Examples might include;
Te²⁻, I⁻, Xe, and Ba²⁺.
Step-by-step explanation
How many electrons in a Cs⁺ ion?
Cs is a group 1 metal. It contains 55 electrons when neutral. A Cs atom loses one electron to form a Cs⁺ ion. The Cs⁺ ion shall contain 54 electrons.
By the Aufbau principle, electrons in an ion or atom will arrange to minimize their energy. The configuration of the 54 electrons in Cs⁺ will be the same as that in any other stable ion or atom with the same number of electrons.
What ions or atoms contain 54 electrons?
The atomic number of Xenon is 54. A neutral xenon atom contains 54 electrons. An Xe atom will be isoelectronic with the Cs⁺ ion.
Metal atoms from one period below Xe lose all their valence electrons to become isoelectronic with Xe and hence Cs⁺. For example, Ba is in group 2. Its atom contains two valence electrons when neutral. The Ba atom loses both to form the Ba²⁺ ion. The Ba atom contains 56 electrons when neutral. Ba²⁺ will contain 54, the same as Cs⁺. As a result, Ba²⁺ is also isoelectronic with Cs⁺.
Non-metal atoms from the same period as Xe gain electrons to become isoelectronic with Xe. For example, the atomic number of Te is 52. A Te atom gains two electrons to form a Te²⁻ ion. Each Te²⁻ ion will contain 54 electrons, which is the same as the number of electrons in a Cs⁺ ion. Te²⁻ will be isoelectronic with Cs⁺.
Similarly, an iodine atom (atomic number 53) is one electron away from Cs⁺. It gains one electron to form the I⁻ ion. The I⁻ ion will be isoelectronic as Cs⁺.