NOMENCLATURE OF IONS AND COMPOUNDS

Unit Introduction

You have spent the past few units learning about elements, ions, and chemical bonds. The purpose of this unit is to introduce you to the system of naming ions, ionic compounds, and covalent compounds.

Monatomic Ions

Recall from previous units that ions are elements with a positive or negative charge due to the imbalance of electrons. A monatomic ion is an ion of a single atom. If you look at the periodic table, you can predict the charge of a monoatomic ion based on the element’s group. Check out the periodic table below to see these charges.

You can see that elements in Group 1 have 1 valence electron, and when that electron is lost, the element becomes a cation with a charge of 1+. When Group 2 elements ionize, they have a charge of 2+. Transition metals in groups 3-12 have variable charges. Group 13 elements lose 3 electrons for a charge of 3+. The elements in group 14 can either lose or gain electrons, which is why the charges 4+ and 4- are both at the top of the chart. Group 15 elements gain 3 electrons to bring the charge to 3-. Elements of group 16 have 6 valence electrons, and as they gain 2 more, the charge equals 2-. Group 17 elements have 7 valence electrons and only need to gain 1 for a complete outer shell, and the addition of 1 more electron makes the charge 1-. Finally, the elements of group 18 have complete outer shells, do not commonly ionize, and have a charge of 0.

The symbol for a monatomic ion will be the element’s chemical symbol followed by the charge. The chemical symbol is denoted by one or two letters. The first letter is always capitalized and the second letter (if there is one) is always lower-case. Here are some examples of monatomic ions and their abbreviations.

Element	Symbol	Charge	Ion
Aluminum	Al	3+	Al^3+
Hydrogen	H	1+	H^+
Chlorine	Cl	1-	Cl^-
Oxygen	O	2-	O^2-
Calcium	Ca	2+	Ca^2+

When monatomic cations are named, there are no special rules. If hydrogen is referenced, you can simply call it hydrogen or a hydrogen ion. However, if the cation is polyvalent, meaning it can have different charges, you need to name it specifically with its charge. For example, iron or Fe can ionize in 2+ or 3+ forms. In this case, Fe²⁺ should be called “iron two plus” or “iron two” and Fe³⁺ should be called “iron three plus” or “iron three, to indicate the number of electrons lost and the ion’s charge.

When monatomic anions are named, usually the suffix -ide replaces the last part of the element’s name. For example, a chlorine atom is called chloride, and an oxygen ion is called oxide.

Ionic Compounds

Ionic compounds are formed when a cation and an anion form a chemical bond through the transfer of electrons. When ionic compounds are named, there are some rules that need to be followed:

· name the cation first, then the anion

· ionic compounds form with a net charge of zero

· write ionic compounds with the lowest possible integer value, ex: NaCl instead of Na₃Cl₃, etc.

What would the name be for the compound AlCl₃? The cation is aluminum, and the anion is chloride. Because aluminum has a 3+ charge, three chlorine atoms are needed to each accept one of aluminum’s valence electrons, which is why chlorine is followed by a subscript 3. One Al³⁺ ion plus three Cl^- ions will combine with a net charge of zero. The name of this compound, AlCl₃, is aluminum chloride.

The next image shows a few more examples of ionic compounds with the names and formulas indicated, as well as an explanation of the transfer of electrons.

For more information on naming monatomic ions and ionic compounds, please read the following resource from Khan Academy. Answer the practice questions on the website as you read through the lesson to check your understanding.

https://www.khanacademy.org/science/chemistry/atomic-structure-and-properties/names-and-formulas-of-ionic-compounds/a/naming-monatomic-ions-and-ionic-compounds

Watch the following video clip about naming ions and ionic compounds:

Polyatomic Ions

A polyatomic ion is an ion that contains more than one atom. It can be a diatomic molecule or a compound molecule. The reason it is considered an ion is because it is charged, due to an imbalance of electrons and protons. Reference the following site for more information and a list of common polyatomic ions.

https://www.khanacademy.org/science/chemistry/atomic-structure-and-properties/names-and-formulas-of-ionic-compounds/a/polyatomic-ions

Binary Covalent Compounds

A covalent compound is formed when two nonmetals join in a chemical bond through the sharing of electrons. The word binary means two. A binary covalent compound is made by combining two elements. One nonmetal will act as the positive charge and another nonmetal will act as the negative charge. The naming of covalent compounds follows a set of rules:

· write the name of the first element

· add a prefix to the first element name (if more than 1) to indicate the number of atoms

· write the root of the second element and add “-ide” suffix

· add a prefix to the second element name to indicate the number of atoms.

Here are a few examples of covalent compounds with the formulas and names listed. Can you predict the name of the last compound?

Follow the rules above to give a name to chemical formula H₂O.

Name of first element – hydrogen

With prefix – dihydrogen

Name of second element – oxide

With prefix – monoxide

Put it all together – dihydrogen monoxide. What is this commonly known as? Water!

Often, binary covalent compounds have common names in addition to the system names. Here is a table showing examples of compounds with their system names and common names. This table also shows how only two elements can combine in different ratios to form several unique compounds.

FORMULA	COMMON NAME	SYSTEM NAME
N2O	nitrous oxide	dinitrogen monoxide
NO	nitric oxide	nitrogen monoxide
N2O3	nitrous anhydride	dinitrogen trioxide
NO2	nitrogen dioxide	nitrogen dioxide
N2O4	nitrogen tetroxide	dinitrogen tetroxide
N2O5	nitric anhydride	dinitrogen pentoxide
NO3	nitrogen trioxide	nitrogen trioxide

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