Chemistry: Covalent Compounds

When you tutor high school chemistry, writing chemical formulas comes up every year.  Having investigated ionic compounds, let’s sort out covalent ones.

If you look in the Chemistry category, you’ll see various posts about writing chemical formulas for ionic compounds.  Those posts explain that, for ionic compounds, matching the combining capacities of the metals and the nonmetals is key to writing proper formulas.  Another way to look at it is that the charges must be balanced in a viable ionic compound formula.

With covalent compounds, we needn’t worry about combining capacities; the name of the compound tells all.  That’s because covalent names include prefixes that tell us the number of each type of atom involved:

prefix number
mono 1
di 2
tri 3
tetra 4
penta 5
hexa 6
hepta 7
octa 8
nona 9
deca 10

Take, for example, dinitrogen pentoxide. The di before nitrogen tells us there are two nitrogens; the penta before oxide tells us there are five oxygens.  Therefore, we have

dinitrogen pentoxide: N2O5

Going the other way, here is a point to mind: The prefix mono is rarely used. For example:

SF :  sulphur hexafluoride.

Note that we don’t call it monosulphur hexafluoride, even though there is only one sulphur. However, we do call CO carbon monoxide. To my knowledge, mono is only used when another related compound is more common. In the case of carbon monoxide, we use mono to distinguish it from carbon dioxide, which of course is more often mentioned. Whatever the reason, the prefix mono is seldom used, even when there is only one of that atom.  Other examples:

CCl4 : carbon tetrachloride

BF3 : boron trifluoride

Of course, one question that might evolve:  “How can I tell if a compound is ionic or covalent?”  The simple answer:  If the compound is ionic, its name starts with a metal.  If it’s covalent, it starts with a nonmetal.  I explain how to tell a metal from a nonmetal here.

Remember:  only covalent compounds use the numeric prefixes in the table above; ionic compounds don’t use them.

Source: Chemistry, Charles E. Mortimer, Sixth Edition, Wadsworth, Inc., 1986.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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