanium, leading to germane, GeH4, Tin, stannane, SnH4, and lead, plumbane, PbH4. Like silane, these are colorless, pungent, toxic, flammable gases. It is surprising that plumbane, with a hefty molecular weight of 211, should be a gas - but it is small and symmetric, having the same compact shape as methane, and thus does not liquify until -13 degrees C. At your next party you can casually ask your guests if there is a gas that contains lead - indeed there is.

Move to column V and start at the top. Nitrogen combines with hydrogen to make ammonia, NH3. This is a colorless gas with a strong, pungent odor. Ammonia dissolves in water to form ammonium hydroxide. Most hydrides dissolve in water to produce acids, like hydrogen chloride forming hydrochloric acid, as described earlier, but aqueous ammonium hydroxide is a base. Dissolved ammonium hydroxide is sometimes referred to as simply "ammonia" in casual conversation, as in, “This cleaner contains ammonia.” Indeed, the smell of ammonia is familiar to all, thanks to these household cleaners. This is something you don't want in your lungs, yet that is precisely what happens if you take a deep breath of ammonia gas, hence its toxicity. NH3 mingles with the water in your breath and in your cells and becomes ammonium hydroxide, which eats away at your lungs. 4% concentration can be lethal in 10 minutes.

Phosphine, PH3, is a colorless gas that smells a bit like fish or garlic. It is very flammable and very toxic. The heat of a summer's day can ignite a mixture of phosphine and air, and the stunningly low concentration of 11 parts per million can be lethal in four hours.

Traveling down the periodic table, arsine, AsH3, stibine, SbH3, and bismuthine, BiH3, are similar to phosphine - almost as toxic and almost as flammable.

The next column, column VI, begins with oxygen at the top, leading to water, which is familiar to all, so let's descend to sulfur, producing hydrogen sulfide. This is a colorless gas with the distinctive odor of rotten eggs. It burns with a blue flame, yielding water and sulfur dioxide. The latter is another gas, with the distinct smell of a lit match, and its own toxicity. Aside from the fire hazard, H2S is toxic on its own. 0.07% concentration is lethal in an hour. Though not as effective as chlorine, hydrogen sulfide was used, on two occasions, as a chemical weapon during World War I.

The issue of sulfurous poisoning was addressed, tangentially, in the 1968 film Hellfighters, starring John Wayne. This is not one of the ten John Wayne movies that they show over and over again, so if it happens to come across your tv or entertainment service, you might want to watch it, but not with high expectations. Roger Ebert called it a “Slow moving, talkative, badly plotted bore”. Watch it anyways, at least once, it's not that bad. Chance Buckman, played by Wayne, leads a team that extinguishes oil well fires around the world. Vera Miles plays Madelyn, Buckman's ex-wife. Divorced for 22 years, she is still very much a part of his life, and she goes to Venezuela to watch him battle a particularly dangerous fire, five oil wells in a tight line burning all at once. Though not central to the plot, these oil wells vent hydrogen sulfide into the air, and are called "sour". This is not unusual; in 1975 an oil drilling operation in Denver City Texas released hydrogen sulfide into the air and killed nine people. But in the movie, the oil wells are burning. The raging fire oxidizes H2S into H2SO4, hydrogen sulfide into hydrogen sulfate - so declares the movie script, which identifies hydrogen sulfate as the offending poisonous gas. It's plausible I suppose - H2SO4 is a liquid at room temperature, but heat and wind could disperse it as a vapor or aerosol through the air, like steam from a pan of boiling water. H2SO4 combines with water in the lungs or the eyes to make sulfuric acid, which is extremely corrosive. Buckman elaborates, “Eight men were in the field house when it caught fire 100 yards away. Seven are dead, and the eighth is blind.” With this as backdrop, Buckman orders gas masks for his team, and takes extra precautions. As always, he knows exactly what to do, and shows no fear.

Move down the column of elements and land on selenium, giving Hydrogen selenide. This is a colorless gas that smells like decayed horseradish. Because of the heavy metal selenium, H2Se is even more toxic than H2S. Six parts per million will kill a rat in an hour. The story is very much the same for the next two compounds: H2Te, hydrogen telluride, and H2Po, hydrogen polonide, but only if you can keep them together long enough to make some measurements. They tend to separate into their constituent elements spontaneously, and H2Po is radioactive, as is everything containing polonium.

I'm going to sweep through column VII in one go. We've already described elemental chlorine reacting with water to produce hydrogen chloride gas, which then dissolves in water to produce hydrochloric acid. This acid is ferocious - thus the use of chlorine gas as a chemical weapon in World War I. The other halogen hydrides are similar. These gases become acids in the lungs and are extremely toxic. However, they are not flammable, because fluorine, chlorine, and bromine bind more tightly than oxygen. All other things being equal, it takes energy to dislodge a chlorine atom and replace it with an oxygen atom. The smells of these gases are similar - pungent and acrid, like a swimming pool magnified a thousand fold. 0.1% concentration is generally deadly. At lower concentrations these gases can still damage the eyes, the corneas in particular, resulting in permanent blindness. The last compound, hydrogen astatide, HAt, exists only briefly in the lab, so measurements are not practical. It decomposes quickly into its constituent elements, and astatine is radioactive with a short halflife.

