Й is for iod

This letter is a variation on the last one, but shorter in sound. It is called an "i kratkaye" or "short ee." It often forms diphthongs, meaning it combines with another vowel to make one sound. In handwriting, it is the same as Ии, but with the short sign on top: Йй.

Йод (iod, pronounced "yode") is iodine, which was used as an antiseptic in years gone by. Few other words start with Йй!