Here's how to determine the number of electrons in the chlorine-35 ion (³⁵Cl¹⁻):

* Atomic Number: Chlorine (Cl) has an atomic number of 17. This means a neutral chlorine atom has 17 protons and 17 electrons.

* Charge: The superscript "1-" indicates that the chlorine atom has gained one extra electron.

* Total Electrons: Therefore, the ³⁵Cl¹⁻ ion has 17 + 1 = 18 electrons.