2019: The Year of the Periodic Table

The Periodic Table

2019 is the International Year of the Periodic Table

In case you missed it, 2019 is officially the International Year of the Periodic Table, marking 150 years since Dmitri Mendeleev discovered the “Periodic System”.

Well, this is a chemistry blog, so it would be pretty remiss not to say something about that, wouldn’t it? So, here’s a really quick summary of how we got to the periodic table we all know and love…

Around 400 BCE, the Greek philosopher Democritus (along with a couple of others) suggested that everything was composed of indivisible particles, which he called “atoms” (from the Greek atomos). The term ‘elements’ (stoicheia) was first used around 360 BCE by Plato, although at that time he believed matter to be made up of tiny units of fire, air, water and earth.

Skipping over a few centuries of pursuing what was, we know now, a bit of a dead-end in terms of the whole earth, air, fire and water thing, in 1661, Robert Boyle was probably the first to state that elements were the building blocks of matter and were irreducible but, and this was the crucial bit, that we didn’t know what all the elements were, or even how many there might be.

Antoine Lavoisier wrote one of the first lists of chemical elements.

Antoine Lavoisier (yep, him again) wrote one of the first lists of chemical elements, in his 1789 Elements of Chemistry. He listed 33 of them, including some that turned out not to be elements, such as light.

Things moved on pretty quickly after that. Just thirty years later, Jöns Jakob Berzelius had worked out the atomic weights for 45 of the 49 elements that were known at that point.

So it was that by the 1810s, chemists knew of 50 or so chemical elements, and had atomic weights for most of them. It was becoming clear that more elements were going to turn up, and the big question became: how do we organise this ever-increasing list? It was a tricky problem. Imagine trying to put together a jigsaw puzzle where two-thirds of the pieces are missing, there’s no picture on the box, and a few pieces have been tossed in from other puzzles for good measure.

Enter Johann Döbereiner, who in 1817 noticed that there were patterns in certain groups of elements, which he called triads. For example, he spotted that lithium, sodium and potassium behaved in similar ways, and realised that if you worked out the average atomic mass of lithium and potassium, you got a value that was close to that of sodium’s. At the time he could only find a few triads like this, but it was enough to suggest that there must be some sort of structure underlying the list of elements.

In 1826 Jean-Baptiste Dumas (why do all these chemists have first names starting with J?) perfected a method for measuring vapour densities, and worked out new atomic mass values for 30 elements. He also set the value for hydrogen at 1, in other words, placing hydrogen as the “first” element.

Newland’s table of the elements had “periods” going down and “groups” going across, but otherwise looks quite familiar.

Next up was John Newlands (another J!), who published his “Law of Octaves” in 1865. Arranging the elements in order of atomic mass, he noticed that properties seemed to be repeating in groups of eight. His rows and columns were reversed compared to what we use today — he had groups going across, and periods going down — but apart from that the arrangement he ended up with is decidedly familiar. Other chemists, though, didn’t appreciate the musical reference, and didn’t take Newlands very seriously.

Which brings us, finally, to Dmitri Mendeleev (various other spellings of his name exist, including Dmitry Mendeleyev, but Dmitri Mendeleev seems to be the most accepted one). His early life history is a movie-worthy story (I won’t go into that else we’ll be here all day, but check it out, it’s really quite amazing). When he was just 35 he made a formal presentation to the Russian Chemical Society, titled The Dependence between the Properties of the Atomic Weights of the Elements, which made a number of important points. He noted, as Newlands had already suggested, that there were repeating patterns in the elements, or periodicity, and that there did indeed seem to be connections between sequences of atomic weights and chemical properties.

Dmitri Mendeleev suggested there were many elements yet to be discovered.

Most famously, Mendeleev suggested that there were many elements yet to be discovered, and he even went so far as to predict the properties of some of them. For example, he said there would be an element with similar properties to silicon with an atomic weight of 70, which he called ekasilicon. The element was duly discovered, in 1886 by Clemens Winkler, and named germanium, in honor of Germany: Winkler’s homeland. Germanium turns out to have an atomic mass of 72.6.

