So you want to name a compound or identify a formula huh? Well first we need to know what type of compound we’re talking about (See the rules for determining the type of compound on the previous post).
Here is a quick rundown of the rules for naming Binary Compounds.
Nonmetal + Nonmetal = Molecular Compound
When naming molecular compounds, we use prefixes to identify how many of each type of element are present in the compound.
- Mono * 6. Hexa
- Di 7. Hepta
- Tri 8. Octa
- Tetra 9. Nona
- Penta 10. Deca
*The only time we vary from this is when there is only one of the first type of element we don’t add mono. For example, CO is Carbon monoxide, not Monocarbon monoxide.
The final rule for naming Molecular compounds is that the second element, when it is a nonmetal, the ending is changed to the suffix “-ide”. For example, Sulfur would be sulfide.
So for example, P4O10 would be Tetraphosphorus Decaoxide
Metal + Nonmetal = Ionic Compound
To name an ionic compound, we do not need to use prefixes to name our compounds. Instead, we have to identify the charge of the elements we’re working with.
If the element is a metal, it will is a Cation (+). If the element is a nonmetal it is an Anion (-).
The charge is usually written on the periodic table, but it can also be deduced from the placement of the element on the periodic table. Alkali Metals are all +1, Alkaline Earth Metals are all +2, and Group 3B is +3. Likewise the Halogen group (7A) is -1, the Oxygen group (6A) is -2, and the Nitrogen group (5A) is -3.
After you determine the charge of the elements, you then find the “Common Charge” in order to make the compound balanced (same number of +/- charges on each side). Lets take Magnesium fluoride for example.
Mg has a charge of +2; while F has a charge of -1. In order to make the charges balance, we can add atoms to one side or the other. In this case, we need another F -1 in order to make the compound balance. So the common charge of Magnesium fluoride is ±2, and the formula would be written MgF2