Question: What characteristic of Mendeleev's periodic table implicitly predicted the properties of elements that had not yet been discovered?
About the article "Periodic table", section "Mendeleev's table".

Candidate passages:
An important breakthrough in making sense of the list of known chemical elements (as well as in understanding the internal structure of atoms) was Dmitri Mendeleev's development of the first modern periodic table, or the periodic classification of the elements.
Mendeleev, a Russian chemist, felt that there was some type of order to the elements and he spent more than thirteen years of his life collecting data and assembling the concept, initially with the idea of resolving some of the disorder in the field for his students.
Mendeleev found that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements.
Mendeleev's law allowed him to build up a systematic periodic table of all the 66 elements then known based on atomic mass, which he published in "Principles of Chemistry" in 1869.
His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties.
Mendeleev had such faith in the validity of the periodic law that he proposed changes to the generally accepted values for the atomic weight of a few elements and, in his version of the periodic table of 1871, predicted the locations within the table of unknown elements together with their properties.
He even predicted the likely properties of three yet-to-be-discovered elements, which he called ekaboron (Eb), ekaaluminium (Ea), and ekasilicon (Es), which proved to be good predictors of the properties of scandium, gallium, and germanium, respectively, which each fill the spot in the periodic table assigned by Mendeleev.
At first the periodic system did not raise interest among chemists.
However, with the discovery of the predicted elements, notably gallium in 1875, scandium in 1879, and germanium in 1886, it began to win wide acceptance.
The subsequent proof of many of his predictions within his lifetime brought fame to Mendeleev as the founder of the periodic law.
This organization surpassed earlier attempts at classification by Alexandre-Émile Béguyer de Chancourtois, who published the telluric helix, an early, three-dimensional version of the periodic table of the elements in 1862, John Newlands, who proposed the law of octaves (a precursor to the periodic law) in 1864, and Lothar Meyer, who developed an early version of the periodic table with 28 elements organized by valence in 1864.
Mendeleev's table did not include any of the noble gases, however, which had not yet been discovered.
Gradually the periodic law and table became the framework for a great part of chemical theory.
By the time Mendeleyev died in 1907, he enjoyed international recognition and had received distinctions and awards from many countries.
In 1873, Jacobus Henricus van 't Hoff and Joseph Achille Le Bel, working independently, developed a model of chemical bonding that explained the chirality experiments of Pasteur and provided a physical cause for optical activity in chiral compounds.
van 't Hoff's publication, called V"oorstel tot Uitbreiding der Tegenwoordige in de Scheikunde gebruikte Structuurformules in de Ruimte", etc.
(Proposal for the development of 3-dimensional chemical structural formulae) and consisting of twelve pages text and one page diagrams, gave the impetus to the development of stereochemistry.
The concept of the "asymmetrical carbon atom", dealt with in this publication, supplied an explanation of the occurrence of numerous isomers, inexplicable by means of the then current structural formulae.
At the same time he pointed out the existence of relationship between optical activity and the presence of an asymmetrical carbon atom. 
Russian chemistry professor Dmitri Mendeleev and German chemist Julius Lothar Meyer independently published their periodic tables in 1869 and 1870, respectively.
Mendeleev's table was his first published version; that of Meyer was an expanded version of his (Meyer's) table of 1864.
They both constructed their tables by listing the elements in rows or columns in order of atomic weight and starting a new row or column when the characteristics of the elements began to repeat.
The recognition and acceptance afforded to Mendeleev's table came from two decisions he made.
The first was to leave gaps in the table when it seemed that the corresponding element had not yet been discovered.
Mendeleev was not the first chemist to do so, but he was the first to be recognized as using the trends in his periodic table to predict the properties of those missing elements, such as gallium and germanium.
The second decision was to occasionally ignore the order suggested by the atomic weights and switch adjacent elements, such as tellurium and iodine, to better classify them into chemical families.
With the development of theories of atomic structure, it became apparent that Mendeleev had unintentionally listed the elements in order of increasing atomic number or nuclear charge.
The significance of atomic numbers to the organization of the periodic table was not appreciated until the existence and properties of protons and neutrons became understood.
Mendeleev's periodic tables used atomic weight instead of atomic number to organize the elements, information determinable to fair precision in his time.
Atomic weight worked well enough in most cases to (as noted) give a presentation that was able to predict the properties of missing elements more accurately than any other method then known.
Substitution of atomic numbers, once understood, gave a definitive, integer-based sequence for the elements, still used today even as new synthetic elements are being produced and studied.
The existence of an element between thorium and uranium was predicted by Mendeleev in 1871.
In 1900 William Crookes isolated protactinium as a radioactive material from uranium which he could not identify.
Different isotopes of protactinium were identified in Germany in 1913 and in 1918, but the name "protactinium" was not given until 1948.
Since the 1950s thorium, uranium and protactinium have been classified as actinides, hence protactinium does not occupy the place of eka-tantalum in what is now called Group 5.
Eka-tantalum is actually dubnium.
Mendeleev's 1869 table had implicitly predicted a heavier analog of titanium and zirconium, but in 1871 he placed lanthanum in that spot.
The 1923 discovery of hafnium validated Mendeleev's original 1869 prediction. 
Professor Dmitri Mendeleev published the first periodic table of the chemical elements in 1869 based on properties which appeared with some regularity as he laid out the elements from lightest to heaviest.
When Mendeleev proposed his periodic table, he noted gaps in the table, and predicted that as-yet-unknown elements existed with properties appropriate to fill those gaps. 
The four predicted elements lighter than the rare earth elements, "ekaboron" ("Eb"), "ekaaluminium" ("Ea"), "ekamanganese" ("Em"), and "ekasilicon" ("Es"), proved to be good predictors of the properties of scandium, gallium, technetium and germanium respectively, which each fill the spot in the periodic table assigned by Mendeleev.
Initial versions of the periodic table did not give the rare earth elements the treatment now given them, helping to explain both why Mendeleev’s predictions for heavier unknown elements did not fare as well as those for the lighter ones and why they are not as well known or documented.
Scandium oxide was isolated in late 1879 by Lars Fredrick Nilson; Per Teodor Cleve recognized the correspondence and notified Mendeleev late in that year.
Mendeleev had predicted an atomic mass of 44 for ekaboron in 1871 while scandium has an atomic mass of 44.955910.
In 1871 Mendeleev predicted the existence of a yet-undiscovered element he named eka-aluminium (because of its proximity to aluminium in the periodic table).
The table below compares the qualities of the element predicted by Mendeleev with actual characteristics of Gallium (discovered in 1875 by Paul Emile Lecoq de Boisbaudran).
Technetium was isolated by Carlo Perrier and Emilio Segrè in 1937, well after Mendeleev’s lifetime, from samples of molybdenum that had been bombarded with deuterium nuclei in a cyclotron by Ernest Lawrence.
Mendeleev had predicted an atomic mass of 100 for ekamanganese in 1871 and the most stable isotope of technetium is 98Tc.
Germanium was isolated in 1886, and provided the best confirmation of the theory up to that time, due to its contrasting more clearly with its neighboring elements than the two previously confirmed predictions of Mendeleev do with theirs.