Mendeleev also predicted the existence of gallium, which he named ekaaluminium, and predicted, amongst other things, that it would have an atomic weight of 68 and a density of 5.9 g/cm3. When the element was duly discovered by the French chemist Paul Emile Lecoq de Boisbaudran, he first determined its density to be 4.7 g/cm3. Mendeleev was so sure of his prediction that he wrote to Lecoq and told him to check again. It turned out that Mendeleev was right: gallium’s density is actually 5.9 g/cm3 (and its atomic weight is 69.7).

Despite constructing the one thing that every chemist over the last 150 years has spent years of their life poring over, Mendeleev was never awarded the Nobel Prize for Chemistry. He was nominated in 1906, but the story goes that Svante Arrhenius — who had a lot of influence in the Royal Swedish Academy of Sciences — held a grudge against Mendeleev because he’d been critical of Arrhenius’s dissociation theory, and argued that the periodic system had been around for far too long by 1906 to be recognised for the prize. Instead, the Academy awarded the Nobel to Henri Moissan, for his work on isolating fluorine from its compounds (no doubt impressive, not to mention dangerous, chemistry).

Henry Moseley

Henry Moseley proposed that atomic number was equal to the number of protons in the nucleus of an atom.

Mendeleev died in 1907 at the age of 72, just before the discovery of the proton and Henry Moseley’s work, in 1913, which proposed that the atomic numbers of elements should be equal to the number of positive charges (protons) they contained in their nuclei. This discovery would have pleased Mendeleev, who had already suggested, based on their properties, that some elements shouldn’t be placed in the periodic table strictly in order of atomic weight.

After which, of course, came the discovery of the neutron — which would finally clear up the whole atomic mass/atomic number thing — atomic orbital theory, and the discovery of super-heavy elements. The most recent additions to the modern periodic table were the official names, in 2016, of the final four elements of period 7: nihonium (113), moscovium (115), tennessine (117) and oganesson (118).

Which brings us up to date. For now…


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Any old ions?

The other day, thanks to an expedition to the swimming pool, I found myself drying my hair twice in one day. As I did so it occurred to me that the process was a lot faster at home with my old, battered hair dryer that it had been in the pool changing rooms.

I pondered variables (what can I say, I’m a scientist). I hadn’t washed my hair at the pool, I’d just rinsed it (hence the need to wash it again later) which meant no shampoo and, critically, no conditioner. Does conditioner’s presumably slightly hydrophobic nature help your hair to shrug off extra moisture? This didn’t seem like it ought to make that much difference. It seemed much more likely that the difference was simply due to the hair dryer itself.

hair-dryer

My battered old hair dryer.

And this got me thinking about the nature of my much-loved, rather battered, bright red hair dryer.

I’m going to ‘fess up here. I didn’t buy this hair dryer because I’d carefully researched its specifications and features and decided it was the best model for the job. No, if I’m honest I bought it because it was red and all the others were boring black and silver. Rational eh?

However, I do remember something about this particular hair dryer, and you can just about see a reference to this feature on the photo. Namely, it apparently contains an “ionic generator”. The little dial that you can see in the middle of the photo (set to orange, which if I recall correctly means, ‘maximum ions’) apparently adjusts the ion levels.

At the time I did try to find out exactly what the technology might be. I recall it was difficult – there didn’t seem to be much information out there – and since to be honest I didn’t care that much so long as it dried my hair, and it wasn’t particularly expensive (and it was red, RED!) I bought it anyway.

It only takes a quick glance at Amazon to see that the idea has not gone away. There are lots of ‘ionic’ hair dryers on the market, making claims such as, “Ionic conditioning with 90 per cent more ions“, “Heat-balancing ionic technology for condition and shine and a frizz-free finish“, “stylish dryer with ionic technology–seals in moisture to the hair cuticle for increased shine and silky, glossy hair” and the simple “4X More Ions“.

Hm.

Ok, well first of all what are ions? Whereas most people have a faint idea what atoms and molecules are, far fewer are confident to describe ions – despite the fact that they are firmly a part of the compulsory GCSE Science syllabus and were, of course, also included in  O-level Chemistry before that. Exactly why this should be is tricky to explain. Possibly it’s simply because ‘atoms’ and ‘molecules’ do occasionally crop up in everyday speech, whereas ions are that bit more obscure. Possibly it’s because children learn about atoms in the most basic terms quite early on, and come back to the idea regularly, but ions only turn up relatively briefly (unless, of course, you choose to study A-level Chemistry). There may be an element (hoho) of confusion over the fact that element 26 is called ‘iron’, which in most English accents sounds the same as ‘ion’. And just to really confound everyone, there are such things as iron ions.

