# According to what criteria was the periodic table of elements ordered?

I was wondering why the elements in the periodic table were disposed the way they are. I understand, of course, that they are put in increasing atomic number fashion, but I'd like to know more about the topic. Could you link some reference that examine in detail the structure of the periodic table?

• It was realized that (what is now) every 8th element exhibited similar chemical behavior and the periodic table was arranged as such. It was later realized from quantum mechanics why these behaviors were so similar (the discovery and understanding of orbitals, electron configuration, etc.). – LordStryker Sep 4 '14 at 19:14
• Elements were ordered by the increase of their atomic weight, not numbers. – RBW Sep 4 '14 at 20:00
• sounds like a duplicate of this: chemistry.stackexchange.com/questions/15883/… Maybe you mean something else, in which case please clarify the question. – user137 Sep 4 '14 at 22:50
• I'm not asking how Mendeleev found out atomic weight, but what are the other chemical properties to consider when you build the periodic table. If increasing atomic weight were the only criterion, it would just be one long string of elements, or a rectangle. – Nicol Sep 5 '14 at 5:50
• @Nicol All the Noble gases have virtually zero reactivity with anything we know of. We know why this is now (because their valence space is filled) but back then nobody knew WHY. They just knew that there were some elements that had the same chemical behavior (in this case, atoms that don't react). Rinse and repeat for other groups in the periodic table. – LordStryker Sep 5 '14 at 13:08

I do not address the question of history, of Mendeleyev's investigations, that is answered in How was Mendeleev able to develop his table?

Modern understanding of basic principles follows. Let us have a piece of periodic table

  ... A0 B0 ...
... A1 B1 ...
.............


Element "B0" must have the next atomic number after "A0", i.e. one more electron. It is different chemically, although may have (albeit not necessarily) similar physical properties.

Element "A1" must be chemically similar to "A0", although physical properties may differ significantly (as a general rule, ".1" elements are denser than ".0"). It happens that for each element there exist its "analog" in the next period, that has similar chemical properties (based on similar outer-shell electron configuration). This pattern, once noticed, permitted for good predictions (search for Mendeleyev's "eka" elements in Internet for more information).

Periods have different lengths. Analog's atomic number may be 8, 18, or 32 ahead off the prototype's atomic number (the number of electrons expended to "raise" all the configuration one shell up). This difference is due to quantummechanical effects.

The vertical columns are families, which are elements that share similar characteristics. For example on the far right side of the periodic table are the alkali metals, known for being extremely reactive. and on the opposite side are the noble gases, which are known for reacting to very few substances. The horizontal periods are organized by the atomic number; the number of protons in an particle.

Dimitri Mendeleev purposely left spaces in the periodic table because he knew there were elements that had not yet been discovered, that would need a place on the periodic table.