# What Is the Modern Periodic Law?

Periodic law, also known as Mendeleev's law, is the concept that the chemical and physical properties of elements are based on an element's atomic weight when arranged by increasing periodic atomic number. Periodic law was first developed in 1869 when Dimitri Mendeleev and Lothar Meyer each developed a periodic table on their own and noticed similarities in elements with comparable atomic masses. Both men organized the elements by their masses and noticed certain properties recur periodically.

Mendeleev organized his periodic table by listing the elements with like characteristics into columns called groups. He left blank spaces for the elements that had yet to be discovered, such as scandium, gallium, technetium and germanium. Noble gases were also left off of Mendeleev’s table because they had yet to be discovered. Meyer also organized his chart according to atomic mass, but based his periodic law on the molar volume of the elements, which is the atomic mass divided by the density. Mendeleev’s table, however, is still considered noteworthy because of the accuracy of the elements’ atomic masses.

Mendeleev assumed that atomic weight was inaccurate, which was his main reason for not organizing his periodic table by increasing atomic weight. In 1913, Henry Moseley hypothesized that the properties of the elements were based on its nuclear charge, not its nuclear weight. He X-rayed the elements and used the following formula to confirm his suspicions: V = A(Z-b)2.

(V is equal to X-ray frequency. Z is the atomic number, and A and b are constants.) After his experiment, Moseley revised periodic law to state the following: "Similar properties recur periodically when elements are arranged according to increasing atomic number."

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