Arrange this isoelectronic series in order of increasing atomic radius: Se^2- , Sr²⁺ , Rb⁺ , Br^- .

Isoelectronic species are atoms or ions that have the same number of electrons and, therefore, the same electronic configuration. In this question, Se^2-, Sr²⁺, Rb⁺, and Br^- all have 36 electrons, making them isoelectronic. Understanding this concept is crucial for comparing their atomic radii, as they share similar electron arrangements.

Atomic radius refers to the size of an atom, typically measured from the nucleus to the outermost electron shell. In general, atomic radius increases down a group in the periodic table due to the addition of electron shells, while it decreases across a period due to increased nuclear charge. For isoelectronic species, the effective nuclear charge experienced by the electrons influences their size, with more protons pulling electrons closer and resulting in a smaller radius.

Effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in a multi-electron atom, accounting for both the total nuclear charge and the shielding effect of inner electrons. In the context of isoelectronic species, the greater the number of protons in the nucleus, the higher the Z_eff, leading to a smaller atomic radius. Thus, comparing the atomic radii of the given ions involves analyzing their respective nuclear charges and how they affect the size of the electron cloud.