Module 5 Lesson 3 - Read
Read: Chemical Nomenclature
Overview
Think about all the kinds of substances you come into contact with every day. Do you know the chemical names for all of them? You may not know them all, but you might know some of the common everyday substances: carbon dioxide, sodium chloride, dihydrogen monoxide (H2O), propane, and more. Within this lesson, the rules and trends for naming substances and writing their formulas will be explained.
Relationship Between Substances and Nomenclature
Chemical nomenclature is a fancy word for the systematic naming and formula writing system for chemical substances. Before learning to name compounds and write their formulas, it is important to learn how they all fit together.
Above is a flowchart showing the breakdown of pure substances into two categories: elements and compounds. Each of the five subcategories (atom, diatomic molecule, molecule, ionic compound, and acid/base) have their own unique naming systems.
Naming Atoms and Diatomic Molecules
The name for atoms and diatomic molecules are the names from the periodic table. For these, simply find the atom on the periodic table and copy its name.
Examples: Ca is calcium, O2 is oxygen, Al is aluminum, and H2 is hydrogen.
Writing the Formula for Atoms and Diatomic Molecules
The formula for an atom or diatomic molecule is the same as the symbol found on the periodic table, with the exception of seven elements. There are seven elements that are actually found in pairs in nature, so you must write the subscript 2. These seven elements are known as diatomic molecules, as di- means two.
Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine are found in pairs in nature.
So their formulas would be written: Br2 I2 N2 Cl2 H2 O2 F2
Names and Formulas of Binary Ionic Compounds
Ionic compounds are formed through one or more ionic bonds. With an ionic bond, electrons are transferred from one atom to another in order to gain stability. After the transferring of electrons has occurred, they will both hold a charge. One atom holds a negative charge and the other atom a positive charge. Like magnets, they will attract each other and form a strong ionic bond. Ionic bonds are usually formed between one metal and one nonmetal. Metals are found on the left side of the periodic table, and nonmetals are found on the right side of the periodic table (the exception is hydrogen).
Naming Binary Ionic Compounds
Name the cation (metal) first, then name the anion last and change its ending to “-ide”.
NaCl is named sodium chloride
K2O is named potassium oxide
Writing the Formula for Binary Ionic Compounds
The first step to writing the formula for ionic compounds comes from determining the charges of the cation and anion. The last step would be balancing the charges by determining how many of each atom is needed to make a neutral compound. Use the following periodic table to quickly determine the ion charges for the most common elements.
Calcium bromide is written CaBr2. Calcium is the cation with a charge of +2. Bromine is an anion with a charge of -1. In order to neutralize the calcium, two bromines would be needed, thus the final formula is written with two atoms of bromine for one atom of calcium.
Names and Formulas of Binary Molecular Compounds
Molecular compounds are composed of only nonmetals which are found on the right side of the periodic table, plus hydrogen. Molecules form when non-metal atoms covalently bond through the sharing of their electrons. The following periodic table shows the location of non-metal atoms.
Naming Binary Molecules
A binary molecular compound is a two-element molecule. The naming system for covalent molecules is quite simple in comparison to ionic compounds, acids, and bases. There are ten prefixes that tell how many atoms of each element are present in the compound. Knowing those ten prefixes is the key to naming binary covalent molecules.
| 1 | mon- |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
| 7 | hepta- |
| 8 | octa- |
| 9 | nona- |
| 10 | deca- |
Naming the first element in the molecule: prefix + element name.
For the first element, find the prefix corresponding to the number of atoms of that element present in the molecule. Then, attach the prefix to the name of the element. The only exception is not to use a prefix if there is only one atom for the first element in the molecule.
In N2O4 the first element is called dinitrogen.
In CO2 the first element is called carbon (not monocarbon).
Naming the second element in the molecule: prefix + element name with “-ide” suffix.
For the second element, first find the prefix corresponding to the number of atoms of that element present in the molecule. Next, attach the prefix to the name of the element. Then, change the ending of the element to -ide.
In N2O4 the second element is called tetraoxide (oxygen is changed to oxide).
In CO the second element is called monoxide.
