How did the periodic table change over time

The periodic table has long-since filled in Mendeleev's gaps and has added new elements.It has even changed the weights of other elements.

Explanation:

The first periodic table in the "rows and columns" form we see today was invented by Dmitri Mendeleev in 1869. It included the properties of all of the known elements of that time. Mendeleev's left gaps in the periodic table as he knew there were yet to be other elements discovered and that has now been filled with the other discovered elements.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.