let's talk about how that happened. if we look at the periodic table from around 1870, we can see elements designated by their two-letter symbols arranged in a table.
check out the entry of the third column, fifth row. there's a dash there. from that placeholder springs the brilliance of Mendeleev. By putting it there he was making a statement. you all haven't discovered this element yet, in the meantime I'll give it a name.
based on where it was located he was able to tell us alot about it. he could predict all these things simply from where the blank spots were, and his understanding of how the elements behaved.
Paul Lecoq de boisdaudran(2018) a few years after this prediction, a French guy named Paul Lecoq discovered a new element and named it Gallium after Gaul. it's one step away from aluminum on the table, it's eka-aluminum. Mendeleev was right!
gallium's atomic weight is 69.72, a cubic cm of it is 5.9g its a solid metal at room temp but it melts at 30c. not only did mendeleev nail gallium but he predicted many other elements that werent known at the time.
Dmitri Mendeleev's theory was arranging the sixty three elements based on atomic mass into a periodic table. dmitri listed the elements in order by their atomic weight and put them in columns based on their physical and chemical properties. the periodic table is used by students and chemists around the world to understand chemical reactions.