What did mendeleev do with the elements that did not fit?

Answer 1

Atomic weights were not always known to the same precision that they are today. There were small errors of measurement associated with their determination, and larger errors -- a factor of 1.5 or 2 -- associated with an incorrect attribution of valence.

Mendeleev first formulated the Periodic Law, and then arranged the elements in a table according to the known values of atomic weight.

In doing this he first came across beryllium -- an element with chemical properties very similar to aluminium. This element had been assumed to have a valence of 3 (like aluminium) and an atomic weight of 13.5 g/mol. Mendeleev realised that this could not be the case, but that if he assigned it a valence of 2 and an atomic weight of 9 g/mol, it fitted into the same group as magnesium and calcium. In some ways this was not appropriate, because chemically beryllium had a much closer resemblance to aluminium than to magnesium. However some of the compounds of beryllium had crystal structures isomorphous with the corresponding magnesium compounds, and this convinced Mendeleev that he was right to make this change in the molecular weight (and valence) assignement. Today we know that he was quite correct in making this change.

The modern basis of the periodic law expresses it in terms of atomic number rather than atomic weight. There are just three cases among the elements where the atomic number order does not follow the atomic weight.

(1) element 18 is argon, atomic weight 40.0 g/mol; element 19 is potassium, atomic weight 39.1 g/mol. But fortunately for Mendeleev's peace of mind and the general recognition of the periodic law among chemists argon had not yet been discovered!

(2) element 27 is cobalt, modern atomic weight 58.9 g/mol; element 28 is nickel, modern atomic weight 58.7 g/mol. These two elements are very similar, and their atomic weights were at that time within experimental error of each other, so Mendeleev assigned both of the elements the same atomic weight, and correctly placed cobalt before nickel in his 1871 table.

(3) element 52 is tellurium, modern atomic weight 127.6 g/mol; element 53 is iodine, modern atomic weight 126.9 g/mol. This was a real problem for Mendeleev. Iodine was a very common material, but tellurium was rather rare and infrequently encountered.

It was very clear that iodine was closely related in chemistry to bromine and chlorine, while tellurium obviously fitted in with sulfur and selenium. But the then accepted values of atomic weight had Te at 128 g/mol while I was at 127 g/mol.

Mendeleev was deeply convinced that the atomic weight of Te should be less than that of I. Scraping the bottom of the barrel, he was able to come up with a determination by a Czech(?) mining engineer that put the atomic weight of Te at 125 g/mol. A heated debate ensued with several carefully carried out determinations suggesting values between 127.5 and 128.0 g/mol , and Mendeleev insisting that such a value could not be correct because "the Periodic Law is a Law of Nature".

Answer 2

Mendeleev categorized elements according to Atomic Mass because he believed elements showed recurring properties according to atomic mass. H.G.J. Mosley discovered that the nuclear charge increased by one for each element and, therefore, elements arranged by increasing nuclear charge rather than atomic mass can better explain repeating properties on the Periodic Table.

Answer 3

Because his table was based on the atomic weight of the elements.

Answer 4

Mendeleev's decision to leave gaps in his periodic table was supported by the discovery of gallium

Answer 5

Every element fits into the periodic table of elements devised by Mendeleev.

Answer 6

he didn't know everything.

Answer 7

because