This Christmas morning, millions around the world looked in their stockings & hoped not to see coal. – yet they’d probably love to see the same element, carbon in a different arrangement – because diamonds and coal are the made up of the same thing! I didn’t get coal in my stocking. Or diamonds. But I did get another “allotrope” of carbon – graphite (Snoopy pencils 🙂 ) What’s the difference – all aboard the CARBON ALLOTROPE Express to find out!
We’ve talked a lot about hydrocarbons – carbon hooked up to hydrogens – and how they make great skeletons for “organic” biochemical molecules like proteins & DNA, which are based on hydrocarbons with some other elements, like oxygen and nitrogen, sprinkled in as parts of “functional groups” in place of hydrogen. But turns out carbon can be pretty cool all by itself.
The basic unit of all these elements is an atom. And in a SOLID, atoms are stuck in place & only have enough energy to vibrate. In a CRYSTALLINE solid, the place they’re stuck in follows a repeating floor plan (think of a brick wall or a tiled floor in 3D). The same element can have multiple such floor plans leading to crystals w/different properties – we call these different arrangements ALLOTROPES & carbon (C) has multiple. 3 of the main carbon crystal allotropes are DIAMOND, GRAPHITE, & FULLERENES.
Note that I did *not* say coal. COAL *is* a solid form of C, BUT it is NOT crystalline. Instead it’s an AMORPHOUS SOLID (no defined shape) – instead of following a floor plan, the C atoms in coal stick together more “willy-nilly” (protein crystallographers know all too well the difference between a crystal and an amorphous clump of protein gunk! The technique of x-ray crystallography for determining protein structure relies on protein molecules organizing in a perfectly-repeating pattern, so that, when we beam x-rays at them, those rays bounce off the atoms of each copy the same way & their signals add up – so we can capture the reflected rays on a detector and work backwards to figure out the protein structure. So we spend a lot of time trying to find the right conditions for crystallization – testing out a bunch of different “cocktails” of different salts, etc. – and looking at well after well under a microscope to look for the crisp edges of crystals but mostly seeing gunky stuff)
But anyways, back to coal…Its “random” carbon packing makes it easy to incorporate impurities because the carbons don’t have to fit into a carefully “calculated” repeating structure. So coal’s usually not *pure* carbon – instead it often also contains hydrogen, oxygen, nitrogen, & sulfur (Sulfur has a rotten-egg smell so maybe if someone’s been really naughty Santa leaves them high-sulfur coal…)
Those other elements have lots of opportunities to sneak in because coal is formed over long periods of time when dead things (which contain those other elements in addition to carbon) decay & new things pile on top of them & start squeezing out moisture. But as the decaying material (peat) gets subjected to higher and higher pressure & temperatures it’s not just moisture getting squeezed out – some impurities do too, so more pressure, less moisture & impurities. And coal can be classified accordingly.
🔹ANTHRACITE (hard coal) is the “five star” coal variety. It’s formed under very high pressure & is 86%-98% C (by weight)
🔹BITUMINOUS COAL (soft coal), the most common form, is formed under lower pressures, so it’s only 69%-86% C
🔹SUB-BITUMINOUS COAL & LIGNITE have even less C, though definition-wise, a rock must be combustible & have at least 50% C to be called coal
As we talked about the other day with candles, combustion involves breaking things down by reacting them with oxygen. When you break down carbon this way, you don’t just create heat and light – you also produce carbon dioxide. This carbon dioxide is a big problem because it traps heat, contributing to global warming. And there are other additional problems to burning coal – when you burn it you release those impurities, which contribute can react with oxygen to form pollutants like sulfur oxide (which can cause acid rain) & nitrogen oxides which contributes to smog & react with other pollutants to make worse pollutants like ozone. And some of the non-totally-combusted coal escapes as soot.
Enough with the naughty “presents” – let’s talk carbon’s crystalline forms!
