How to Write Formulas & Name Compounds
How to Write Formulas & Name Compounds
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In Chemistry, learning how to write chemical formulas and name chemical compounds is a necessary skill for any student! The process not only helps in communicating chemical information easier but also helps you understand the attributes and behaviours of different substances.
In chemical formulas you’ve probably noticed some small numbers on the bottom right in an element in a compound. Those are called subscripts. Subscripts which are the numbers on the bottom right show how many atoms of each element are in a compound, for example the compound carbon dioxide (CO₂) consists of one carbon atom (C) and two oxygen atoms (O₂).
Other than subscripts you must’ve also seen numbers on the top right of an ion with a positive (+) or negative (-) charge, that is called a superscript. Superscripts are put on ions to show their charge and how much of it. Take a Beryllium Ion (Be ) for example, the subscript shows that Beryllium has a charge of positive two!
VERY IMPORTANT RULE FOR COMPOUND NAMING: USE LOWER CASE FOR ALL COMPOUND NAMES
There are certain rules you have to follow in order to name chemical compounds
Ionic Compounds 🧪
Ionic compounds are formed from metals and nonmetals where electrons are transferred.
Rules:
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The name of the metal (or the cation) comes first.
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The name of the nonmetal (or anion) comes after the cation and change the ending to “ide”
Example:
Compound: KF
Name: potassium fluoride
Multivalent Metals 🧪
Multivalent metals are cations that have more than 1 charge. Be careful when naming multivalent metals in compounds because you need to use roman numerals in brackets after the name of the cation to show what charge it is. In this case you need to select one of the charges that helps balance out the charge depending on the formula.
List of Roman Numerals
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I
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II
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III
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IV (think about 1 before 5)
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V
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VI (think about 1 after 5)
Take iron (II) sulfide (FeS) for example
As you can see iron is a multivalent metal with charges and sulfide is a monovalent (one charge) anion. You’re probably wondering now, how did you get the roman numeral 2? If you look at your periodic table, Iron has two possible charges which are 2+ and 3+ and we know that a sulfur (S) ion has a charge of 2-. Let’s now check how many ions of each element do we have! Since there are no subscripts, there is 1 Iron ion and 1 Sulfur ion.
Let’s look at the charge balancing!
Fe Total Positive Charge =
S Total Negative Charge = 2-
So we need a charge of 2+ from the iron to balance out the 2- from the sulfur. Lucky us! Iron has a possible charge of 2+! Therefore the charge of Iron in this specific compound is 2+ or II
Fe Total Positive Charge = 2+
S Total Negative Charge = 2-
That way they balance each other out!
So for the name it should be iron (II) sulfide!
Covalent Compounds:
Covalent compounds are formed from nonmetals through sharing electrons. The naming rule is VERY different from what we saw in naming ionic compounds because you need to use prefixes before the elements.
2-
2+
2+
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Rules
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The first element keeps its name (only if there is one atom)
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Second element’s name also ends with “ide”
For example, CO₂ is carbon dioxide, and N O is dinitrogen trioxide.
Finally, here are the mistakes that students often make.
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Forgetting to balance charges in ionic formulas.
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Misusing prefixes in covalent compound names.
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Not using Roman numerals for multivalent metals.
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Using upper case in compound names
Being aware of these mistakes can save you from errors!
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