Mendeleev's predicted elements
Dmitri Mendeleev published a periodic table of the chemical elements in 1869 based on properties that 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-then-unknown elements existed with properties appropriate to fill those gaps. He named them eka-boron, eka-aluminium and eka-silicon with atomic masses 44, 68 and 72 respectively.
2019
2019 has been designated as International Year of the Periodic Table of Chemical Elements by the United Nations General Assembly given that it coincides with the 150th anniversary of its creation by Dmitri Mendeleev in 1869.