The periodic table of elements; the foundational testament of all chemistry…
is… wrong.
Obviously it’s not wrong-wrong. That’s just me trying to get your attention.
It’s more like it’s… Well, you’ll see.
Why a table? Arranging chemical elements in
a neat table came about because many of them have similar properties. As the elements get heavier,
the same qualities come around again periodically. Dmitri Mendeleev usually gets the credit for the
biggest leaps, here, as his version not only recognised periodic trends,
but used those trends to make predictions about elements that were not yet discovered.
He didn’t get everything right, and his version took a few decades to become fully accepted,
but he successfully predicted the existence of 8 then-unknown elements using their periodic nature.
Today, we have a table of elements that arranges the elements by increasing atomic number,
and organises them in columns, known as Groups, according to those recurring properties.
Group 18, on the right side of the table are the Noble Gases. These are very unreactive,
and they don’t really form compounds unless you push them into exceptional circumstances.
Group 17 are the halogens; fluorine, chlorine, bromine and iodide.
They form very stable, singly charged ions. And trends like this continue for each group.
Not every element in a column is identical, though; there are often big changes. Silicon,
for instance, will form four bonds just like carbon, but its size and electronic properties
stop it forming long chains, and prevents it taking part in complex organic-like chemistry.
One important point is that the table also deeply reflects the electronic structure of atoms.
Each similar element in a group has the same number of electrons in their outer-most shell
of electrons. Those noble gases have full outer shells, that contributes to their low reactivity.
The halogens are one electron short, which is why it’s easy form them to gain an
extra one to make an ion. But… the first two elements,
hydrogen and helium, occupy a row of their own. The very first shell of an atom becomes full
after just two electrons, meaning we restart on next line down after just the two elements.
And that raises the issue of what column to align them in because the next row down,
lithium to neon, has 8 members. Helium, thankfully, is fairly unambiguous.
It’s unreactive, it has a full, closed outer shell of electrons. It’s very obviously a noble gas,
we put it in Group 18 off on the right. But hydrogen… this is the problem.
It doesn’t really go… anywhere. So, let’s unpack a few options.
First, the obvious. Route One, the no-brainer. Put it in Group 1.
This makes some atomic sense. Hydrogen is the first element of its row
with one outer electron, it must go in the first position, in Group 1, where the others
also have one outer electron. This makes the singly charged positive ion fairly common.
Perfectly logical. Except… the point of a periodic
table is to give us information about those recurring, periodic properties of elements. This
is how Mendeleev and others began arranging things and making sense of the properties of elements.
And if we look down Group 1, we see a lot of metals. Metals are shiny, conductive, crystalline…
but hydrogen isn’t. It’s a gas. It’s molecular. Ah, but you can compress hydrogen into liquid
metallic hydrogen, such as in the core of gas giant planets!
Yes, we can. But we’ve done that with several elements. We can get metallic
allotropes of carbon, and even phosphorus. This isn’t really an argument for anything.
What else?
We can drop the Group 1 metals into water and they form alkaline solutions – which is
why they’re sometimes called alkali metals. These are the famous school chemistry demonstrations
of sodium and potassium in water.
Hydrogen… not so much. It doesn’t really react with water from its elemental form.
The hydrogen with a plus-one charge does look a little like the alkali metal ions,
with their plus one charges, but its behaviour, and how we make it, is very different.
The hydrogen ion, on its own, is also a naked proton. It’s a sub-atomic particle.
We can’t isolate it chemically. So, its stability, and very existence,
is a very different story to say, sodium ions. So, putting hydrogen here in Group 1
looks like it satisfies everything… …but it doesn’t match any reactivity
trend. It looks neat, but it is, in a word, wrong. It is a superficial position at best.
Now, let’s look at another equally valid argument from electronic structure,
but one that isn’t quite as popular. As well as having just one electron,
hydrogen also happens to need only one electron to form a full, completed shell of valance
electrons. It can also form negatively charged hydrides… just like those Group 17 halogens.
It looks a little weird, but I think we’re on slightly firmer ground here.
Each natural state of the halogens is as a diatomic molecule. Like hydrogen. Each of
them forms one conventional bond. Like hydrogen. And, really interestingly, if you’re into your
inorganic and organometallic chemistry, hydride ligands can act very similarly to halogens.
They have the same negative charge for electron counting, they can even bridge
between metal centres in bimetallic compounds, just like halide ligands.
So, it’s not entirely mad to put it here, but it doesn’t fit all the reactivity.
The hydrogen halides are all acidic, while a hydrogen…
hydride… That’s just H2. It isn’t acidic at all. And hydrogen isn’t very electronegative. That’s
the ability for an element to draw electrons to it, and a key
property of halogens is that they do that. This also means we don’t usually see free
hydride floating around in solutions the same way we see free halides.
So, it’s an interesting argument to put it in Group 17,
and I honestly think it’s a stronger argument than Group 1, but it’s by no means perfect.
