Mendeleev arranged the elements by atomic mass and then separated them into various groups. What was his criterion for which elements were grouped together?

Mendeleev's periodic table was an early attempt at organizing the known chemical elements in a systematic manner. He arranged the elements in order of increasing atomic mass and noticed that elements with similar properties occurred at regular intervals.

Mendeleev grouped the elements together based on their similarities in chemical and physical properties. He noticed that when elements were arranged in order of increasing atomic mass, elements with similar properties tended to appear in the same vertical column. This allowed him to identify groups or families of elements that shared common characteristics, such as reactivity, melting point, and boiling point.

Mendeleev's criterion for grouping elements together enabled him to predict the properties of undiscovered elements based on their position in his periodic table. This predictive power was a major breakthrough in the study of elements, and it ultimately led to the further development of the modern periodic table, which is organized by atomic number rather than atomic mass. To sum up, Mendeleev's criterion for grouping elements together in his periodic table was their similarities in chemical and physical properties when arranged in order of increasing atomic mass. This organization allowed him to identify groups or families of elements sharing common characteristics, and to predict the properties of undiscovered elements based on their position in the periodic table.