But I think the most likely is that ions are a bit tricky to understand.

I’ll have a go.

Ions are charged particles.

There, that was easy, wasn’t it?

What do you mean, what does ‘charged’ mean? It means they have either a positive or negative charge.

What do you mean, ‘what does that mean’?

Oh all right. All right. Back to basics.

helium atom

A helium atom containing a tiny nucleus made up of two protons and two neutrons (red and blue), surrounded by an ‘electron cloud’. 1 fm = 0.0000000000010 millimetres.

First of all we need to understand a bit about atoms. Atoms are made up of two parts. There is the nucleus, which is made up of protons and neutrons (except for hydrogen’s nucleus, which is just a proton) and then, whizzing around that, are electrons. Electrons are quite fiddly things that behave frankly very oddly. In particular, they don’t actually drift around atoms in stately orbits as shown in most diagrams. In fact, they are sort of there and sort of not-there at the same time, and chemists talk about an ‘electron cloud’ as a result. An electron cloud need not contain lots of electrons (this depends on the size of the atom) – it just describes an area where you might find one or more electrons.

Anyway, that’s all a bit complicated and for our purposes it doesn’t really matter – all we need to know is that there’s a nucleus in the middle and electrons around it.

Electrons have a negative charge, protons have a positive charge, and neutrons have no charge. It’s quite difficult to rigorously define what I mean by ‘charge’ without getting into some tricky maths and physics. If you are ok with the idea of negative numbers (who hasn’t had an overdraft at some point or another?) then think of it like this: electrons are -1 and protons are +1 (and neutrons are 0). If you have one proton and one electron, the overall ‘balance’ is zero – their charges cancel each other out. In the case of helium, there are two protons and two electrons. This neat bit of balancing is no accident: it’s the case for all atoms. Carbon has 6 protons and 6 electrons. Oxygen has 8 protons and 8 electrons. Calcium has 20 protons and 20 electrons, and so on.

If the electrons and protons aren’t balanced for some reason (usually as a result of a chemical reaction) then the thing that you were calling an atom a moment ago stops being an atom and becomes, wait for it….

An ion!

Oxygen atoms have 8 protons and 8 electrons, but oxygen ions (properly called oxide ions) have 8 protons and 10 electrons. Which means they have a bit more minus than plus. They are, if you like, a bit overdrawn. If you add it up, you find the number works out as -2. And so we say that oxide ions have a charge of -2, and chemists (who are lazy) write this as O2-. Which is not, we must be careful here, the same thing as O2. That means two oxygen atoms joined together, to make an oxygen molecule. What do you mean it’s confusing?

One more example then. Calcium atoms have 20 protons and 20 electrons, but calcium ions have 20 protons and 18 electrons. Add that up and you get +2. We say that calcium ions have a charge of +2, and write Ca2+(and there’s no such thing as Ca2, so that’s one less thing to worry about).

Where have we got to? Ions are charged particles, and that means that they either have a positive charge or a negative charge. These charges are typically between 1 and 3, positive or negative.

mineral water label

Fizzy mineral water, chock full of lovely ions.

Ions are very important, because they form during chemical reactions and many everyday substances are made up of ions. For example table salt, sodium chloride, is made up of Cl ions and Na+ ions.

Tap water, and indeed bottled water, are full of ions. Tap water has chloride ions (chlorinated water is a jolly good thing, assuming you don’t want typhoid, and is definitely not harmful regardless of what your nearest quack might try and tell you). It might also have fluoride ions, which are also very good for your general health (again, there’s lots of nonsense spread about this). Both tap and mineral water usually contain some sodium ions and some calcium ions. The ion balance does affect the taste – the more sodium there is the more salty the water tastes, for example – but that’s about it really. The ions don’t give the water any special properties except, perhaps, the ability to conduct electricity (which pure water, as in just H2O, actually does really badly).

Having explained ions, let’s get back to hair dryers for a moment. Ionic hair dryers claim to produce streams of negatively-charged ions. They usually claim to use something like the mineral tourmaline to do this, but despite much searching I struggled to find out much about how this was supposed to work or, most crucially, what the negative ions actually are. Negative ions are not a thing in and of themselves. They must be ions formed from atoms, so which element? Or elements?

