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Trigonal planar
SeO3-2 stands for Selinite Ion.
SeO3 would be acidic, tending to make selenic acid H2SeO4
trigonal planar
polar
Lithium is Li^+ , and selenite is SeO3^2-. It will take 2 Li^+ for each SeO3^2-, so the formula is Li2SeO3
Fe2(SeO3)3
Sulfur tetraoxide is a trigonal planar. There is the sulfur in the middle and three oxygen that surrounds it with all of them a double bond linking them to the sulfur.
Selenite ion is the compound of SeO3.
SeO2
Gypsum has the chemical formula CaSO4.2H2O A selenite has the chemical formula Me(SeO3)-, where Me is a metal. As gypsum selenites are salts.
SeO2 is the chemical formula for selenium dioxide.