Is triodide ion polar?

No, triiodide is nonpolar. Polarity depends on the difference in electronegativity between two bonded atoms. Triiodide is composed of three iodine atoms which have identical electronegativities. There is no permanent build up of electron density on one end of the I—I bonds because each iodine is just as good at attracting electrons as its neighbor. Triiodide is nonpolar for the same reason that molecular iodine (I2) is nonpolar. Polarization occurs when you have a significant difference in electronegativity between two bonded atoms, for example the H—O bonds in water.