Lesson 10
Home Table of Contents Preliminary Information Types of Names Types of Chemicals Formulas & Composition Balanced Equations Weight Relationships Molecular Properties Additional Challenges Wrap-Up

 

What's in a name? Quite a bit. Even the type of name can tell you something because different types of names are associated with different types of compounds. We will deal with four types of names throughout this lesson.

Prefix Names

Prefix names are used primarily with covalent compounds. The name tells which elements are present. The most electronegative element is last in the name and has an "-ide" ending. The prefixes tell how many of each atom are in the formula. The prefixes used are "mon-" or "mono-" for one, "di-" for two, "tri- " for three, "tetra-" for four, "penta-" for five, "hexa-" for six, "hepta-" for seven, "octa-" for eight, "nona-" for nine, and "deca-" for ten. The absence of a prefix generally means there is one of those atoms in the formula if it is a prefix name. The "a-" and "o-" endings of the prefixes are generally dropped when attached to "oxide."

Some examples of prefix names are dinitrogen tetroxide and phosphorus trichloride. The interpretation of prefix names to get a formula is quite straightforward. Simply write down the elements present and use the prefixes to show how many of each atom you have.

N2O4
PCl3

 

Stock Names

Stock names can be used for covalent and ionic compounds when the first element in the name can have more than one charge or oxidation state. They are used mostly for ionic compounds containing transition metals. The charge or oxidation state of the first element is shown in parentheses using Roman numerals at the end of the name of that element.

 

Oxidation States

Oxidation state is very much like the charge on an ion but it can apply to any element whether it is metal or nonmetal, ionic or covalent.

Let me show you what I mean using this example. FeCl2 is an ionic compound.

FeCl2

Iron is in the form of positively charged cations. Chlorine is in the form of negatively charged chloride ions. The charge on iron cannot be determined from the periodic table, but it can be figured out from the formula of this compound. Chloride ion has a -1 charge. We also say it has a -1 oxidation state. The two chloride ions balance the charge on the iron, therefore the iron has to have a +2 charge. We also call that a +2 oxidation state.
Fe2+ Cl-
The name of this iron is iron(II) and the name of the compound is iron(II) chloride.
iron(II) chloride

 

SCl2 is a covalent molecular compound, not an ionic compound.

SCl2

         
: Cl : S : Cl :
         

But chlorine is more electronegative than sulfur and the shared electrons are pulled away from sulfur and toward chlorine making the sulfur somewhat positive and the chlorine somewhat negative. In a sense the chlorine is gaining an electron, although not completely.

: Cl :

S

: Cl :
In that sense chlorine has an oxidation state of -1 even though it cannot be considered to be an ion with a charge of -1. Similarly the sulfur can be considered to have lost two electrons, even though not completely. Therefore sulfur is said to have an oxidation state of +2, even though it cannot be considered to be a cation with a +2 charge.
Cl- S2+ Cl-
The Stock name for this compound is sulfur(II) chloride.

sulfur(II) chloride

 

If the Roman numeral is not needed because the element has only one possible positive oxidation state (as in BaCl2) it is not used. Thus the name for BaCl2 is barium chloride - no parentheses and no Roman numerals.
BaCl2
barium chloride

 

 

Interpretation

The interpretation of Stock names to get formulas is more involved than it was for prefix names. You have to do some figuring. Essentially treat it like an ionic compound and match the charges. Iron is +3 and oxygen is -2, so the formula for the compound is Fe2O3.
iron(III) oxide
Fe3+     O2-
Fe2O3

 

In carbon(II) oxide, carbon is +2 and oxygen is -2. Even though these do not exist as ions we can match the oxidation states to get the formula CO. Notice that even though there is a "two" in the name there is no "two" in the formula. By the way, the prefix name for this compound is carbon monoxide.
carbon(II) oxide
C2+     O2-
CO

 

 

Latin Names

Latin names are the old names used for both ionic and covalent compounds to indicate high and low oxidation states of the first element in the name. These names only work well if there are only two oxidation states for the element or at least only two common oxidation states. You also need to know what those common oxidation states are. Some examples are ferrous nitrate, cupric chloride and nitrous oxide.

To ge the formula from a Latin name you have to determine the oxidation state (from memory or look it up) then treat the compound as if it were ionic.

Ferrous is the lower oxidation state of iron and that happens to be +2. Nitrate has a -1 charge. Thus the formula for ferrous nitrate is Fe(NO3)2.
ferrous nitrate
Fe2+     NO3-
Fe(NO3)2

 

Cupric is the higher oxidation state of copper and that also happens to be +2. Chloride has a -1 charge. Thus the formula for cupric chloride is CuCl2.
cupric chloride
Cu2+     Cl-
CuCl2

 

Nitrogen has about five or six oxidation states to choose from. Nitrous happens to indicate the +1 oxidation state. Oxide has a -2 oxidation state. Therefor the formula for nitrous oxide is N2O.
nitrous oxide
N+     O2-
N2O

 

 

Simple Names

Simple names are used primarily for ionic compounds but also for several covalent compounds. Simple names are used when only one possible combination of the elements exists. These names are like Stock names without the Roman numerals because they are not needed.

Because the names do not give you any clues as to how many atoms there are or what oxidation states they have, you have to figure these things out based on what you know about the elements, usually from their position on the periodic table.

Barium chloride is a typical example. Because of the positions of barium and chlorine on the periodic table we know we have an ionic compound and that barium ion has a +2 charge and chloride ion has a -1 charge. These will combine to make the formula BaCl2.
barium chloride
Ba2+     Cl-
BaCl2

 

Hydrogen sulfide is another example. Hydrogen sulfide is a covalent molecular compound. Hydrogen needs one more electron and sulfur needs two more electrons. Therefore, they combine in a two-to-one ratio to make H2S.
H S
needs 1 e- needs 2 e-
H2S

 

 

Hydrates

Sometimes molecules of water are incorporated into the crystals of ionic compounds in a very systematic fashion. When this happens, the number of water molecules can be included in both the name and the formula.

Cupric sulfate pentahydrate is an example. The first part of the name is interpreted just as before. Cupric ion has a +2 charge. Sulfate ion has a -2 charge. Therefore we have CuSO4. Pentahydrate means five water molecules and these are tacked onto the formula using (dot) 5H2O.
cupric sulfate pentahydrate
CuSO45H2O

 

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