STC_Naming and Writing Formulas Lesson

Naming and Writing Formulas

Now that you know that ions are formed by atoms, you are ready to write and name chemical formulas. The system of naming chemical compounds is called nomenclature. In order to write a chemical formula, you first need to know what type of chemical it is, because the rules for naming are different. There are two main types of bonds: ionic and covalent. Ionic compounds are formed by bonds between metals, which lose electrons to become stable and nonmetals, which gain electrons. Covalent bonds form molecules by sharing electrons. By simply looking at the first element in the formula, you can quickly identify a compound (ionic) or a molecule (covalent). If the first element listed is a metal, you have an ionic compound. If the first element listed is a nonmetal, you have a molecule. Look at the examples provided, then try them on your own. (Remember that metals are located on the LEFT side of the periodic table!)

You will learn more about bonding later. Right now we will learn how to name and write the formulas of compounds and molecules. We will begin with ionic compounds.

Naming Binary Ionic Compounds

We will start by naming the easiest type of ionic compounds to name, binary. Binary compounds only contain two elements. To name these compounds:

Name the positive ion first

Recall that postive (+) ions are formed by metals and are called cations.

Name the negative ion last, changing the ending to "ide."

Here are some examples:

NaCl (table salt) = sodium chloride

AlBr3 = aluminum bromide

Li2O= lithium oxide

***Notice that we do not need to mention the numbers in the formulas here. You will learn why in a minute.

Writing Formulas for Binary Ionic Compounds

Now let's turn this around a write the formula from the name. Watch the following to learn how to write binary ionic compounds.

Writing and Naming Compounds with Transition Metals

Have you noticed that we haven't named any compounds that contain transition metals? That's because they require a different rule. Since the charge of transition metals is not known from their position on the periodic table, the charge must be indicated somehow in the name of the compound. Using this system, known as the Stock System, ions that have multiple charges available are shown with a Roman numeral that indicates the charge (note that there is no space between the element name and the Roman numeral).  

Watch this to learn how to write and name compounds containing transition (and some post transition metals).

To name a compound with a transition metal, you will mentally un-criss-cross the charges to determine the charge on the metal. Then, that metal charge is written in Roman numerals in parenthesis behind the metal. Watch the following to see how.

Writing and Naming Ternary Ionic Compounds

Compounds that are made from more than two elements are called ternary compounds. Most often, these compounds are made from an ion made up of more than one element, called a polyatomic ion. The group of atoms act like a single ion and are treated like one.  

The following doc is a list of common polyatomic ions. You will need to memorize these! I suggest that you print this list and make a mark each time you come across that particular ion. This will help you narrow down this list and to memorize the ones most commonly seen on the AP Exam. If you have a great memory, go ahead and memorize them all. But, before you do, take a look at this shortcut in this video of how to write and name compounds with polyatomic ions.

Remember, the name of the polyatomic ion is not changed when naming the compound!

Writing and Naming Molecules

Now that you know how to name all types of ionic compounds, let's learn how to name molecules. Remember that molecules come from covalent bonds, formed by a non-metal and a non-metal. In this type of bond, electrons are shared. So, we can't use the criss-cross method as we did before. Later you will learn about the theory behind how these molecules are formed. Right now, you only need to know that they are formed by sharing electrons and how to write and name them.

When naming molecules between two nonmetals, a system is used that names the number of atoms in a molecule using Greek prefixes.

Here is how you name molecules: 1. Write the prefix that shows the number of atoms of the first nonmetal, followed by the name of this nonmetal. (You don't need to put a prefix if there is only one of these atoms.) 2. Write the prefix for the second nonmetal, followed by this nonmetal, changing the ending to -ide. The prefixes are: mono - 1 hexa - 6 di - 2 hepta - 7 tri-3 octa - 8 tetra- 4 nona 9 penta - 5 deca - 10

Examples:

CO = carbon monoxide

CO2 = carbon dioxide

N2O = dinitrogen monoxide

Monocarbon monoxide or monocarbon dioxide are incorrect, mono is only used on the second atom.

Fun Practice!

Remember to work on the module practice problems as you complete each section of content.  

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