Here's how to determine the number of electrons in a scandium ion:

* Scandium's Atomic Number: Scandium (Sc) has an atomic number of 21. This means a neutral scandium atom has 21 protons and 21 electrons.

* Scandium Ion: Scandium typically forms a +3 ion (Sc³⁺). This means it loses 3 electrons.

* Electron Count: A scandium ion (Sc³⁺) therefore has 21 - 3 = 18 electrons.