Gaps in the periodic table were first used by Dimtri Mendeleev when he could not find an element to fit the patterns of the groups and periods he was using. Mendeleev knew there would be undiscovered elements so used these gaps to prompt other scientists to start looking for them.
Over time as scientific techniques and machinery advanced this allowed scientists to discover more about the mysteries of the atom. In the process, new elements have been discovered and the gaps in the periodic table completed.
A very famous example is Gallium also known as element 31. By looking at the metals either side of element 31 and the group it was in. Mendeleev hypothesized its properties to be a soft silvery metal with a melting point of around 30 degrees and named it Eka-Aluminium as it was just below Aluminium in Group 13. It wasn’t until 1875 when the french scientist Francois Lecoq de Boisbaudran discovered Gallium, with nearly exactly the same properties as Mendeleev predicted.
In more recent times, the discovery of radioactive elements and decay experiments have lead to the discovery of increasingly larger atoms with very high Atomic Numbers which require specialist equipment and techniques to discover. This was made possible by the invention of the cyclotron one of the first particle accelerators invented by Ernest Lawrence in 1920. A particle accelerator allowed scientists to accelerate atoms at high speeds and collide them together creating new elements with previously unseen masses, opening the door to create synthetic elements in the lab.
Dmitri Mendeleev was a Russian scientist who in 1869 proposed the first structure of the periodic table
Eka aluminium was the term used to first describe the gap in the periodic table below aluminium before the element gallium was discovered
Francois Lecoq de Boisbaudran was a french scientist credited with discovering the element gallium
Atomic number is the number of protons in the nucleus
A particle accelerator is a machine designed to speed up atoms before colliding them together to create new elements with a large mass
Ernest Lawrence was an American scientist who is credited with inventing the first particle accelerator called a cyclotron