The correct answer is option 1. 63.
Dmitri Mendeleev developed the periodic table in 1869. At that time, the number of known elements was approximately 63. Mendeleev's work was based on the observation of recurring patterns in the properties of these known elements when they were arranged in order of increasing atomic mass.
Mendeleev noticed that when the elements were arranged this way, elements with similar properties appeared at regular intervals. He used this observation to construct his periodic table, organizing the elements into rows and columns. He arranged the elements into horizontal rows called periods and vertical columns called groups.
Mendeleev left gaps in his periodic table for elements that were yet to be discovered. He predicted the properties of these undiscovered elements based on the properties of neighboring elements. This bold step of leaving gaps and predicting the properties of undiscovered elements based on the periodicity of the known elements was one of the most significant contributions of Mendeleev's periodic table.
Over time, as scientists discovered new elements, Mendeleev's periodic table was refined and expanded. However, his original insights into the periodicity of the elements laid the foundation for the modern periodic table, which is arranged based on increasing atomic number rather than atomic mass, as proposed by Henry Moseley later on. |
