Comment by adrian_b

I do not know to which Wikipedia entries you have looked, but these give enough examples:

https://en.wikipedia.org/wiki/Aperiodic_tiling

https://en.wikipedia.org/wiki/Quasicrystal

https://en.wikipedia.org/wiki/Aperiodic_crystal

A couple of days ago there have been 2 HN threads about quasicrystals, one being about quasicrystals that are found in some very rare natural minerals, which form in special conditions, like meteorite impacts.

Some people before Mendeleev have thought about periodicity of the chemical properties with the atomic mass, but the genius of Mendeleev consisted mainly that he had much more trust in the idea that periodicity must exist.

So while his predecessors were discouraged by the discrepances between periodicity and the known chemical properties, Mendeleev assumed that periodicity is true and any facts that appear to contradict it must be caused either by earlier experimental mistakes that have produced wrong values for some chemical properties of the known elements or by the fact that some chemical elements have not been discovered yet, so empty spaces must be reserved for them in the periodic table.

Nonetheless, the periodic table that comes from Mendeleev has remained somewhat misleading until today, because it was based mainly on the periodicity of valence, which was indeed the most important chemical property for the chemical researchers of the 19th century, which were interested in laboratory experiments made for the discovery of new chemical substances and elements and for the investigation of their properties.

For practical applications, e.g. for the modern chemical industry or metallurgy, valence, which determines the ratios in which elements may combine to form substances, is only one of the properties of interest. The chemical behavior of elements is mainly determined by 3 characteristics, valence, i.e. the number of electrons on the outer layer, atomic size and electronegativity. All 3 properties are approximately periodic, but the quasi-periods vary slightly and a big cycle that goes between 2 noble gases is frequently segmented in 2 or more minor cycles within which properties vary monotonically, but they jump at boundaries. For example, the electronegativity grows from alkaline metals until Cu/Ag/Au, then it jumps down to Zn/Cd/Hg, then it grows again until the noble gases, after which it jumps downwards again.

The result is that for each of the 3 essential properties of a certain chemical element there may be different chemical elements in the next "period" that resemble best with it and only one of those is located in the same "group".

The classification of chemical elements in "groups" is only partially useful, because to really understand chemistry you must also understand the other kinds of similarities between elements, which group the elements in a different way than the periodic table of Mendeleev.

For instance, given the 3 elements carbon, oxygen and sulfur, it is impossible to say which pair of them contains more similar elements, so they can be grouped together. Oxygen and sulfur are in the same Mendeleev group, differing from carbon. However, carbon and sulfur have almost the same electronegativity, differing from oxygen, which causes a lot of similarity between many of their chemical compounds, e.g. between carbonates and sulfates. Moreover, carbon and oxygen have closer atomic sizes, differing from sulfur, which also explains many chemical properties, e.g. why the carbonate ion is CO3, while the sulfate ion is SO4.

A similar discussion can be done about almost any chemical elements, e.g. for some properties silicon resembles germanium and selenium resembles tellurium, because they are in the same Mendeleev groups, while for other properties silicon resembles selenium and germanium resembles tellurium.

In conclusion, the periodic table of Mendeleev provides only a fraction of what must be known about the periodicity of properties among chemical elements.