But.. seeing hydrogen and helium together does suggest another option… Group 1 and 2 for both.
Okay, this makes structural sense. Both elements only have s electrons,
so they feel quite at home at the top of the s block of the periodic table.
Well, yes, that’s going to work… but only structurally.
We still have all the problems of claiming hydrogen is an alkali metal, when it clearly
isn’t, and now we’ve taken an incredibly obvious noble gas candidate, and stashed it
far away from its very chemically-similar cousins, and putting it with other highly reactive metals.
So, no. That’s not going to work. We don’t often see this as a
serious layout for those reasons. A more popular option is to throw
our hands up, not care, and put hydrogen out on its own at the top. Not connected to anything.
Ignore the problem. But this does break
with the idea that we can align elements by properties. Putting hydrogen in its
own removes our ability to visually organise and predict properties. It doesn’t tell us anything.
So, it’s a solution, and you can certainly see it on some older posters of the periodic table,
but not really recommended by chemists and I think it’s simply gone out of fashion in recent years.
Now… these options are all a bit… imperfect. But… there is another
option that not many people are aware of. And it is almost certainly going to make
some people very, very angry. But I’ve found it hard to argue against.
It’s a little out-of-the-box, and you have to approach it from an odd angle to see it,
but bear with me for a moment…
Group 14. Above carbon.
Yes, I know what you’re thinking. This is madness. This is a stupid position. It’s an
aesthetic nightmare sticking out like that. It puts hydrogen in the p block, which is silly…
…but let’s build up the argument, bit by bit.
That single electron in hydrogen can be considered as one valance electron;
or one valance electron short of a full, complete shell; or... and this is the out-of-the box
bit… because that first row can only take two electrons, it can be considered half filled.
These half-filled points in each row of the periodic table
are often interesting places. Properties that usually vary smoothly across a row will often
see a slight step-change at that mid-point. The half-filled point for the second row
would be at carbon, which has four electrons out of a possible eight in its valance shell.
So, there is an electronic structure rationale. A bit of a stretch, but it’s definitely there.
But more importantly, let’s look at properties.
First, electronegativity, the ability for an atom to draw electrons toward itself. This
property helps explain how elements behave, so it’s a very important consideration.
If you put hydrogen in Group 1, the electronegativity sticks out significantly. It’s
way too electronegative compared to the alkali metals. Shunt it over to Group 17, and again
it doesn’t fit. It’s not electronegative enough to be a halogen, which is the problem we saw earlier.
But if you look at the numbers, it’s pretty close to carbon. Almost
bang on the trend to be in Group 14. This similarity in electronegativity leads to
the non-polar nature of the carbon-hydrogen bond. Let’s talk details.
The bond enthalpy – the energy required to break a bond – for di-hydrogen molecule,
is 436 kilojoules per mole, which is a hundred kilojoules higher than a typical carbon-carbon
bond, which in turn is a hundred kilojoules higher than typical silicon-silicon bonds.
Also, the typical carbon-hydrogen bond is 413 kilojoules per mole. This makes the C-H bond and
the H-H bond more similar than you might expect. And I should know, my PhD thesis rests on the idea
that these bonds are similar and react similarly towards the same catalysts.
There is also very similar combustion chemistry. We know that alkanes – organic chemicals of just
carbon and hydrogen – burn to form carbon dioxide and water. Well, H2 fits that trend;
it’s an alkane of zero length.
We can also draw a strong relationship between water, and alcohols, if we conclude
that hydrogen and carbon are chemically similar enough to belong to the same group.
Hydrogen fits really well in Group 14. At least, it fits better than the other options.
So… ultimately… which one is right?
None of them, really. Or all of them.
And this is the important thing. The periodic table is something we have made.
And we made it to help make sense of patterns we observe
in nature. The periodic table is slightly isomorphic with electronic structure, but it’s
not the full quantum mechanical description – the map is very much not the territory.
The table is a tool; which we use to make predictions about the nature
of elements and atoms. And the question is how well that tool helps us do that.
There have been some other attempts to make completely different layouts,
which can help smooth over the discrepancies of the conventional table.
Some circular periodic tables exist to better reflect atomic structure. Some
three-dimensional pyramid-shaped tables exist to try and make sense of the weird electron-filling
rules that the elements seem to follow. But… for the majority of these alternative
tables, the main weakness is that they’re just not very readable.
And if your table isn’t reada ble; it’s not going to be a practical and useful tool.
So, that’s really the key take home message. How well can we use this tool. And providing
you can use it and understand what it means: you can stick hydrogen wherever the hell you like.
Probably not above Boron. I have absolutely no idea what is going on in this one,
but someone uploaded it to Wikimedia Commons and makes no sense at all. And I have tried,
I really have tried, to make sense of it or look up the rationale, but I’m stuck.
So, if you know, please leave a comment, like, share subscribe and all that.