Writing Formulas Review
Ionic Compounds
1) a metal and nonmetal compound. Metals are on the left of the periodic table, nonmetals are on the right of
the stair step line.
a) The metal forms a positive ion called a cation.
b) The nonmetal forms a negative ion called an anion.
c) The cation is always written first in a compound.
2) All group 1 metals form a 1+ ion, all group 2 metals form a 2+ ion, B and Al form 3+ ions, Group 15 down
to the metalloid line form 3- ions, Group 16 down to the metalloid line form 2- ions, and Group 17 form 1ions. All other elements may form more than one ion except Ag, Zn, and Cd, which always form Ag1+,
Zn2+ and Cd2+
3) Polyatomic Ions – YOU MUST MEMORIZE!!!! Ammonium, nitrite, nitrate, sulfite, sulfate, hydrogen
sulfate (bisulfate), hydroxide, cyanide, phosphate, hydrogen phosphate (biphosphate), dihydrogen
phosphate, carbonate, hydrogen carbonate (bicarbonate), hypochlorite, chlorite, chlorate, perchlorate,
acetate, permanganate, dichromate, chromate, peroxide, oxalate (C2O42-), phosphite (PO33-)
a) Note: You must memorize the formula, the charge and the name!
4) Transition metals and metal below the metalloid line on the periodic table can have more than one charge.
You will know the charge by the Roman numeral in the formula’s name. Ex. Iron (II) oxide = Fe2+, Lead
(IV) oxide = Pb4+.
Once you know the cation and anion’s charges, you may put the compound’s formula together by balancing the
charges. The charges must add up to zero in the final compound. The compound may not have any charges on
it. You may use the criss-cross method if you like. However, you must remember to reduce the subscripts.
Ex. Fe3+ and O2-  Fe2O3 2 and 3 cannot be reduced, the formula is correct.

Ex. Pb4+ and O2-  Pb2O4; However 2 and 4 can be reduced, so the correct
formula is PbO2.
Remember, if you need more than one polyatomic ion you must put parenthesis around it and put how many
you need outside of the parenthesis. You may never change any subscript in the polyatomic ion.
Molecular formulas
 Made up of two nonmetals
 You will recognize their names because they use prefixes. Mono-1, di-2, tri-3, tetra-4, penta-5, hexa-6,
hepta-7, octa-8, nona-9, deca-10. Whatever prefix the element has is how many atoms are in the formula.
Diphosphorous pentoxide = P2O5.
Acids
 Has the word acid attached to it. Will always have an H+ as the cation.
 If is has the word hydro then an element name ending in –ic and the word acid, it is binary. It contains H+
cation and a nonmetal with it’s normal charge for the anion. Yes, you must still balance charges.
Hydrosulfuric acid = H+ and S2- = H2S
 If it does not have the word hydro at the beginning, then it is H+ and a polatomic ion. If the root word ends
in –ic followed by the word acid, then the polyatomic ion ends in
– ate. Ex. Sulfuric acid – must have
come from H+ and the sulfate ion (SO42-). Therefore the formula for sulfuric acid is H2SO4, remember to
balance your charges.
 If there is no hydro and the acid’s root name ends in –ous, then the polyatomic ion in the acid ends in –ite.
Ex. Sulfurous Acid – must have come from H+ and the sulfite ion (SO32-). Therefore the formula for
sulfurous acid is H2SO3.
Review Practice
Write the formula for the following compounds:
1) Sodium nitrite - _________________________
26) Carbon disulfide - _______________________
2) Sodium carbonate - ______________________
27) Iron (II) sulfate - ________________________
3) Disulfur dichloride - _____________________
28) Iron (III) phosphate - _____________________
4) Sodium sulfate - ________________________
29) Acetic acid - ____________________________
5) Nitrogen triflouride - _____________________
30) Iron (III) bromide - ______________________
6) Potassium hydroxide - ____________________
31) Carbon dioxide - ________________________
7) Potassium nitrate - _______________________
32) Strontium chloride - _____________________
8) Potassium sulfite - _______________________
33) Strontium hydroxide - ____________________
9) Carbonic acid - _________________________
34) Strontium nitrate - _______________________
10) Potassium phosphate - ___________________
35) Dichlorine heptoxide - ____________________
11) Cadmium oxalate - ______________________
36) Phosphoric acid - ________________________
12) Hydrochloric acid - _____________________
37) Strontium sulfite - _______________________
13) Diphosphorous trioxide - _________________
38) Phosphorous acid - ______________________
14) Cadmium carbonate - ____________________
39) Strontium sulfide - ______________________
15) Cadmium sulfate - ______________________
40) Iron (III) sulfate - _______________________
16) Carbon tetrabromide - ___________________
41) Iron (III) phosphate - ____________________
17) Cadmium phosphate - ___________________
42) Mercury (II) bromide - ___________________
18) Aluminum bromide - ____________________
43) Mercury (II) carbonate - __________________
19) Aluminum nitrate - _____________________
44) Mercury (II) sulfide - ____________________
20) Nitric acid - ___________________________
45) Dinitrogen tetroxide - ____________________
21) Aluminum nitrate - _____________________
46) Bismuth (III) telluride - __________________
22) Iron (II) oxide - ________________________
47) Nickel (II) fluoride - _____________________
23) Iron (II) hydroxide - _____________________
48) Cobalt (III) oxide - ______________________
24) Carbon monoxide - _____________________
49) Zinc cyanide - __________________________
25) Iron (II) carbonate - _____________________
50) Silver perchlorate - ______________________