Going back to the atoms – atoms are made up of a central nucleus containing positive protons & neutral neutrons surrounded by a “cloud” of negative electrons (e⁻ ) & atoms interact w/one another through their outer (valence) e⁻ which they can share to form strong covalent bonds. You need 2 electrons to make a single bond (usually one shared by each binding partner), and 4 for a double. C has 4 valence e⁻, so it can covalently bond to up to 4 other Cs
DIAMOND: In diamond, each C’s covalently bonded to this max (4 other C) to form a tetrahedron & the tetrahedrons join together to form a 3D network
🔹diamond’s very strong, unlike my delicate protein crystals, because unlike protein crystals, where the “building blocks” are protein molecules & only connect to one another through weak intERmolecular forces that involve partial charge based attractions, not actual sharing, each repeating unit of a diamond is a single C atom & they connect to one another through strong covalent bonds (like those that hold the atoms of each individual protein together). So diamond’s like 1 huge molecule but it’s not a true molecule because it doesn’t have a set # of atoms.
🔹In order for a material to conduct electricity, e- must be able to move freely through it. In diamond, all the e- are tightly held, so diamond does NOT conduct electricity
Diamonds are coal are both made from carbon, but they’re *not* made from each other – diamonds formed much deeper in the mantle part of Earth’s crust where pressures & temps are way higher. They formed hundreds of millions to billions of years ago and only made it to mineable depths thanks to really deep volcanic eruptions. Super powerful ones that blasted the pre-made diamonds through Earth’s crust so fast that they didn’t have time to incorporate impurities along the way and, even though there are more potential impurities in this higher up stuff, the diamonds are so tightly made that they’re not gonna let anything else in between their carbons. As a result you end up with pristine diamonds sitting there for millions to billions of years.
GRAPHITE: Graphite is made up of stacks of GRAPHENE -> 1-layer sheets of covalently-bonded C
🔸 unlike in diamond, in graphene, each C is only covalently bonded to 3 other C, in a flat honeycomb crystal lattice layout. Since it’s only using 3 valence e- for this “next-door-neighbor” bonding, the 4th gets “delocalized” (no longer owned by it’s original owner). Instead, it goes into a “shared” pool where it can move freely among the sheet, so electricity can move through the planes of layers (but NOT between layers)
🔸 atoms within a graphene sheet are actually held together tighter than atoms in a diamond, BUT diamond’s bonds extend in 3D whereas graphene’s strong bonds are only 2D – they’re restricted to 1 sheet. So why don’t the layers just fall apart? The same reason geckos can walk on walls – van der Waals forces! even “owned” e- are constantly moving around randomly (though their turf’s restricted) – sometimes they’ll randomly clump more to one side, leading to partial charges that can lead to temporary charge-charge interactions w/other atoms in other sheets. And you have lots of surface area to have these interactions, so it’s kinda like how if you alternate the pages of 2 phone books, the friction between the pages can make for a super super strong joining. So the layers don’t just fall apart but, since the individual interactions are weak, if you apply some force, the layers can glide over each other – & off onto your paper from your pencil!
FULLERENES: Fullerenes are a family of carbon-only structures with the shape of hollow spheres, ellipsoids, or tubes whose walls are made up of 5, 6, or 7-C rings. They have a wide range of sizes that depends upon the # of C atoms
🔺 BUCKMINSTERFULLERINE (C60)(BUCKYBALLS) & relatives are closed, highly symmetrical spheres that look like soccer balls
🔺CARBON NANOTUBES (CNTS) (aka buckytubes) are like graphene rolled up into a tube and capped w/half a buckyball. Their diameter is ~50,000X smaller than human hair, but can be “really long” – long as in cm range (this may seem short but considering the dimeter…). They’re strong & efficient heat conductors, making them “all the buzz” in nano materials
🔺CARBON NANOBUDS are buckyball-like “buds” covalently attached to outer side walls of carbon nanotube (like a warty tube)