Compound Naming Guide

Writing Formulas for Molecular Compounds 

Order of elements for writing: 

1. Carbon 

2. Hydrogen 

3. All other nonmetals in order of decreasing electronegativity. 

Subscripts indicate the quantity of each element per molecule in chemical formulas. 

Writing Names for Molecular Compounds 

Prefixes indicate the quantity of each element per molecule:

• Mono-, Di-, Tri-, Tetra-, Penta-, Hexa-, Hepta-, Octa-, Nona-, Deca-.

• Exceptions:

  • No "mono-" for the first element (e.g. carbon monoxide).

  • Vowel adjustment for oxygen (e.g., monoxide, not monooxide).

In binary molecular compounds, the second element ends with the suffix "-ide." 

• Examples: 

  • Hydrogen → Hydride 

  • Carbon → Carbide 

  • Nitrogen → Nitride 

  • Oxygen → Oxide 

  • Fluorine → Fluoride 

Writing Formulas for Ionic Compounds 

Order of elements for writing: 

1. Cation (the positively-charged ion) comes first. 

2. Anion (the negatively-charged ion) comes second. 

Formula Unit: The simplified ratio of ions in an ionic compound. 

Subscripts indicate the quantity of each element per formula unit in chemical formulas. 

Writing Names for Ionic Compounds 

Cations: 

• If it is a representative element, you generally only have to write its name. 

  • Exceptions are Sn (+2, +4), Pb (+2, +4), Sb (+3, +5), Bi (+3, +5), and Tl (+1, +3), since they have multiple oxidation states. 

• If it is a transitional metal, you generally have to specify its charge due to varied oxidation states. 

  • Exceptions are Ag (+1) and Zn (+2), since they have only one oxidation state. 

• For polyatomic ions, use their common name (don't modify). 

Anions: 

• If it is an element, the prefix "-ide" is used. 

• For polyatomic ions, use their common name (don't modify). 

Use the subscripts to determine the charges of the ions, as they must cancel out to create a neutral formula unit.

• For example, to find out the charge of iron cations in Fe₂O₃, we know that oxygen has a charge of -2. With 3 oxygen ions, the total negative charge is -6, so the iron ions must contribute +6. Since there are 2 iron ions, each iron ion must have a charge of +3.

Writing Formulas for Acids 

Note: Acids are typically used while aqueous. This means that they are dissociated in water as a solution, which donates protons to increase H₃O⁺ concentration. 

• Binary Acids (hydrogen + one other component):

1. Hydrogen is written first. 

2. The other nonmetal or particle is written second. 

• Oxyacids (hydrogen + polyatomic ion with oxygen):

1. Hydrogen is written first.

2. The polyatomic ion is written second. 

Writing Names for Acids 

Binary Acids: 

• Start with the "hydro-" prefix. 

• Write the root word of the other element. 

• End with the "ic" suffix. 

• Follow with the word "acid."

Oxyacids: 

• Write the name of the polyatomic. 

• If the polyatomic ion ends in "-ate", change the suffix to "-ic". 

  • Exception: Phosphoric Acid (H₃PO₄) comes from phosphate (PO₄³⁻), but we keep the "oric" despite the rule. 

• If the polyatomic ion ends in "-ite", change the suffix to "-ous".