Zecorium  
Celestio  
Derpynio  
Raritio  
Maudanio  
Octavio  
XD

I’m not sure the groupings make sense in terms of what a periodic table represents, though I see why they were done. But with a periodic table, the properties of elements group in columns, not rows. We represent it starting with hydrogen because hydrogen is the smallest element, but technically it’s a circle; you should be able to split it between any two columns and it still makes sense. In other words, as you go across, you come to similar elements in similar orders. Ignoring hydrogen and helium which are wierd, on the second row starting with the upper left, you have a fairly reactive metal, a slightly less reactive metal, a bunch of empty spaces, then a polyvalent jack-of-all-trades, a fairly reactive non-metal, a more reactive non-metal, a hideously reactive non-metal, then a completely inert gas. Go to the next line and it’s basically the same thing, just the metals get more reactive, the non-metals less. The first two columns of elements are increasingly reactive metals. The last column is the non-reactive noble gasses, but the three prior to that are (left to right) increasingly reactive non-metals that get (top to bottom) decreasingly reactive. So in the bottom row you’d have a hideously reactive metal, a slightly less hideously reactive metal, a bunch of polyvalent highly reactive metals, a fairly inert slightly metallic non-metal, a barely reactive non-metal, a mostly non-reactive non-metal, and a slightly reactive non-metal, then a completely inert non-metal.
 
So assuming you start with pegasi in the top left, having Rainbow Dash then Fluttershy doesn’t really make sense. You’d probably start with Scootaloo in the top left, going down to Rainbow Dash in the bottom left. Then start column 2 with Fluttershy and end with Spitfire. (Alternatively, Fluttershy below Scootaloo atop Dash’s column and Derpy atop Spitfire’s column.) One above Dash and Fluttershy would probably be Cloudchaser and Flitter (or Cloudchaser and Thunderlane, or Fleetfoot and Cloudchaser). The exact details there are just stylistic.
 
Jumping to the other side where the Noble Gasses are, top right down to bottom right would probably be Celestia, Luna, Twilight Sparkle, Cadance, Shining Armor. (Alternatively, put Twilight at the top of the second to last column, move up Cadance and Shining, and put Prince Blueblood at the bottom of the Noble Gasses.)
 
One column over from there you’d want powerful unicorns, starting (unlike the pegasi) with the most powerful at the top (Trixie with alicorn amulets), going down to probably Sweetie Belle at the bottom left corner of the right side where it joins the transition elements.
 
I’d probably want the first column of the non-metals to be major background ponies (Derpy, Lyra, Bon Bon, Berry Punch, etc.)
 
Transition elements would then be miscellaneous background ponies across the top, going down to increasingly bad villains (most of the heavier transition elements, really all of them except Bismuth, are quite toxic). Except, the two separate bars are the rare earth elements, so there I’d probably put the Apple Family and Pie family.

Why are there odd numbers of ponies in each row of the p-block equivalent and the f-block equivalent? That would require the electron subshells responsible for those blocks to have orbitals that can only fill halfway, which makes no sense.

@Background Pony #26D9  
Not that I care about Spike, but I do agree on background ponies comment. There are bg pony toys in the works that are put on front over actual characters that had voices. Never made sense to me.

@Background Pony #2EBA  
You just called anyone who doesn’t include spike in their images jerks. How about you not insult people over such petty shit, hmm?

I just don’t get why Spike is often ignored in favor of background ponies or 3 minute characters for whatever reason.  
Its one thing to just not include him, that’s fine, but adding instead Derpy or Lyra or someone in his place is just…wrong.  
The two aren’t even really characters…

@Millennial Dan  
“a silly Spikey-Wikey fanboy.”
 
And how about we not insult each other, hm?

@Millennial Dan  
Just seein’ if you were an easy catch.

@Background Pony #2EBA  

 
That’s just pathetic, but what do I expect from a silly Spikey-Wikey fanboy.

@Millennial Dan  
That’s because Fleetfoot is a million times better than Sunset Shimmer in every way.

@Background Pony #2EBA  
At least he wasn’t forgotten here… heck, even Fleetfoot was included for some reason.

@Millennial Dan  
I’d like to say you now know what being a Spike fan is like…but that’s all the time.

You don’t have Sunset in here? There’s no excuse for that.

So, Sombra is Promethium?

@BK_Leonidas  
@0particle
 
At least she didn’t forget Spike like a lot of jerks easily would have.

this makes me really really angry

So Celestia has no family cadance is radioactive and almost everything is made out of Diamond tiara? Pretty accurate

No Thunderlane.
 
Also, if Celestia is in Hydrogen’s place, does that mean she has no family?

Does this mean that ponies like Cadence are radioactive?

Hmm, maybe they should have gone with trying to match pony names up with the symbols of the real elements. That’s an interesting challenge. Perhaps I’ll take that one up.

Those noble gases don’t look at all stable.

Can’t even make an abbreviation for Derpy?

@0particle  
No Harshwhinny either :(

No Blueblood! :c

I thought this was a Smash Bros. character select screen, at first glance. I’ve been playing that game WAY too much…..