Because those elements were not discovered at the time Mendeleev proposed the periodic table.
Mendeleev left gaps for eight elements in his periodic table that were yet to be discovered. These gaps eventually corresponded to the elements gallium, germanium, scandium, technetium, francium, and technetium.
Mendeleev left blank spaces in his periodic table to account for elements that had not yet been discovered. He predicted the properties of these missing elements based on the patterns he observed in the table. This empty spaces allowed for the periodic table to be a predictive tool for the discovery of new elements.
There are more elements and it's organized differently
The long dashes on Mendeleev's periodic table represent gaps where elements had not yet been discovered but were predicted to exist based on the periodic pattern of elements. Mendeleev used these gaps to accurately predict the properties of the missing elements, leading to the discovery of new elements.
Mendeleev left blank spaces because certain elements on the Periodic Table were not known at that time. He knew that in the future, those elements would be found and placed on the periodic table.
Leaving spaces and gaps in the periodic table allowed for prediction of new elements and their properties, which were later discovered and filled in. This vindicated the decision to leave gaps, as it demonstrated the power of the periodic table in organizing and predicting the properties of elements.
The gaps represented the elements which were not discovered. When the elements were invented, they were placed into these gaps.
The gaps in the periodic table left by Mendeleev were eventually filled in as new elements were discovered. Scientists continued to study and experiment, leading to the discovery and verification of elements that fit into the empty spaces in the periodic table.
There were gaps in the periodic table a hundred years ago because investigators had not discovered some elements yet, though there was a "place" for them in the array. Recall that through the genius of Mendeleyev, the elements had been given an "order" owing to their chemical properties. By creating a "chart" (the first periodic table), Mendeleyev could see where elements "would be" though none had yet been discovered. He could predict that eventually an element would appear "here" on the table and would have a "these" properties because of the arrangements in the array or table of element.
Dmitri Mendeleev predicted the existence and properties of several elements that were later discovered and filled gaps in the periodic table. He also predicted the properties of these elements based on their position in the periodic table.
When Mendeleev proposed his periodic table, he noted gaps in the table, and ... Initial versions of the periodic table did not give the rare earth elements the ... that these atomic elements would be discovered, that he included them in later.
Dmitri Mendeleev noticed gaps in the periodic table when he arranged elements by increasing atomic mass and saw that certain elements with similar properties did not fit into the pattern. He predicted the existence and properties of these missing elements based on the trends in the known elements around them.
Mendeleev left gaps for eight elements in his periodic table that were yet to be discovered. These gaps eventually corresponded to the elements gallium, germanium, scandium, technetium, francium, and technetium.
It was recognized by the inventor of the Periodic table that these empty spaces are for elements/isotopes that would be discovered someday. Dmitri Mendeleev is generally credited with the publication, in 1869, of the first widely recognized periodic table. He developed his table to illustrate periodic trends in the properties of the then-known elements. Mendeleev also predicted some properties of then-unknown elements that would be expected to fill gaps in this table.
Mendeleev left blank spaces in his periodic table to account for elements that had not yet been discovered. He predicted the properties of these missing elements based on the patterns he observed in the table. This empty spaces allowed for the periodic table to be a predictive tool for the discovery of new elements.
The gaps in the periodic table provided a framework for scientists to predict the properties of undiscovered elements based on the trends and patterns observed in the known elements. By identifying missing elements in the periodic table, scientists were able to focus their search on regions where certain properties were expected based on the elements surrounding the gaps. This guided experimental efforts to discover and synthesize new elements.
There are more elements and it's organized differently