Barrons SAT Subject Test Chemistry, 13th Edition

The final formula is CaSO 4. Notice that the subscripts “2” are omitted.

Example 2

Use the crisscross method to write the formula for iron(III) sulfate, a category II
cation and a polyatomic ion.

The final formula is Fe 2 (SO 4 ) 3.

Examples of Ionic Compounds with Polyatomic Ions and Either Category I or

II Cations

Name

Formula

Comment

Sodium sulfate

Na 2 SO 4

Category I—the Na + always is 1+

Potassium dihydrogen

phosphate

KH 2 PO 4

The H 2 PO 4 − ion has a 1− charge and the K+, from category I, is 1+

Iron(III) nitrate

Fe(NO 3 ) 3

Category II—transition metal, must contain a Roman numeral

Cesium perchlorate

CsClO 4

The per- prefix is used because the polyatomic ion has 1 more oxygen

than the chlorate ion

Manganese(II) hydroxide

Mn(OH) 2

Category II—transition metal, must contain a Roman numeral

Category III—Binary Covalent Compounds

Binary covalent compounds are formed between two nonmetals. Although these
compounds do not contain ions, they are named very similarly to binary ionic
compounds. To name binary covalent compounds, use these steps.

1. The first element in the formula is named first, using the full elemental
   name.