Complex sentence: In 1869, when Dmitri Mendeleev published his periodic table, the space under iodine was empty; after Niels Bohr established the physical basis of the classification of chemical elements, it was suggested that the fifth halogen belonged there.
Split into simpler sentences: In 1869, when Dmitri Mendeleev published his Periodic Table, the space under iodine was empty. After Niels Bohr established the physical basis of the classification of chemical elements, it was suggested that the fifth halogen belonged there.