How did Mendeleev arrange the periodic table?

Mendeleev arranged the periodic table based on atomic weight, while also considering the chemical properties of the elements.

Dmitri Mendeleev, a Russian chemist, is credited with the creation of the first widely recognised periodic table in 1869. His innovative approach was to arrange the elements by their atomic weight, which is the total weight of the protons and neutrons in an atom. However, Mendeleev didn't just stop at atomic weight. He also took into account the chemical properties of the elements, grouping together those with similar characteristics.

Mendeleev's periodic table was not just a simple list of elements. It was a grid-like structure, with elements placed into rows and columns. The rows, or periods, represented the energy levels of the atoms, while the columns, or groups, contained elements with similar properties. This arrangement allowed Mendeleev to highlight the periodicity of the elements, or the repeating patterns in their properties.

One of the most remarkable aspects of Mendeleev's periodic table was his prediction of the existence of undiscovered elements. He left gaps in his table where he believed new elements would fit, based on the patterns he observed. He even predicted the properties of these unknown elements. When these elements were later discovered and their properties measured, Mendeleev's predictions were found to be remarkably accurate.

Mendeleev's periodic table has been refined and expanded over the years, with the modern periodic table now arranged by atomic number (the number of protons in an atom) rather than atomic weight. However, the basic principles of Mendeleev's arrangement - the grouping of elements by similar properties and the recognition of periodicity - remain at the heart of the periodic table today.