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Demitri Mendeleev is one of the greatest chemists in history. He is the creator of the periodic table. His periodic table predicted undiscovered elements and their chemical and physical properties.
Mendeleev's Discovery
Mendeleev published his periodic table in 1869. However, Mendeleev was not the first chemist to propose a periodic table. At least five other chemists published a periodic table before him. We've forgotten many of the early periodic tables. Some lacked the insights of Mendeleev's table. However, a few got discredited based more on politics than science. In the 1800s, academic fame was important for getting papers published and distributed. Sadly, some of the earlier discoverers of the periodic table lacked academic standing. They did not gain recognition for their insights.
Folklore tells us that Mendeleev discovered the periodic table in a dream. However, he had been working on classifying the elements based on patterns. He noticed that many of the elements had shared chemical and physical properties. At the time, we had not discovered the existence of protons, neutrons, and electrons. Chemists thought that atoms were indivisible because of Dalton's atomic theory. The only measurement Mendeleev had was atomic mass. He began to arrange elements with similar properties into columns. He based his arrangement on the elements' atomic masses. He also used atomic mass to organize the elements into rows. As he arranged the table he realized his periodic law. His periodic law stated that the best way to arrange the elements is by atomic mass. When arranged, the elements' physical and chemical properties will go through periodic cycles. Today we would call these periodic trends or periodicity.
Mendeleev's fame comes from his audacity. He left blank spaces for undiscovered elements. He also had the courage to predict the properties of these undiscovered elements. Six years after Mendeleev published his periodic table, a French chemist discovered Gallium. As chemists tested the properties of Gallium, they discovered that Mendeleev was correct! He predicted Gallium's atomic mass, state of matter, melting point, density, and common oxidation states. Mendeleev's periodic table became indispensable.
Mendeleev's Incorrect Predictions
Mendeleev did get a few things wrong. The most significant is that he did not predict the noble gasses. Chemists at the time did not know about valence electrons. Instead, they used oxide and hydride formation as the basis of bonding. He could not predict the existence of colorless, odorless, and unreactive elements.
He also held too tightly to his pattern. He did know that some elements could bond with four oxygens. He placed these elements in group eight of his table. But, these elements are transition metals. Today we know that the transition metals have d-orbitals. The d-orbitals can hold ten electrons. Mendeleev basically noticed the octet rule. However, he did not consider that some elements may be exceptions to the octet rule. He included elements like Copper and Zinc with the alkali and alkaline-earth metals.
He was also limited by only having atomic masses. A few elements with a higher atomic number weigh less. For example, the atomic mass of Tellurium is 127. But, the atomic mass of Iodine is 126. Mendeleev proposed that the experimental values were wrong. He argued that Tellurium would have an atomic mass of 125 and Iodine an atomic mass of 127. Today we know that isotopes are the reason for the differences in atomic weights.
