How to calculate the place of of a element if know their atomic number in a periodic table?


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  • $\begingroup$ Try writing the electronic configuration of that element. It will give you some clues. $\endgroup$ – Apoorv Potnis Feb 18 '18 at 17:21

You will not fail following the numbers of periodicity from one electron in the outer electronic shell to the completely filled shells. Shells are filled

  • in the first shell with 2 electrons ($\ce{_2He}$),
  • in the second shell with 8 more electrons ($\ce{_10Ne}$)
  • in the third shell with again 8 more electrons ($\ce{_18Ar}$)
  • in the fourth shell with 18 more electrons ($\ce{_36Kr}$)
  • in the fifth shell with again 18 more electrons ($\ce{_54Xe}$)
  • in the sixth shell with 32 more electrons ($\ce{_86Rn}$)
  • in the seventh shell with - you guess - again 32 electrons.

The positions of elements were arranged empirical by their chemical properties by Mendeleev in 1865. Years later it became clear that atoms consist of electrons, arranged around the nucleus and the mentioned above electron configurations are inert or Noble gases.

Unfortunately physics claims shells to have $2n^2$ electrons (where $n$ is the number of the shell). Which brings you to the point to learn about Aufbau principle and Madelung rule.

  • 1
    $\begingroup$ Well, physics is perfectly correct, as long as you consider a shell to be represented by a single principal quantum number $n$. Nobody said that shells must correspond exactly to rows in the Periodic Table, which is what you seem to be talking about. $\endgroup$ – orthocresol Feb 18 '18 at 20:21

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