Most discussions of chemistry use the words “atom” and “molecule” so the use of the word “element” in Periodic Table of the Elements might seem a bit, well, weird.
Our explanation for that is, the words “element” and “compound” were used when we didn’t know much at all about chemicals and we were trying to figure it all out.
An element was a “thing” that couldn’t be change into several things. Water was a compound because we knew it could be changed into two things, and it appeared those things were Elements–we couldn’t change them into other things.
OK, now for the Periodic Table: Mendeleyev noticed a periodicity to the Elements. If one kept going in increasing weight, and did a “wrap around” at certain points, things could be organized with rows and columns, and things in the same column were alike in several ways. His confidence in what he had found was so strong, that he pointed out that he could see two things that hadn’t been discovered yet, and he told what their properties would be. That–doesn’t happen much, if ever–in science. Usually we end up with two explanations that both make sense and we have to do work to eliminate one; said another we, we know nothing until we’ve done all the work.
Regarding the “wrap-around” idea. You can do something somewhat similar by writing numbers on a strip of paper. Write the numbers equally spaced apart of do something like numbers from 1 to 40. Once the numbers are all written, coil the strip so that 11 is under 1, 12 is under 2, 21 is under 11, 22 is under 12, etc. Since the strip only has numbers there will be only one criterium for numbers to be in the same “family”, the last digit (for example, one family is 2,22,32).
Rob Sterling 