One thing I never learned in high school chemistry (er, I mean, one of the many things I never learned in high school chemistry) was why the periodic table of the elements is shaped the way it is. Is there a reason scientists have settled on its current U shape instead of, say, a nice orderly square?
Along similar lines, why are the lanthanides and actinides always placed in two rows below the main chart. Are they being called out as different, or are the two lines just a space-saving move along the lines of placing Alaska and Hawaii in unused corners of U.S. maps?
Regarding Lanthanides and Actinides, IIRC those two rose somehow fit in the middle of the chart, which would make it too wide. Except it would also ruin the periods of the other elements. The chart needs to be 3-dimensional, it seems.
It has to do with the number of electrons that can fit into each energy level - or, if all you remember is the “planetary” model, the number of electrons that can fit in each orbit. The first level can only hold two electrons, so there are only two elements in that level: Hydrogen, which normally has only one electron, and helium, which has two.
The next level can hold eight electrons, and so there are eight elements in this row. After this it gets a little trickier. The third level can hold 18 electrons, but there are only 8 elements in the third row. So here’s a new rule: the outer shell or “orbit” can only have 8 electrons in it. So the third row of the table has eight elements. When you get to the fourth electron shell, the first two electrons added go into this fourth shell, but then electrons start to fill up the remaining spots in the third shell. These are the transition metals that occupy the middle of the periodic table.
The lanthanides and actinides are a similar story. They should actually be placed in the middle of the table as well, after (intuitively ) lanthanum and actinium. They occur when the fourth electron level is filled up with (I think) 32 electrons. So, instead of the U shape we see, the periodic table should really have more of a steep left side and then a terrace effect, but you’d never be able to fit it on a standard page with the lanthanides and actinides taking up the left-middle, and the transition metals on the right-middle.
The nice thing about the periodic table is that you can get all sorts of information off it just by looking at the organization. You can get a basic idea of atom size, ion size, electronegativity, valence electron number, outer electron shell, and other nifty things.
With regards to the lanthanides and the actinides, there are two reasons that I know of of why they are pulled out. One, as said before, was a space concern. Second, they have what is known as the f-block orbitals as the outer electron shell. If you look, you’ll see that the first two periods are s-orbitals, the last 6 periods are the p-orbitals, the ten in the middle are d-orbitals, and the lanthanides and actinides are f-orbitals. They pull those 28 out (two rows of 14 periods) because of their similarity to each other that way.
It has to do with the number of electrons that can fit into each energy level - or, if all you remember is the “planetary” model, the number of electrons that can fit in each orbit. The first level can only hold two electrons, so there are only two elements in that level: Hydrogen, which normally has only one electron, and helium, which has two.
The next level can hold eight electrons, and so there are eight elements in this row. After this it gets a little trickier. The third level can hold 18 electrons, but there are only 8 elements in the third row. So here’s a new rule: the outer shell or “orbit” can only have 8 electrons in it. So the third row of the table has eight elements. When you get to the fourth electron shell, the first two electrons added go into this fourth shell, but then electrons start to fill up the remaining spots in the third shell. These are the transition metals that occupy the middle of the periodic table.
The lanthanides and actinides are a similar story. They should actually be placed in the middle of the table as well, after (intuitively ) lanthanum and actinium. They occur when the fourth electron level is filled up with (I think) 32 electrons. So, instead of the U shape we see, the periodic table should really have more of a steep left side and then a terrace effect, but you’d never be able to fit it on a standard page with the lanthanides and actinides taking up the left-middle, and the transition metals on the right-middle.
The nice thing about the periodic table is that you can get all sorts of information off it just by looking at the organization. You can get a basic idea of atom size, ion size, electronegativity, valence electron number, outer electron shell, and other nifty things.
With regards to the lanthanides and the actinides, there are two reasons that I know of of why they are pulled out. One, as said before, was a space concern. Second, they have what is known as the f-block orbitals as the outer electron shell. If you look, you’ll see that the first two periods are s-orbitals, the last 6 periods are the p-orbitals, the ten in the middle are d-orbitals, and the lanthanides and actinides are f-orbitals. They pull those 28 out (two rows of 14 periods) because of their similarity to each other that way.
The periodic table is arranged according to the periodically repeated arrangement of the element’s electrons. Each column represents a “family” with a similar electron configuration; the noble gases at the far right have the maximum number of electrons in their outer level and are all very stable, especially helium, and the alkali metals on the far left all have one electron in their outer level and are all very reactive. Read in rows, the elements are arranged by atomic weight and valence. The Lanthanides and Actinides do indeed go in the middle of the top chart, but doing it that way would make it too long to fit in a classroom, and then the top ones wouldn’t match up. It should be 3-D.
The U shape comes from how the electrons arrange themselves in levels and sublevels. The first level, one, only contains sublevel s, two electons. Hyyrdogen one electron, unstable, Helium two electrons, stable. Level one is now full; go to level two. Level two has sublevel s and sublevel p, which holds six electrons. So Lithium has three electrons, two go into Level one, one goes into level two. Keep going across the rows until levels one and two are full, two electrons in level one and eight in level two . Since all levels are full you would expect that element to be very stable…surprise! It’s neon, we’re back at the stable Noble gases. It keeps going like that…level three has three sublevels, s with one, p with two, and new improved sublvel d, with ten. It keeps increasing like that for a while, with each level containing a new sublevel. The end result: a U-shaped periodic table.
Mendeleev had a square periodic table, but it didn’t quite work right; the properties in the columuns didn’t quite match the atomic masses he had the rows arranged in.
Now whatddya wanna bet once I post this someone beat me to it?
It’ll give you your answer AND the answer to questions you didn’t know you have about the arrangement of the Periodic Table.
(Aside: I’ve memorized the Periodic Table by attending countless boring meetings. At each one, I’ve attempted to reproduce the entire table in my notepad; unfortunately, with time I’ve become pretty good at it and it is no longer the effective device to help me get through meetings it once was.)
IIRC there is more to it than that, though. In terms of the rows, the above is correct, but in terms of the columns, it gets more complicated. I think it was found that the lanthanides and actinides share certain characteristics with scandium and yttrium. So in a certain way, the whole La and Ac rows should be in columns below Sc and Y, but in other ways they need to continue in a row following Barium and Radium, respectively.
I don’t have the book that I read this in, so the explanation is really vague, and I don’t particularly care to look it all up right now, but this page offers a good summary of the history of the Mendeleev periodic table (which is essentially the one we use now): http://www.chemsoc.org/viselements/pages/history.html
Also, the Oliver Sacks book “Uncle Tungsten: Memoirs of a Chemical Boyhood” has a great chapter or two on the periodic table, and the whole book is a fun way of looking at chemistry - I recommend it (and indeed my first comment was from that book, but I leant it to my brother :(!)
Thanks to all for the info! And thanks for the book recommendations. Somehow, you knew that I didn’t know what to get with my Amazon.com gift certificate.
) lanthanum and actinium. They occur when the fourth electron level is filled up with (I think) 32 electrons. So, instead of the U shape we see, the periodic table should really have more of a steep left side and then a terrace effect, but you’d never be able to fit it on a standard page with the lanthanides and actinides taking up the left-middle, and the transition metals on the right-middle.