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i dont understand this paragraph on medeleev

The paragraph is this (the sentence I dont understand is in capital letters):

Mendeleev swapped elements around and left gaps, and assuming that the atomic mass measurements were incorrect and that some elements were yet to be discovered. He even predicted properties of the missing elements from group trends - a remarkable insight at the time. It was not unil protons were not discovered in the early 1900s, THAT THE REAL REASON FOR THE ORDER IN MENDELEEV'S TABLE WAS REVEALED.

wasn't Mendeleev's table in order of atomic mass? Aren't we using the periodic table in order of atomic number now? What does it mean by revealed?

This was from OCR chem, spec 2016

Reply 1

benjaminfinch

What I think it's trying to say is:

Mendeleev organised his table in terms of patterns of properties, not atomic mass.

http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/patterns/periodictablerev4.shtml

If you look here you can see that it looks very similar to our current one. The reasons for the properties were due to the number of protons, and that this meant a corresponding number of electrons. Does this make sense?

Reply 2

Frank Peters

Original post by benjaminfinch

the last bit didnt...

Reply 3

benjaminfinch

Basically, he knew there were patterns, but didn't know what caused the patterns. We later found out that protons caused the patterns, therefore the discovery of protons shed light on why the periodic table of Mendeleev was arranged as it was.

Reply 4

uncleleech

Imagine you have 5 objects and are asked to sort them. You can only see the properties of them and not what makes them. You might arrange them by their physical properties (melting point, what they interact with etc.) This is what Mendeleev did with the elements. It wasn't until later that we discovered ways to see what makes up the elements. It just happened that the number of protons (and electrons) affected the properties of the atoms.

Mendeleev did what humans (and especially scientists) are good at and found patterns that could be arranged. It could later be explained by further scientific research.