Arrange the following atoms or ions in order of decreasing ionization energy: Sr2+, Se2-, Rb+, Br-?

Ionization energy : Sr²⁺ > Rb⁺ > Br⁻ > Se²⁻

Explanation 1 "
All of Sr²⁺, Se²⁻, Rb⁺ and Br⁻ are isoelectronic and have the electronic configuration of krypton.
In each atom, electrons in the outermost shell is attracted by nuclear charge. Therefore, the greater the nuclear charge, the stronger the force to attract electrons in the outermost shell is, and leading to a greater ionization energy.
Atomic number: Sr (38) > Rb (37) > Br (35) > Se (34)
Nuclear charge: Sr²⁺ (38) > Rb⁺ (37) > Br⁻ (35) > Se²⁻ (34)
Hence, ionization energy : Sr²⁺ > Rb⁺ > Br⁻ > Se²⁻

Explanation 2 :
All of Sr²⁺, Se²⁻, Rb⁺ and Br⁻ are isoelectronic and have the electronic configuration of krypton.
Sr²⁺ and Rb⁺ have 2 positive charges and 1 positive charge respectively to attract the outgoing electron, while Br⁻ and Se²⁻ have 1 negative charge and 2 negative charges respectively to repel the outgoing electron.
Hence, ionization energy : Sr²⁺ > Rb⁺ > Br⁻ > Se²⁻