University High School Chemistry: Covalent Nomenclature

Covalent Nomenclature

Chapter: Covalent Bonds and Formulas

The Covalent Bond

Lesson Objectives

The student will:

explain what covalent bonds are.
explain why covalent bonds are formed.
compare covalent bonds with ionic bonds in terms of how their definitions and how they are formed.

Introduction

As we saw in the chapter “Ionic Bonds and Formulas,” metallic atoms can transfer one or more electrons to nonmetallic atoms, producing positively charged cations and negatively charged anions. The attractive force between these oppositely charged ions is called an ionic bond. However, chemical bonding does not require the complete transfer of electrons from one atom to another. When a bond forms between two nonmetallic atoms, neither has a low enough electronegativity to completely give up an electron to its partner. Instead, the atoms overlap their orbitals, and the electrons residing in these shared orbitals can be considered to be in the valence shells of both atoms at the same time. These atoms are now in a covalent bond, held together by the attraction of both nuclei to the shared electrons.

Ionic versus Covalent Bonding

The way that atoms bind together is due to a combination of factors: the electrical attraction and repulsion between atoms, the arrangement of electrons in atoms, and the natural tendency for matter to achieve the lowest potential energy possible. In most cases, these factors favor atoms that have obtained a complete octet of valence electrons. In ionic bonding, the atoms acquired this octet by gaining or losing electrons, while in covalent bonding, the atoms acquire the noble gas electron configuration by sharing electrons.

As you may recall from the discussion of ionic bonds in the chapter “Ionic Bonds and Formulas,” ionic bonds form between metals and nonmetals. Nonmetals, which have high electronegativity, are able to take electrons away from metals. The oppositely charged metal and nonmetal ions will then be attracted to each other. In covalent bonds, electrons are shared, meaning that metals will form few, if any, covalent bonds. Metals do not hold on to electrons with enough strength to participate in covalent bonding. For a covalent bond to form, we need two atoms that both attract electrons strongly, or two atoms with high electronegativity. Hence, the great majority of covalent bonds will form between two nonmetals. When both atoms in a bond are from the right side of the periodic table, the bond is most likely to be covalent.

An animation showing ionic and covalent bonding (2a) is available at http://www.youtube.com/watch?v=QqjcCvzWwww (1:57).

Covalent Formulas and Nomenclature

Lesson Objectives

The student will:

list the Greek prefixes from 1 to 10.
provide the correct formulas for binary covalent compounds.
name binary covalent compounds using the IUPAC nomenclature system.

Vocabulary

chemical nomenclature

Introduction

The systematic procedure for naming chemical compounds, or the chemical nomenclature, is different for different types of compounds. In the chapter “Ionic Bonds and Formulas,” we have discussed the procedures for naming binary ionic compounds, ionic compounds involving polyatomic ions, and ionic compounds involving metals with variable oxidation states. In this section, we will describe chemical nomenclature for covalently bonded compounds. Because of the large numbers of covalent compounds that may form between the same two elements, the nomenclature system for covalent compounds is somewhat different to the nomenclature system for ionic compounds.

In naming ionic compounds, there is no need to indicate the number of atoms of each element in a formula because, for most cases, there is only one possible compound that can form from the ions present. When aluminum combines with sulfur, the only possible compound is aluminum sulfide, $\text{Al}_2\text{S}_3$ . The only exception to this is a few metals with variable oxidation numbers, and these are handled by indicating the oxidation number of the metal with Roman numerals, such as in iron(II) chloride, $\text{FeCl}_2$ .

With covalent compounds, however, we have a very different situation. There are six different covalent compounds that can form between nitrogen and oxygen, and for two of them, nitrogen has the same oxidation number. Therefore, the Roman numeral system will not work. Instead, chemists devised a nomenclature system for covalent compounds that would indicate how many atoms of each element is present in a molecule of the compound.

Greek Prefixes

In naming binary covalent compounds, four rules apply:

The first element in the formula is named first using the normal name of the element.
The second element is named as if it were an anion. There are no ions in these compounds, but we use the “-ide” ending on the second element as if it were an anion.
Greek prefixes, shown in Table below, are used for each element to indicate the number of atoms of that element present in the compound.
The prefix "mono-" is never used for naming the first element. For example, $\text{CO}$ is called carbon monoxide, not monocarbon monoxide.

Greek Prefixes
Prefix	Number Indicated
Mono-	1
Di-	2
Tri-	3
Tetra-	4
Penta-	5
Hexa-	6
Hepta-	7
Octa-	8
Nona-	9
Deca-	10

Examples:

$\begin{array} {ll} \text{N}_2\text{O} & \text{dinitrogen monoxide}\\ \text{NO} & \text{nitrogen monoxide}\\ \text{NO}_2 & \text{nitrogen dioxide}\\ \text{N}_2\text{O}_3 & \text{dinitrogen trioxide}\\ \text{N}_2\text{O}_4 & \text{dinitrogen tetroxide}\\ \text{N}_2\text{O}_5 & \text{dinitrogen pentoxide}\\ \text{SF}_6 & \text{sulfur hexafluoride}\\ \text{CO}_2 & \text{carbon dioxide}\\ \text{P}_4\text{O}_{10} & \text{tetraphosphorus decaoxide}\\ \text{P}_2\text{S}_5 & \text{diphosphorus pentasulfide} \end{array}$