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Because you watch SciShow, Brilliant is offering you a  30 day free trial and 20% off an annual premium subscription  at Brilliant.org/SciShow. 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, you can take the Brilliant  course all about Molecules! This course was made in collaboration with another YouTube channel, MinuteEarth.

And, as you know, there are some  really good science communicators on YouTube, but the partnership is  what really sets this course apart, because Brilliant makes  online learning interactive, with thousands of lessons in  science, computer science, and math. At the end of this particular Brilliant course, you’ll know what different molecules  are just by looking at their diagrams. And real learning like this makes us proud to have Brilliant supporting this SciShow video.

To try Brilliant with your first 30 days free, you can go to Brilliant.org/SciShow or click  the link in the description down below. That link also gives you 20% off an  annual premium Brilliant subscription. Thanks for watching! [♪ OUTRO]