When writing the formula, translate the given prefixes and write the formula with the correct number of atoms.
Carbon dioxide would be CO2.
Nitrogen trihydride would be NH3.
Diphosphorus trioxide would be P2O3.
Names and Formulas of Acids and Bases
Acids are molecules that start with the element hydrogen. The only exception is H2O, which is water. Although water starts with H, it is not an acid. Naming a simple binary acid is not complicated. The prefix “hydro-” is added to the beginning of the name, and the suffix “-ic acid” is added to the end.
HCl is named hydrochloric acid. The “chlor” in hydrochloric acid tells that chlorine is bonded to hydrogen.
H2Se is named hydroselenic acid.
When writing a formula for a binary acid, two things must be taken into account. First is to determine which element is bonded to hydrogen. Once that element is found, the anion charge of the element must also be determined. Hydrogen in acid has a +1 charge, so in order to write the correct formula, the charges must balance.
The formula for hydrofluoric acid is HF. The “fluor” in hydrofluoric acid tells you fluorine is bonded to hydrogen. Fluorine has an ion charge of -1. Hydrogen has an ion charge of +1. Only one atom of each element is needed in order to produce a neutral acid.
Bases are compounds that contain the anion hydroxide (OH-). Any ionic compound that contains the polyatomic anion hydroxide is considered a base.
Naming a base: name the cation + hydroxide.
Ca(OH)2 is named calcium hydroxide.
KOH is named potassium hydroxide.
Hydroxide has a charge of -1. In order to write the formula for the base, the charge of the cation must be determined. Then the formula must be written so when both elements are combined, they cancel each other's charges.
The formula for magnesium hydroxide is Mg(OH)2. Magnesium has an ionic charge of +2, and hydroxide has an ionic charge of -1. You need two hydroxide molecules in order to cancel out magnesium’s charge.
Expand: Determining Which System to Use
Discover
It is not easy to learn how to properly name and write different types of substances. It is even harder to determine which naming system to use. Within this section, an in-depth explanation of how to determine the naming system will be given, along with some more guided practice of naming and formula writing.
Knowing When to Use Each System
Study the following flowchart:
This flowchart allows you to see how all four types of compounds within this lesson relate to one another. Acids usually start with hydrogen (except water). Binary molecular compounds are composed of only nonmetals. Ionic compounds are composed of one metal and one nonmetal. Bases are ionic compounds with a hydroxide anion. When seen through the flowchart, their relationships are easier to see.
Practice Naming and Writing Ionic Compounds
What is the name for AlF3?
Answer: aluminum fluoride
AlF3 is an ionic compound, so the rules for naming ionic compounds must be followed. First, name the cation using the name from the periodic table. Al is aluminum. Then, name the anion, but change its ending to -ide. F is fluorine, but in an ionic compound, its name is fluoride.
What is the formula for lithium oxide?
Answer: Li2O
Lithium oxide is an ionic compound (lithium is a metal and oxygen is a nonmetal), so the rules for writing formulas for ionic compounds must be followed. First, determine the charges of the cation and anion. The cation is lithium (because it was written first), and it has an ionic charge of +1. The anion is oxygen (because it was written last) and it has an ionic charge of -2. You need two atoms of lithium to balance the -2 charge of oxygen, thus the written formula has two atoms of lithium for one atom of oxygen.
Practice Naming and Writing Binary Molecular Compounds
What is the name for PF5?
Answer: phosphorus pentafluoride
Phosphorus pentafluoride is a binary molecular compound (two element compound that only contains nonmetals), so the rules for naming molecular compounds must be followed. First, the number of atoms for each element must be determined, and matched to the prefixes. There is only one phosphorus, so its name is phosphorus (not monophosphorus). There are five atoms of fluorine, so the prefix to use is penta.
What is the formula for sulfur hexafluoride?
Answer: SF6
Sulfur hexafluoride is a binary molecular compound, so the rules for writing formulas for binary molecular compounds must be followed. Sulfur is written without a prefix which indicates that there is only one atom present. Fluoride was written with the prefix hexa- which means six. So, there are six atoms of fluorine in the formula.
Module 5 Lesson 3 of 5