Here's why:

* Isoelectronic means that two species have the same number of electrons. However, they don't necessarily have the same number of protons.

* Molar mass is the mass of one mole of a substance, which is directly related to the number of protons in the nucleus of an atom.

Examples:

* N³⁻, O²⁻, F⁻ are all isoelectronic, meaning they have 10 electrons each. However, their molar masses are different:

* Nitrogen (N): 14.01 g/mol

* Oxygen (O): 16.00 g/mol

* Fluorine (F): 19.00 g/mol

The difference in molar mass arises because each element has a different number of protons.

In summary: While isoelectronic species have the same number of electrons, their molar masses can be different due to variations in the number of protons in their atoms.