Which isoelectronic ion is the biggest?

Why isoelectronic ion with higher positive charge has smaller size?

• Answer:

  The traditional explanation for this is that the electrostatic attraction of the electrons to the nucleus of an atom is reduced by repulsion of the other electrons present in an atom without positive charge. If an isoelectronic ion has a positive charge, the effect of repulsions is reduced because of the smaller number of electrons, and the resulting stronger attraction to the positively charged nucleus pulls the outer electrons closer. However, all such questions about the properties of atoms should preferably be answered in a purely mathematical manner by quantum mechanics.