After much hunting I eventually came upon an interesting piece written by Andrew Alden, at about.com. He explains that tourmaline has an interesting trick called pyroelectricity, which means that it does become charged when heated. The ancient Greeks even knew about this: in 314 BC Theoprastus noticed that tourmaline (called lyngourion at the time) attracted sawdust and bits of straw when heated.

Ed Trollope, from Things We Don’t Know, helpfully explained this pyroelectric effect as follows: the crystal structure of tourmaline becomes polarised (in other words the charges already in the structure become unevenly distributed) if you change its temperature. This results in a voltage across the crystal, which in turn leads to a small current being generated.

But I’m still unclear what the ions, if they exist, actually are. And the problem is that if you search for this, the first umpteen links are all pure and utter nonsense. Tourmaline is a complicated mineral that contains a whole host of different metal ions as well as oxygen, OH (hydroxide) and fluorine. Does it produce oxide ions, O2-? These are very reactive and wouldn’t hang around for any useful length of time. And if they did they would surely be harmful. Presumably they would cause the production of ozone (definitely not a good thing). Are the manufacturers using the word ‘ions’ when they actually mean ‘electrons’? They are not the same thing of course, but perhaps ‘ions’ seemed like a friendlier word.

Electrons would reduce static in your hair, but then static is short-lived anyway. Would it speed up drying time? None of the explanations I’ve actually seen for this including, most memorably, “The negative ions break down water molecules to one-fifth of their size” (errrr, what?), provide a really satisfactory, scientific explanation as to why it should. Or why it should ‘seal water into the hair’, whatever that means. It is feasible that the reduction of static could help keep the hair strands separate, which might help, but surely brushing or even running your fingers through your hair would have a much bigger effect. What I’m also not clear on is whether the tourmaline in your hair dryer carries on producing streams of ions/electrons indefinitely, or whether it becomes degraded over time. Which you would expect, if the charged particles are coming from the tourmaline itself. Is my battered old hair dryer really doing anything at all anymore, if it ever did?

It’s all very unsatisfactory. My best guess? Ionic hair dryers do reduce static build-up in hair, which would leave it smoother immediately after drying. Conditioner and styling products will also help with this mind you, and will probably have a more significant effect. The rest, I strongly suspect, is pure woo. And my hair dryer dries my hair faster simply because it runs hotter and with a faster airflow than the cheap, basic models in the swimming pool changing rooms.

But if you know better, I’d genuinely love to hear from you.

Why weigh atoms that way?

A couple of days ago I was listening to the latest Radiolab podcast. If you’ve never listened to one of these, you really should. They are beautifully produced and, without fail, utterly fascinating. Over the last year or so I’ve learned about a possible cure for a disease with a 100% mortality rate, an apocryphal Russian story about horses frozen into a block of ice, and a new theory for the end of the dinosaurs where, if I understood it correctly, they were essentially grilled to death. Episodes of Radiolab always feel like a thoroughly good use of brain-time.

Anyway, if you’re still with me and haven’t dashed off to immediately download some of these little gems, the most recent episode is about weights and measures and how we’ve standardised them over the years. In particular the kilogram, which is the last physical standard in use, although possibly not for long (listen to the podcast).

shutterstock_27019597_Alhovik_mod_2

So what are the scales made of…?

This got me thinking about atoms and, in particular, how we decide their mass. This matters you see, because the mass of atoms tells us chemists how much stuff to use. If I want a saline solution with a particular concentration, all I need do is look up the numbers on the periodic table, weigh out the appropriate amount of salt and dilute it with the appropriate amount of water. And if you’re a patient who needs a saline drip, you’d better hope I did it correctly.

Anyway, if you remember your periodic table (which of course you do, but just in case, here’s a picture) all the elements come with two numbers.

The Periodic Table

One of these numbers is the atomic number, which is the number of protons in the nucleus of each atom of the element. Conveniently, nature has managed to produce an atomic nucleus for each number between 1 and, at last count, 118 and if you ‘read’ the periodic table from left to right, top to bottom, you’ll see the numbers go up one at a time.

