The purpose of this section 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.
Some binary covalent compounds, like 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
You can recognize binary covalent compounds from their formulas, which contain symbols for only two, nonmetallic elements. The general pattern of such formulas is AaBb, 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 compounds 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.
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
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.
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.