Chemistry Help with Naming Compounds
- Dec 21, 2023
- 4 min read
Updated: Jan 22, 2024
Top Mistakes Students Make on Chemistry Exams Series
Nomenclature Nightmare: Navigating the Maze of Naming Compounds
Have all the rules for naming compounds made you dizzy?
Naming compounds can often feel like navigating a maze. It is not uncommon for students to get lost in the labyrinth of rules and exceptions.
This blog post will focus on the common questions students have that cause them to make mistakes when naming compounds.
It is NOT meant to teach the subject but is a follow-up to a lecture on naming compounds.
If you'd like a review on naming compounds; click on the following links to view the videos.
The main areas of confusion students tend to have after listening to a lecture on naming compounds are the following:
Knowing when you state and don't state the number of atoms in a compound.
Knowing when to use Roman numerals and what the numerals stand for.
Remembering the rules for naming aqueous acids (in part 2).
Understand the reason behind the rules to remember them better.
Common Point of Confusion
Chemistry Help with Naming Compounds: When do you state the number of atoms in the name of compound?
The only time you state the number of atoms in the name of compound is if it is a molecular compound. A molecular compound is made only of nonmetal atoms.
The first step in naming compounds is to classify them as a molecular compound, ionic compound, base, or aqueous acid.
Classification depends on whether the compound contains metal or nonmetal atoms.
Molecular compounds are made only of nonmetal atoms. On the periodic table, nonmetals are shown in green.
More than one type of compound is possible between the same type of nonmetals. For example, some of the possible compounds between nitrogen and oxygen include NO, NO2, and N2O5.
Greek prefixes are needed to state the number of nitrogen and oxygens in the compound since there are multiple compounds made only of nitrogen and oxygen.
Click to show Greek Prefixes
Mono: 1
Di: 2
Tri: 3
Tetra: 4
Penta: 5
Hexa: 6
Click to show names of NO, NO2, N2O5
NO: nitrogen oxide
NO2: nitrogen dioxide
N2O5: nitrogen pentoxide
The reason we state the number of atoms in molecular compounds is that we can NOT predict the formula of molecular compounds.
HOWEVER, we do NOT use Greek prefixes to state number in compounds (ionic compounds, bases, and aqueous acids) that are made of a positive and negative ions.
We can predict the formula of ionic compounds or bases using the ion charges. There is only one possible formula for these compounds. Therefore, we don't state the number of ions in the name of ionic compounds.
Ionic compounds
Ionic compounds are made of positive and negative ions.
Metals (shown in blue on the periodic table) will form positive ions while nonmetals (in green) will form negative ions.
How can we predict the formula of an ionic compound from the periodic table?
1.Understand a few generalities from the periodic table.
The two main classifications shown on the periodic table are Family A and Family B. Note that metals can be either Family A or Family B.
The columns for Family A are labeled with roman numerals
2. Determine the ion charge that atoms will form from the periodic table.
a. Metals (blue) will form positive ions.
Family A: Charge is equal to the group number
Family B: We only know the ion will be positive. We can't say the magnitude of charge; if it is +1, +2, or +3.
b. Nonmetals (green) will form negative ions. The ion charge is (Group # - 8).
c. Atoms in Group IV and VIII are unlikely to form ions.
3. Determine the number of ions needed to form an electrically neutral compound. The number will be shown as a subscript in the formula.
There is only one possible compound between a lithium, Li, and a sulfur, S, atom.
It is Li2S. This is the only formula that is possible between Li and S.
It will never be LiS3, Li3S2, LiS2, or any other combination than the one above. Only Li2S is electrically neutral.
This is because Li will always form the +1 ion and S will only form the -2 ion. We know this from the periodic table. Neither atom will form a different ion than those shown on the periodic table above
The name of Li2S is lithium sulfide.
Note: Don't state that there are two lithium ions in the name since we can predict the formula from the periodic table.
The reason we do NOT state the number of ions in a compound made of positive and negative ions (ionic compounds, bases, acqueous acids) is that there is only one possible formula for the compound.
Common Point of Confusion
Chemistry Help with Naming Compounds: When are Roman Numerals Used in Naming Compounds?
Roman numerals are only used to state the charge of a Family B (aka Transition) Metal in Ionic Compounds and Bases.
We can predict the magnitude of ion charge (+1, +2, +3) for Family A metals, but the only item we can predict for Family B metals is that they form a positive ion.
Unlike Family A metals, more than one possible ion can form from Family B (transition) metals. For example, copper, Cu, is a Family B metal which can form both the +1 and +2 ions.
For this reason, we must state whether Cu is the +1 or +2 ion in the name of the compound. We do this with roman numerals.
This requires an extra step. It is necessary to determine the charge on the Family B cation in order to name the compound.
Example: Name CuCl2
Step 1: Find Cu and Cl on the periodic table.
Cl is a nonmetal in Group VII A.
Cu is a transition metal in Family B.
Step 2: Determine the ion charge on both ions. The charge on Cl can be found from periodic table.
The ion charge for nonmetal atoms is (Group # -8).
The ion charge for Cl is (7-8) = -1
Step 3: Determine the ion charge on both ions. The charge on Cu is determined from the formula of the compound.
We know that CuCl2 is an electrically neutral compound. Each Cl has a -1 charge.
The two Cl give a total charge of 2 X (-1) or -2.
This must be balanced by +2.
There is only one Cu so the charge is +2 on copper.
Step 4: The charge for Cu is stated using Roman Numerals in the name.
CuCl2 is copper (II) chloride
Check Your Understanding
Name CaCl2, SCl2, and MnCl2
Click for answer and explanation
CaCl2 is calcium chloride. Ca is a Family A metal. We don't use Roman Numerals in the name since we can predict the charge from the periodic table. Chlorine, Cl, is a nonmetal. CaCl2 is an ionic compound. We can predict the formula from the periodic table and don't state the number of ions in the name.
SCl2 is sulfur dichloride. Both S and Cl are nonmetals. We state the number in the name since we can not determine the formula from the periodic table.
MnCl2 is manganese (II) chloride. Mn is a Family B metal so we must determine the charge from the formula of the compound. Cl is the (-1) ion so the charge on Mn is +2. We don't state the number of ions in an ionic compound.
Conclusion
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