Binary Compounds of Two Nonmetals

Given Name, Write the Formula

The Greek System

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A binary compound is one made of two different elements. There can be one of each element such as in CO or NO. There can also be several of each element such as BF3 or OCl2.

This lesson shows you how to write the formula of a binary compound from its name when two nonmetals are involved. The four formulas above are all examples of this type. Important point to remember: NO metals (which act as the cation) are involved. That means one of the nonmetals will be acting in the positive role while the other is negative.

In fact, you do not even need to know the charges, since the name comes right from the amounts of the two elements involved. Be aware that heavy use of Greek number prefixes are used in this lesson.Here are the first ten:

Number  Prefix  Number  Prefix
one  mono-  six  hexa-
two  di-  seven  hepta-
three  tri-  eight  octa-
four  tetra-  nine  nona-
five  penta-  ten  deca-

Example #1 - write the formula for dinitrogen trioxide.

Example #2 - write the name for carbon monoxide.

Step #1 - the first name will tell you the first element in the formula. In the first example above, it would be N and in the second, C.

If there is a prefix on the name, this gives the subscript on the element. In the first example above, the "di-" tells you there are two nitrogens. Absence of a prefix, as in the second example, says there is only one of that element involved.

Step #2 - the anion name tells you the element; oxide means oxygen. Once again, the prefix will tell you how many of the element are involved. "Tri-" means three and "mono-" means one.

The correct formulas of the two examples are N2O3 and CO.

Note that "monoxide' is written rather than "oxide" when there is one atom of the second element involved. Note also that when one element of the first atom is involved, no "mono-" is used. Monocarbon monoxide is just as wrong as carbon oxide.

Example #3 - write the formula for bromine pentafluoride.

Step #1 - the first symbol is Br and its subscript will be a one, which is understoo to be present.

Step #2 - the second element is fluorine, so F is used. The prefix "penta-" indicates a subscript of 5.

The formula of this compound is BrF5.

Example #4 - write the formula for diphosphorous pentoxide.

Step #1 - the first symbol is P and the subscript is 2.

Step #2 - pentoxide says five oxygens are involved.

The formula of this compound is P2O5.

Example #5 - write the formula for iodine heptafluoride.

Step #1 - the first symbol is I and the subscript is 1. Again it is understood to be there.

Step #2 - heptafluoride says 7 florides are involved.

The formula of this compound is IF7.

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Practice Problems
Write the correct formula for:

1) chlorine monoxide

2) oxygen difluoride

3) boron phosphide

4) dinitrogen monoxide

5) nitrogen trifluoride

6) sulfur tetrachloride

7) xenon trioxide

8) carbon dioxide

9) diphosphorous pentoxide

10) phosphorous trichloride Answers to Set One

Write the correct formula for:

11) sulfur dioxide

12) bromine pentafluoride

13) disulfur dichloride

14) boron trifluoride

15) tetraarsenic decoxide

16) silicon tetrachloride

17) krypton difluoride

18) chlorine monoxide

19) silicon dioxide

20) boron trichloride Answers to Set Two

Write the correct formula for:

21) dinitrogen pentasulfide

22) carbon monoxide

23) sulfur trioxide

24) dinitrogen trioxide

25) dinitrogen monoxide

26) xenon hexafluoride

27) sulfur hexafluoride

28) phosphorous pentachloride

29) nitrogen monoxide

30) bismuth trichloride Answers to Set Three

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