Which of the following isoelectronic species has the smallest atomic radius?
Ar
As we have learned
Atomic Radius -
The distance from the centre of the nucleus to the outermost shell containing the electrons is called the atomic radius. It can be measured by the spectroscopic method and by x-ray diffraction or electron diffraction techniques.
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Isoelectronic species are those atoms or ions which have the same number of electrons. In the case of isoelectronic species as the no. of protons increases atomic radius decreases and the size decreases.
Therefore, Option(4) is correct