In 1869, Dmitri Mendeleev, a Russian chemist, proposed the periodic law and his first periodic table. He published this periodic table two years later in a textbook, The Principles of Chemistry (Osnovy khimii). The series of rows show that chemistry is periodic, meaning that certain properties repeat. Based on the repeating patterns, he left gaps for four elements that he expected would exist. Within 10 years, scientists would discover three of these.