Below is a section of the periodic table, showing the element and its symbol, the associated hydride, its formula, its molecular weight, and its boiling point. Most of these compounds liquify at very cold temperatures, even those with high molecular weight. Water and hydrogen fluoride are the clear exceptions. Despite being small, compact, and lighter than air, they remain liquid at room temperature. Water in particular has a boiling point hundreds of degrees above the other hydrides. Hydrogen sulfide, just below water, has approximately the same shape and size, and is somewhat heavier, so it should have a higher boiling point, or so one would think, yet the boiling point drops by 160 degrees C, or 288 degrees F. chemistry is indeed mysterious. Temperatures in this chart are given in celsius.

Carbon C
CH4 16 -161
Nitrogen N
NH3 17 -33
Oxygen O
H2O 18 100
Fluorine F
hydrogen fluoride
HF 19 19
Silicon Si
SiH4 32 -112
Phosphorous P
PH3 33 -87
Sulfur S
hydrogen sulfide
H2S 34 -60
Chlorine Cl
hydrogen chloride
HCl 36 -85
Germanium Ge
GeH4 76 -88
Arsenic As
AsH3 77 -62
Selenium Se
hydrogen selenide
H2Se 80 -42
Bromine Br
hydrogen bromide
HBr 80 -66
Tin Sn
SnH4 122 -52
Antimony Sb
SbH3 124 -17
Tellurium Te
hydrogen telluride
H2Te 129 -2
Iodine I
hydrogen iodide
HI 127 -35
Lead Pb
PbH4 211 -13
Bismuth Bi
BiH3 211 16
Polonium Po
hydrogen polonide
H2Po 256 36
Astatine At
hydrogen astatide
HAt 211 ?

As mentioned above, water is quite an anomaly. Why should it remain a liquid at such high temperatures? Many papers have been written on this subject. The OH bond is just the right length, the HOH angle is just right, and the dipole moment is just right, so that the molecules stick together and form a liquid. If this was not the case, I don't think life, or at least complex life, would be possible. I realize this is an earth-centered view, but hear me out. Chemistry plods along slowly, if at all, at cryogenic temperatures. Titan contains rivers and lakes of liquid methane, but we don't expect to see any critters there. Reactions move at a glacial speed, if at all. Multiply this by the billions of years that evolution requires to make a multicellular organism, and animals simply can't evolve on a cold planet. At the same time, temperatures above 100 degrees C rattle the delicate molecules of life to pieces. Life exists in a narrow range of temperatures, say -10 to 70 degrees, and life needs some kind of liquid to support its underlying chemistry. If we didn't have water, could any other substance take its place? Is there a planet somewhere with oceans of ethanol, C2H5OH, and fish swimming about? Over geologic time, ethanol will degrade into water, methane, and carbon dioxide, so this picture is not plausible. Other room temperature liquids are similarly unstable over time. An ocean of mercury might persist under an anoxic atmosphere, but no planet contains that much mercury. So I stand by my claim that water is necessary for life, and we're damn lucky water is a liquid, else we would not be here to talk about it. Of course this is just speculation, and we won't know for sure until we send probes to the stars and explore the galaxy.

Let's examine one more compound for the road, this time a metal hydride. diborane has the formula B2H6. It is a colorless gas, hence each diborane molecule is small and self-contained. The boron atom has two electrons that hide in the innermost shell and do not participate in chemistry, leaving three valence electrons, ready to form three bonds. Now this is a puzzle. Take a moment to arrange two borons and six hydrogens into one stable molecule. Remember, it's not BH3, which would be the analog of methane, and entirely intuitive - it's B2H6.

Here is the trick. Join boron and hydrogen with a covalent bond. Each atom contributes one electron, and the resulting electron pair joins the two atoms together. However, this electron pair is also shared by the other boron atom, living in its outermost shell as well. Yes, one electron pair connects three atoms. This is called a banana bond. Diborane has two such bonds, arranging two borons and two hydrogens in a diamond shape. Place two borons next to each other left to right, then place a hydrogen above these two borons forming a triangle, then place another hydrogen below these two atoms forming another triangle. Four atoms are joined together using four electrons, one electron from each atom. Each boron still has two valence electrons remaining, and these are satisfied by two hydrogen atoms on each end. That places four electron pairs in the outermost shell of each boron atom, and these pairs are tetrahedral, just like methane. If the BHBH diamond lies flat on the page, as described above, then the terminal hydrogens to the left of the first boron atom lie above and below the page, completing the tetrahedral structure, and the terminal hydrogens to the right of the second boron atom lie above and below the page as well. This is the diborane molecule.

Diborane is a colorless gas with an odor that has been described as repulsive and sweet. Its boiling point is -92 degrees C, another hydride that is very much a gas anywhere on earth. It is highly flammable, not a surprise there, but it also reacts violently with water, even moisture in the air.

B2H6 + 6H2O = 2B(OH)3 + 6H2

This reaction releases heat, which in turn ignites the liberated hydrogen in the presence of oxygen or air, and boom, an explosion. Beyond this physical hazard, diborane is toxic. 40 parts per million is lethal in four hours.

The next element down in column III in the periodic table is aluminum. This builds a polymer, rather than a self contained molecule, but the next element down, gallium, builds another gas, digallane, Ga2H6. This is extremely difficult to synthesize in the lab, and it breaks down quickly under ambient conditions. When cooled to -50C it becomes a white solid polymer, like its aluminum cousin.