Selenide is an ion. Its chemical formula is Se. Selenides are similar to sulfides. Selenides are reducing agents when pure. They are stronger reducing agents than sulfides. Selenide ores are the source of selenium. Selenides are normally mixed with sulfides. The selenide ion is unstable except in very basic conditions. Normally hydroselenide is made, which is HSe. In acidic conditions, hydrogen selenide gas is made.