Explain how you know that no new element with an atomic number less than 100 will be discovered

The elements are identified by its aomic number and they are placed in the periodic table in ascending order of atomic number.

There is one unique element for a given atomic number.

For example, atomic number 1 identifies Hydrogen and there cannot be another different element with the same atomic number.

All the elements with atomic numbers from 1 to 118 have been discovered. So, you know that there is not room for new products with atomic number equal or less than 118.