email Mark Bishop
 

Binary Covalent Nomenclature

The purpose of this page is to describe the guidelines for constructing the names for binary covalent compounds, which are pure substances that consist of two nonmetallic elements. The water, H₂O, you boil to cook your potatoes and the methane, CH₄, in natural gas that can be burned to heat the water are examples of binary covalent compounds.

 

Converting Formulas to Names

Memorized Names

Some binary covalent compounds, such as water, H₂O, and ammonia, NH₃, are known by common names that chemists have used for years. There is no systematic set of rules underlying these names, so each must simply be memorized. Organic compounds, like methane, CH₄, ethane, C₂H₆, and propane, C₃H₈, are named by a systematic procedure that you might learn later in your chemical education, but for now, it will be useful to memorize some of their names and formulas also.

water - H₂O

ammonia - NH₃

methane - CH₄

ethane - C₂H₆

propane - C₃H₈

Systematic Names

You can recognize binary covalent compounds from their formulas, which contain symbols for only two, nonmetallic elements. The general pattern of such formulas is A_aB_b, where “A” and “B” represent symbols for nonmetals, and “a” and “b” represent subscripts (remember that if one of the subscripts is absent, it is understood to be 1). For example, because nitrogen and oxygen are nonmetallic elements, the formula N₂O₃ represents a binary covalent compound.

Follow these steps to write the names for binary covalent compounds.

If the subscript for the first element is greater than one, indicate the identity of the subscript using one of the prefixes listed below . We do not write mono- at the beginning of a compound’s name.

Example: We start the name for N₂O₃ with di-.

Attach the selected prefix to the name of the first element in the formula. If no prefix is to be used, begin with the name of the first element.

Example: We indicate the N₂ portion of N₂O₃ with dinitrogen.

Select a prefix to identify the subscript for the second element (even if its subscript is understood to be one). Leave the "a" off the end of the prefixes that end in "a" and the “o” off of mono‑ if they are placed in front of an element whose name begins with a vowel (oxygen or iodine).

Example: The name of N₂O₃ grows to dinitrogen tri-.

Write the root of the name of the second element in the formula as shown below.

Example: The name of N₂O₃ becomes dinitrogen triox-.

Add -ide to the end of the name.

Example: The name of N₂O₃ is dinitrogen trioxide.

Prefixes

1 - mon(o)

2 - di

3 - tri

4 - tetr(a)

5 - pent(a)

6 - hex(a)

7 - hept(a)

8 - oct(a)

9 - non(a)

10 - dec(a)

Roots of the Nonmetals

H - hyd

C - carb

N - nitr

P - phosph

As - arsen

O - ox

S - sulf

Se - selen

F - fluor

Cl - chlor

Br - brom

I - iod

Exceptions

Hydrogen atoms always form one covalent bond, and halogen atoms (group 17 or 7A) usually form one bond. Thus hydrogen reacts with halogens to form compounds with the general formula of HX, with the X representing the halogen. Because this is common knowledge among scientists and science students, these compounds are often named without prefixes. For example, HF can be named hydrogen fluoride or hydrogen monofluoride. Likewise, HCl can be named hydrogen chloride or hydrogen monochloride, HBr can be named hydrogen bromide or hydrogen monobromide, and HI can be named hydrogen iodide or hydrogen moniodide. For similar reasons, H₂S can be named hydrogen sulfide or dihydrogen monosulfide.

 

Converting Names to Formulas

The first step in writing formulas when given the systematic name of a binary covalent compound is to recognize the name as representing a binary covalent compound. It will have one of the following general forms.

prefix (name of nonmetal) prefix (root of name of nonmetal)ide (e.g. dinitrogen pentoxide)

or (name of nonmetal) prefix (root of name of nonmetal) ide (e.g. carbon dioxide)

or (name of nonmetal) (root of nonmetal) ide (e.g. hydrogen fluoride)

Follow these steps for writing formulas for binary covalent compounds when you are given a systematic name. Notice that they are the reverse of the steps for writing names from chemical formulas.

Write the symbols for the elements in the order mentioned in the name.

Write subscripts indicated by the prefixes. If the first part of the name has no prefix, assume it is mono-.

Remember that HF, HCl, HBr, HI, and H₂S are often named without prefixes. You will also be expected to write formulas for the compounds whose nonsystematic names are listed above.