Coppery keys to chemistry

Here we are again, confronting another interesting chemistry topic. It’s back to the periodic table today, as we continue our march to the Nobel group. Before we go any further, I feel like I need to restate again what an incredible thing the periodic table is. As much as I joke about how dry the table can be, the fact that we have such a visual, user-friendly and logical method of ordering elements by physical and chemical characteristics might just be one of the most incredible accomplishments of chemistry. No matter how complex the chemistry, everything that happens is based on a relatively small number of laws, some general tendencies of matter, molecules and charges, and the characteristics of elements. The periodic table lays all of those characteristics out in a way that can be almost instantly grasped. No endless lines of text, no subdivided columns, cells and subscripts of subscripts. Can you imagine what a purely text periodic table would look like? Probably nothing good.

Tried to find an image, unable to find someone crazy enough to make an all-text table

 So, today, we’re going to be looking at that upper right section of the marvelous table. One of the fun things about moving towards the right side of the table is the increasingly cool names that the element groupings acquire - none of this “metalloid” business here (descriptive enough, but rather bland). We’re in to the vaguely alchemical-sounding Greek-derived roots here, and today’s sterling example of nifty names in science is the chalcogens.

 

Chalcogen, for those of you not as up on your ancient Greek, translates (roughly) to “copper former”. In addition to the chemical characteristics we’re about to explore, the chalcogens all tend to be found in copper ores. Keep the “-gens” ending in mind, as we’ll be encountering it again next week.

 

They're in here somewhere...

So, besides hanging out with copper, what do the chalcogens have in common? First of all, they are not metals. Additionally, oxygen, sulfur, and selenium all have 4 electrons (out of a maximum of 6) in the outermost valence shell (as does tellurium, but it is counted as a metalloid). Remembering back to what we’ve seen of how atoms achieve that coveted full outermost valence shell, a mostly-full valence shell is much more likely to gain additional electrons than it is to shed electrons.

Chalcogens can either gain two electrons and become an anion with a charge of -2, or else form covalent bonds (typically with other non-metals). In fact, covalent bonds involving chalcogens are probably the single most important bit of chemistry on the face of the Earth. Remember back when we took a look at hydrogen bonding, that weird form of bonding that was only possible because the covalent bonds between oxygen and hydrogen atoms in water molecules contain some charge? Those semi-charged bonds are a direct consequence of the fact that water, as a chalcogen, is more likely to gain an electron than to lose it. Even when in a covalent bond, electrons are going to be drawn to the chalcogen. And that is how we wind up with the partially charged covalent bonds that make hydrogen bonds possible. 

 

Thank you, electron-gaining tendency of chalcogens.

Oxygen is easily the most recognizable element within the chalcogens - the name recognition may have something to do with the fact that we breathe oxygen. Without turning this into a post about human physiology, I’ll say that we, along with quite a few life forms on Earth, use oxygen at a cellular level in order to convert sugar molecules into energy through aerobic respiration. Oxygen is quite reactive, and capable of combining with just about anything. Oxygen is also abundant - it makes up 21% of the atmosphere. This was not always the case. Way back in the Archaean, billions of years ago, there was relatively little oxygen present in the atmosphere, and those organisms in existance practiced anaerobic (no-oxygen) respiration. However, oxygen is by-product of photosynthesis. The rise of photosynthetic organisms raised the oxygen content of the atmosphere, which, in turn, made it possible for more organisms to use the more efficient aerobic respiration, leading to life as we know it.

Not pictured: life as we know it.

Enough about oxygen for now. How about sulfur? Well, it’s also fairly common. Sulfur is the tenth-most abundant element in the universe, and crops up with some regularity in both mineral compounds and fossil fuels. The later can be an issue, as burning fossil fuels releases to the atmosphere, among other things, sulfur. Once there, the sulfur forms acid rain, with corresponding consequences for any ecosystems receiving that low pH precipitation.

How did sulfur wind up in fossil fuels in the first place? Aren’t those largely hydrocarbons? Well, yes, mostly. Fossil fuels are the fossilized remains of plants and animals, which, like most life on Earth, are mostly carbon, hydrogen, oxygen and nitrogen. Those four elements can be expanded out by the addition of sulfur and phosphorus to a group of six elements, which make up the vast bulk of all organic molecules. So, those fossils in the fuel may well have contained sulfur prior to being fossilized.

Alternately, sulfur came into the picture a little bit later. Sulfur belongs to the same group as oxygen, which would imply that sulfur behaves at least somewhat similarly to oxygen. And, in fact, there is an entire class of bacteria that live in non-oxygenated environments, and use sulfur to fulfill much the same role as oxygen in respiration. The process of converting organic matter into fossil fuel commonly involves these sulfur-oxidizing bacteria, leading to sulfur being incorporated in fossil fuels, and released upon burning.

It's round, it's pink, it breathes sulfur. Cool!

How about selenium? Well, selenium is the most metallic of the chalcogens, to the extent that it is occasionally included in the metalloids, and can be used as a semi-conductor. Selenium occurs in minerals (including that aforementioned association with copper ores), and is a nutrient to plants and animals, in small doses. Selenium is toxic at high doses - in certain soils, selenium is highly available, and accumulates in plants. When cattle eat those plants, they develop selenium poisoning - a fun fact admittedly not overly connected to the chalcogenic properties of selenium, but of note to any readers thinking of starting a cattle farm. Test your soils for selenium!

The chalcogens are a small group, but what they lack in numbers, they make up for in how many roles they play in chemistry. It figures that, with elements going up to number 118 (and rising), you're going to get a certain number of overachievers like oxygen. The nonmetals are a real cluster of such overachievers, as you might guess by the fact that we had to separate seven elements over two weeks, when the 38 transition metals all fit neatly into one post. It may be the natural bias towards organic chemistry of an organic life form, but I would argue that much of chemistry is contained within this small group of elements, together with next week's subject, the halogens. Stay tuned!