fluoride and fluorine -- 2/9/2022
Today's selection -- from Chemistry for Breakfast: The Amazing Science of Everyday Life by Mai Thi Nguyen-Kim. Flourine is a highly dangerous gas:
"While I finish my breakfast, let’s start with the difference between fluoride and fluorine -- the perfect topic given that I’m cooking my eggs in a Teflon pan. Keep that in the back of your mind while I explain.
"Fluorides are a form of the element fluorine. If you look at the periodic table ... you’ll find fluorine (F) in the seventh main group, known as the halogens. Fluorine is a gas with an odor similar to chlorine, the halogen used in swimming pools. I hope you’ll never smell it though -- fluorine is pretty damn dangerous.
"What do I mean by ‘pretty damn dangerous’? I mean that even the smallest quantity of fluorine acts aggressively because it’s highly reactive. The general rule is that the easier and faster a substance enters into a chemical reaction with another substance, the more dangerous it is – because it’s less easy to control. There are other reasons why substances can be dangerous or poisonous, but we’ll come to that later.
"In any case, fluorine gas reacts with water to produce fluoric acid. ‘Acid’ sounds scary, and rightly so; if you accidentally poured some on your hand, it wouldn’t just burn your skin, it would eat right through to the bone and dissolve it. Other dangerous acids like hydrochloric acid (the equivalent for chlorine) seem practically harmless in comparison.
"So please keep away from elemental (pure) fluorine and fluoric acid! Don’t worry, there’s nothing you need to do. Luckily, neither substance occurs in nature (or in toothpaste) thanks to another simple, general rule of chemistry: the more reactive a compound, the less common it is in nature. It’s logical really -- if fluorine is so aggressive that it reacts with everything it meets, we can assume that all the fluorine out there has already ‘reacted itself away.’
"However, you can produce fluoric acid in a laboratory not because you're a mad chemist who wants to take over the world, but because of your fried eggs. In a chemical laboratory with the right equipment, you can pair fluoric acid with whatever you want. If you choose the right substance, you can produce polytetrafluoroethylene, PTFE for short, also known as Teflon! And now we're back to my pan and eggs.
"What about the fluorine atoms sitting in my Teflon pan?
"Is there fluorine on my eggs? Good question -- let's dive in.
"Most elements, even the reactive and aggressive ones, have a stable form in which they are unreactive and relaxed. How an atom is made up on the inside determines whether it tends to be aggressive or relaxed. As in life in general, in chemistry it's what's inside that counts. (Well, almost always. In the particle model, what's inside the particles plays no role.)
"We often imagine atoms to be the smallest particles, the smallest components of our world -- but that's not true. Atoms are made up of three different elementary particles: protons, neutrons, and electrons. Protons have a positive charge, valence electrons are extroverted, highly reactive souls who like to take part in chemical reactions.
"Apart from the fact that the valence electrons are farthest from the nucleus, there is something else that makes them restless; while the inner electron shells are full, many elements have a partially empty outermost shell. The number of electrons is limited; as I mentioned previously, it matches the number of protons. Now, I don't mind having a whole row to myself when I go to the movies, but then I'm not an electron. Electrons -- particularly the extroverted valence electrons -- hate empty seats! This is the odd thing about atoms; they really want their shells to be full.
"This desire is expressed in a curious way. Elements with a few empty seats in their valence shell are not so aggressive. But if there's only one empty space to fill or the valence shell contains just one solitary electron, things start to get uncomfortable. Elements that have narrowly missed out on a full shell are the most aggressive. It's like the World Cup, where the runners-up often cry the most -- close but no cigar.
"So, the chemical temperament of an 'atom is linked to how many valence electrons it has. Our fluorine has seven, but its valence shell has space for eight. And that one empty spot makes the fluorine furious. It restlessly searches other atoms and molecules for the missing eighth electron and won't stop until it fills its final spot.
"Fluorine isn't the only element with this problem. Almost every element in the main groups of the periodic table would like to have eight valence electrons. This is known as the octet rule; the name is a bit misleading because it's not a fixed rule (like a law of physics), but simply a model -- although admittedly a very practical model closely connected to the shell model. The octet rule can be used to clarify not only which elements are particularly reactive, but also which reactants go well together. Every element has needs, and fulfilling the octet rule may be one of them. Chemical reactions and bonds satisfy these needs (much like in humans, actually).
"Fluorine is like a baby crying for food. Once fed it’s quiet and peaceful (parents may argue otherwise…). Once fluorine forms a new bond that provides its longed-for eighth electron, nothing much happens.
"So what does this mean for my pan? Teflon is produced by combining fluorine with carbon; one of carbon’s properties is to generously share electrons and shells with other atoms. ... It would take a whole lot of energy to extract the fluorine from this happy state of affairs. To break the bonds, you would need to heat Teflon to over 360°C (680°F); the recommended maximum temperature for Teflon pans is 260°C (500°F) and the optimum temperature for my fried egg is around 83°C (181°F) -- any higher and the egg white solidifies.
"So, according to the octet rule, the fluorine and carbon atoms in my pan have achieved everything an element can possibly achieve: a full valence shell. The chemical bond between fluorine and carbon is truly a model marriage -- neither party has eyes for any other atoms or molecules. Not even for the attractive proteins in the fried egg currently sizzling in my Teflon pan.
"If food sticks to the pan, then the pan molecules and food molecules are simply interacting. Teflon has no interest in my fried egg or any other food. The fluorine in my pan is probably relieved to have left behind its restless youth as fluoric acid. ‘I have everything I want, leave me in Peace, it thinks. ‘Don't mind me, I was never here,’ my fried egg replies, sliding onto my plate without leaving a trace.
"As you read this whole discussion of Teflon, were you thinking to yourself, 'Whoa! What about all the headlines saying Teflon cookware is dangerous to use? Why is Mai cooking with a Teflon pan?' Well, I use Teflon cookware because when it's used correctly, it's safe. Remember those temperatures I mentioned above? It's overheating a Teflon pan that's a bad idea. The gases released by a severely overheated Teflon pan can cause flu-like symptoms."