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Pull up any periodic table and take a close look at hydrogen.
It’s probably sitting up above lithium, at the top of the alkali metals.
But look a little closer.
Maybe it’s shaded a different color than the elements below it.
Maybe it’s floating a slight distance away, with a little drop shadow.
It looks like it’s embarrassed to be there.
Like it’s not really with those guys.
This isn’t a graphic design mistake.
Even though it’s the simplest element, chemists actually don’t agree
on where hydrogen belongs in the periodic table.
There are at least three credible places you can stick it,
and that’s a problem for our understanding of how elements work.
[♪ INTRO]
As fixed and immutable as it may seem hanging on your high school
chemistry classroom wall, the periodic table is really a work in progress.
It does a lot of things we like because they make sense.
Elements are listed in order of ascending atomic number,
and that’s not just for the sake of making a tidy list;
it’s related to their chemical properties.
Atomic number represents the number of protons an atom has,
but also the number of electrons.
And the reason for this would require us to go into quantum mechanics,
but electrons will always fill up in specific patterns.
After a while, the pattern starts over again.
This is because the electrons are going into specific subshells,
which are like distributions of energy around the atom.
When those subshells fill up, the pattern repeats in new subshells.
Which means that by listing the elements in order, something interesting happens.
Electrons are everything in chemistry.
All chemical reactions are just electrons swapping places
until you get down to the nuclear stuff, that is.
And electrons in similar configurations around their atom are going to
act in similar ways, so the fact that the pattern repeats lets us sort elements
together into columns, or groups, and make predictions about their behavior.
These repeating properties are that “periodic” thing in the name.
Lithium and sodium both have only one electron in their outermost subshell.
They react in similar ways, so we put them in the same group: group 1.
Which makes it really annoying that the very
simplest element flouts these periodic rules.
It both does, and doesn’t, act like at least three different groups,
and doesn’t really fit with any of them.
Hydrogen has one lone electron in a shell that has room for a total of two.
That makes it resemble the alkali metals,
like lithium and sodium, because they all have a lone outer electron.
But also like group 14, which starts with carbon,
which have their outer shell half full.
But also also the halogens, starting with fluorine,
because they have room for one more electron.
Let’s quickly look at what makes each of those options both right and wrong.
First, the alkali metals, which is where you usually see hydrogen.
Less because it’s the best option and more because chemists sort of
gave up arguing… with the graphic designers, not with other chemists.
If you had a good chemistry teacher, they probably took out a lump of lithium
or sodium and let you watch it sputter around a beaker
literally catching fire in contact with water.
While hydrogen doesn’t do that, it does behave like the alkali metals in some ways.
All of them tend to lose that one outermost electron
to form an ion with a single positive charge: H plus, Na plus.
They also tend to react with similar partners, like fluorine and chlorine.
But there are differences. Hydrogen, you may have noticed, is not a metal.
Some really intrepid chemists say they’ve convinced it to act like one
under extreme circumstances, which would be
an argument for it being an alkali metal.
But not everyone agrees that counts.
Hydrogen also tends to form really different compounds even when
it’s bonding with the same thing as an alkali metal.
Hydrogen chloride is a corrosive gas; sodium chloride tastes great on your fries.
Speaking of chlorine, let’s skip ahead to the halogens; fluorine, chlorine, on down.
Because hydrogen also forms similar ions to the halogens.
You can get hydrogen to gain an electron and make a hydride ion: H minus.
And halogens also make ions with a single negative charge,
like F minus and Cl minus.
Hydrogen, fluorine, and chlorine also all make gases at room temperature,
and all exist in diatomic form: H two, F two.
On the other hand, hydrogen differs from fluorine in a big way in that fluorine
is the most electronegative element on the periodic table,
meaning the greediest and most likely to hog electrons from its bonding partners.
If hydrogen were placed above fluorine, you’d expect it to be even
more electronegative, but it’s actually much better at sharing.
That brings us back to group 14, the carbon group.
Carbon and hydrogen have very similar electronegativities.
They both tend to make similar sorts of bonds,
including with each other, and they can participate in similar reactions.
But some people will step back and say that comparing an element
with one outer electron to one with four just … isn’t actually the same thing at all.
Like, carbon has this amazing habit of linking hands
and making four bonds at once that you don’t see with hydrogen.
So there just isn’t a clear right answer.
Hail Mary proposals include putting it with both the halogens and the alkali metals,
or sort of linked to them with a dotted line,
or giving up and hucking it somewhere in the middle.
I don’t think this argument is gonna end any time soon.
And this isn’t just some abstract squabble between chemists.
That periodic table was on your classroom wall for a reason: It’s a predictive tool.
Periodic behavior helps us understand and study
what different groups of elements will do.
And a well-constructed table will help you learn those properties
and make predictions about how elements will behave,
whether it’s a teacher asking you the electronegativity of boron
or a chemist trying to work out the ionization energy of dubnium.
If an element is in the wrong place, you’ll end up making wrong predictions.
And if you give up and don’t put hydrogen anywhere,
you can’t predict anything at all.
In the end, no one’s going to get through any study of chemistry
without learning that hydrogen is unique and
has some properties you just can’t predict.
So maybe we should tell it to stop being so embarrassed … it’s just special.
Chemicals are awesome!
Put two of them together and you’ve got a compound, or a molecule.
For more chemistry basics and molecular mischief,
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At the end of this particular Brilliant course,
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Thanks for watching!
[♪ OUTRO]