Monday, November 7, 2011

How We Know: The Periodic Table of Elements

As anyone who has been through high school chemistry should know, the "periodic table" is that not-quite-rectangular chart covered in letters and numbers that symbolize every type of atom we know about.  Some of you may have had to memorize it.  My school provided it for every test where we might need it, which was essentially cheating, since the periodic table itself is just a big cheat sheet that provides a wealth of information if you know how to look for it.

First of all, remember that the table isn't just the elements in numerical order arranged in a strict grid.  It has towers on the left and right sides, and entire extra rows sandwiched in between some elements near the bottom.  These were not arbitrary decisions.  By laying out the table this way, many elements that have similar properties are grouped together, so you know where to look for the noble gasses, for instance, or the alkali metals.

But I don't want to get too much into how we use the periodic table today.  I want to give you two dates, and you'll see the question I'd like to answer.  First, the periodic table as we know it was developed by Dmitri Mendeleev in 1869.  The scanning tunneling microscope, an instrument capable of viewing individual atoms, was first developed in 1981, over 100 years later.  Additionally, when Mendeleev developed his table, only 63 of the 118 elements we know of today were known, and yet, his table did not have to be drastically altered to accommodate the new arrivals as they came.  In some instances, the table actually predicted elements that had yet to be discovered.

Making history doesn't always look like much.

So, how on earth could Mendeleev and other scientists possibly know what they were looking at on the atomic level, in order to produce a chart organizing the elements by such properties as the relative weight of an individual atom?
1.  Our first step is determining that everything is made up of atoms.  This is no easy task, but several notable scientists made headway proving atomic theory.  Antoine Lavoisier's and Joseph Louis Proust's work allowed John Dalton to formulate one of the first atomic theories based on evidence, the evidence of chemical reactions always taking place in simple ratios.

2.  Now we must determine the weights of individual atoms.  This, of course, sounds impossible, yet Dalton himself started this work and published a few atomic weights in 1803.  The weights he found were relative, that is to say he was not weighing them in pounds, ounces, grams, or any other standard unit of measurement, but instead he only knew their weights relative to each other.  He found them based on their reactions to each other.  By carefully measuring what went in and what came out in a series of chemical reactions, performed many times over, he was able to calculate the relative sizes of the individual atoms.

How does that even work?  Well, let's take something easier to imagine.  Pretend someone has given you a dozen boxes, all sealed tightly.  Each box is a different weight, but the person tells you that only wooden blocks of the same size and weight were put in the boxes, in different amounts, to change the wieghts of the boxes.  This person points to a balance beam and asks you to, using the balance beam, figure out how many blocks are in each box.  If, like me, you aren't very good at logic or methodical thinking, this might be an impossible task for you, too, but perhaps now you can see how it could be done by someone like John Dalton in 1803.

3.  The final step assumes we have found the relative weights (with Hydrogen as 1) of several dozen elements, and analyzed them to find their underlying properties (gases, metals, softness, etc.).  Many scientists attempted finding an order to the elements, and while some had noticed groups of elements called triads had similar properties and showed interesting patterns in their atomic weights, few before Mendeleev had arranged all known elements into one cohesive table.  What made Mendeleev's special was that he left gaps in his table if he thought the next known element fit better in the next column, and he switched adjacent elements if he thought they fit better in each other's columns.  He made these decisions purely based on their properties, but it became apparent years later that he had correctly ordered his table by atomic number (number of protons in the nucleus), which wasn't even known at the time, and was the underlying reason behind the elements' different properties.

It was quite the achievement.

So there you have it!  Creating a periodic table with 18th and 19th century technology is not as impossible as I once thought.  (In fact, it already happened!)

And, of course, David Stewart, my older brother and a PhD student in chemistry at the University of Buffalo, who helped me over the hurdle of what the heck Mr. Dalton was up to.

All pictures taken remorselessly from Wikipedia.
Photo of Mendeleev monument by


  1. Good article sir.I have learned some thing here .
    thanks for sharing .wonderful blog
    periodic table

  2. Thank you! It's very encouraging to hear. : )

  3. Thanks for writing so clearly! Me too - learned something!!