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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. |
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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. |
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| 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. |
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| The name of this iron is iron(II) and the name of the compound is iron(II)
chloride. |
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| SCl2 is a covalent molecular compound, not an ionic
compound. |
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·· ··
··
: Cl : S : Cl :
·· ·· ·· |
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| 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 :
·· |
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| 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. |
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| The Stock name for this compound is sulfur(II) chloride. |
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| 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. |
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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 |
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| 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 |
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| 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. |
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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 |
| CuSO4·5H2O |
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E-mail instructor:
Eden Francis
Physical Science
19600 South Molalla Avenue
Oregon City, OR 97045
(503) 657-6958 x 2807
TDD (503) 650-6649 |
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©1998, 1999 Clackamas Community College, Hal Bender
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