The other number, relative atomic mass, is a bit less tidy. It still goes up as you go along the periodic table, but in less regular jumps of roughly between one and three.  Without going into lots of detail, relative atomic mass is standardised against 112 the mass of carbon-12. Which begs the question, why? The more mathematically aware will have clocked that 112 of 12 is, well, 1. So why don’t we compare all the elements to hydrogen, which actually has a mass of 1? Or if that’s infeasible for some reason why not, I don’t know, choose 19 of beryllium-9, or 128 of silicon-28?

Well actually, almost exactly 200 years ago now, atomic mass (called atomic weight, at the time) was originally compared to hydrogen, and it was thought that all elements would have masses which were exact multiples of hydrogen’s.

The problem with this was that as measuring techniques became more sophisticated it became clear that some elements were inconveniently failing to follow the rule. In fact, some were downright contrary, like chlorine which appeared to have a mass which wasn’t even a whole number.

This was, at least partially, sorted out in 1932 when James Chadwick proved the existence of neutrons. The existence of isotopes had already been suggested, but this finally cleared up what the pesky things actually were. It turns out some atoms are fatter than others, having one or two more uncharged particles in their nuclei. This doesn’t change what atom they are – they still have the same number of protons – but it does make them a bit heavier. Take a sample of pure chlorine, for example, and you find that roughly three quarters of the atoms in it have a mass of 35, whereas the other quarter have a mass of 37. These are the isotopes of chlorine: imaginatively named chlorine-35 and chlorine-37. Work out the weighted average of the two and you get 35.5, which is the number you see on periodic tables.

In the mid-20th century something of a minor squabble between chemists and physicists broke out (chemists and physicists often squabble: they’re a bit like the English and the French: they like to visit each other but only so that they can moan about how annoying the other lot are and how badly they do everything). By this time had been a switch from using hydrogen (the lightest element) to oxygen as the standard to which other elemental masses were compared. This was mainly for the convenience of chemical analysis: oxygen combines with a lot of things to make straightforward oxides, whereas hydrides are less common and trickier to work with. Plus, large quantities of hydrogen gas are a bit (in the sense of an elephant being a bit heavy, or cyanide being a bit poisonous) of an explosion risk. Oxygen causes other things to burn jolly nicely, but isn’t actually flammable itself. If you can manage to keep it away from other flammable stuff it’s a far safer option.

The problem was that chemists were using a mass scale based on assigning the number 16 to a natural mixture of oxygen (which contains mostly oxygen-16, with little bits of oxygen-17 and oxygen-18). Physicists, on the other hand, had instead assigned the number 16 to the isotope oxygen-16, which they had isolated using the technique of mass spectrometry.

Josef Mattauch

Physicist Josef Mattauch

You may think the physicists’ method sounds more logical, but the chemists’ reasoning was that in naturally-occurring compounds there would be a mixture of isotopes, so it made sense to use a number based on that mixture since you never actually encounter one atom on its own. Either way, the result was differences in the numbers, admittedly some way down the decimal places, but none the less a difference. Of course it was possible to convert between the two, but at the time scientists were fiddling with such tricksy things as nuclear energy and, of course, bombs. Even a tiny discrepancy in the nth decimal place was potentially catastrophic. Something had to be done.

Edward_Wichers

Chemist Edward Wichers

In 1961 a compromise was agreed, thanks largely to the combined efforts of the physicist Josef Mattauch and the chemist Edward Wichers, who set about persuading their respective groups to be reasonable and play nicely with each other.

The result was that carbon-12 was assigned a mass of exactly 12 and the relative atomic mass scale became based on that. The choice of carbon was, to an extent, somewhat arbitrary. It suited the physicists, who were already using carbon as a standard for mass spectrometry. It fell in between the two previous values (1 for hydrogen and 16 for oxygen), which meant it wouldn’t throw every existing piece of work out by too much. In particular, chemists weren’t keen on switching to the physicists’ method of 116 of the oxygen-16 isotope, because it would change their numbers quite significantly. Switching to 112 of carbon-12 meant, surprisingly, a smaller change. Carbon is also, of course, a naturally abundant element and it was easy to get samples of pure carbon.

And that, as they say, is that. The carbon-12 scale is still used today, over 50 years later, and it’s not going anywhere. Hydrogen is officially 112 the mass of carbon-12, and we use carbon-12 because, basically, it was the only option the chemists and physicists would agree on. Hey, it’s as good a reason as any.