Question

Position of the elements in the periodic elements......

How did the position of \[_{17}^{35}\text{Cl and} \phantom{.} _{17}^{37}\text{Cl}\] get fixed in the modern periodic table?

Answer 1

Mendeleev's periodic table arranged elements by atomic mass, which did not increase uniformly between consecutive elements. This created an issue for isotopes like Chlorine (³⁵Cl and ³⁷Cl) and \(³⁷Cl), which have different masses but identical chemical properties.

Henry Moseley resolved this by discovering that atomic numbers increase consistently by one unit from one element to the next. By organizing the Modern Periodic Table by increasing atomic number rather than atomic mass, the placement discrepancy for isotopes was solved. Since isotopes like \[_{17}^{35}\text{Cl and} \phantom{.} _{17}^{37}\text{Cl}\] share the exact same atomic number, they naturally occupy the same slot and group.