VIII. A Compound by Any Other Name Is . . . A Different Compound!!!
1. A long time ago we talked about Dalton’sLaw of Multiple Proportions which said that you
could get different compounds from the same elements by changing the ratios of atoms of the
elements. For example, you could have C1O1 (although the 1’s are normally not written) and C1O2. As a result of the fact that this regularly occurs in the realm of covalent compounds, it is
necessary for the naming system for these compounds to have a means for distinguishing
between the possibilities. That means is to use numbering prefixes before the names of the
elements. For instance, CO would be named carbon monoxide and CO2 is called carbon dioxide. (These numbering prefixes are also used before the name of the first element in the
compound, unless there is just one atom of that element, in which case the mono- is dropped.)
2. The numbering prefixes are mono- for 1, di- for 2, tri- for 3, tetra- for 4, penta- for 5, hexa- for 6, hepta- for 7, and octa- for 8 (etc., etc.).
1. In ionic compounds, the charges on the ions making up the compound dictate the formula. In
many cases, this means that the ions can only combine in one ratio of atoms. e.g. because Ca is +2 and Cl is –1, these two ions must combine in a 1:2 ratio giving the formula CaCl2. Since
there is only one possible combination of these elements, it is redundant to call it calcium dichloride. Therefore, numbering prefixes are not used when naming these compounds. (There
are cases where there can be different combinations, but this is handled in a different way.)
2. If the formula for an ionic compound has just two symbols in it, then naming the compound is a
simple matter of finding the names that each of the symbols represent, placing the name of the
metal or positive ion first and the non-metal or negative ion second, and, finally, modifying the
ending of the name of the second compound to the suffix –ide.
3. If the formula for an ionic compound has more than two symbols, then somewhere in those set
of symbols, the formula for a polyatomic ion is lurking (usually at the end, and usually involv-
ing one or more oxygens). It then becomes your job to ferret out that polyatomic ion and write
its name exactly as it appears on your polyatomic ion sheet (usually ending in –ite and –ate).
4. An example of the first situation (2.) would be Na2O, which would be named sodium oxide;
an example of the 2nd (3.) would be Na2SO3, which would be named sodium sulfite.
D. Write the formal name for each of the following binary covalent compounds:
E. Write the formal name for each of the following binary ionic compounds:
8. Ba(HSO3)2 ____________________________
9. (NH4)2CO3 ____________________________
VI. A Compound by Any Other Name Is . . . A Different Compound!!!
Sometimes we forget that most of what we study in science are the devices that man has designed
to try to explain things that nature has created. It is essential that we don’t lose sight of that, because it
indicates that keeping in mind how nature works will help us better understand these man-made devices.
For instance, nature follows a simple principle that dictates that all matter tends towards the most stable state. We have already run across this when we discussed radioactivity: It is a phenomenon in which an
unstable nucleus tries to change itself into a stable nucleus through the release of some form of radiation.
More recently, we have encountered this in the realm of chemical bonding, which is a process by
which an atom with an incomplete outer shell attempts to modify its electronic structure to make the
valence shell complete (“full”). In the case of covalent bonding, an atom does this by forming the
required number of bonds to complete its octet (e.g. nitrogen, which has 5 electrons will form 3 bonds to
make it feel like it has a total of 8). In the case of ionic bonding, one atom will lose electrons and another
atom will gain electrons until both have their desired electronic structures. Since the number of electrons
one atom will want to lose (e.g. sodium wants to lose 1) may not equal the number of electrons another
atom wants to gain (e.g. oxygen wants to gain 2), then nature compensates for this by allowing different
numbers of atoms to combine until everyone is stable (in this case, 2sodium atoms will need to combine
with 1oxygen atom). The ionic bonding process just described also makes sure that matter obeys another
“rule” of nature: The Law of Conservation of Energy which says that energy (which includes electrical
energy in the form of electrical charges) cannot be created or destroyed.
So nature has been going along making the necessary changes to adhere to the principle of moving
towards stability for billions of years – long before man ever existed. Then, along comes man, who thinks
too much and he develops this discipline called science, and one particular branch of it – called chemistry
– tries to represent the products of this movement towards stability – namely, compounds. And the
devices for representing these compounds are things that he calls formulas and names. All that you have
to remember as a student is that those formulas and names have to indicate the way nature dictates things.
This means that, the compound formed by sodium and oxygen (which it was noted above requires 2
sodium atoms for every 1 oxygen atom), must be described by the formula Na2O1 [or simply Na2O]
where the subscripts2 and 1 show the proper ratio of atoms of the two elements. It’s that simple!
Several years ago, some high-school students out near Pittsburgh, PA caused quite a stir in their
community when they passed out pamphlets detailing research by the military into a secret substance
that posed a serious threat to our society. They identified the substance as dihyrdogen monoxide and
people in the community flooded the phone lines of the municipal center demanding information on
this material. The joke was on them as you will hopefully understand by the time you get through this
section, because this is the formal chemical name of a familiar com) . . .
1. You may recall a lab from earlier in this year where you prepared one gas (from a reaction
between vinegar and baking soda) and your teacher prepared a second gas for you and then you
compared their properties. Their physical properties were quite similar, but their chemical
properties were quite different. The interesting feature of this experiment was that both gases
represented compounds of carbon and oxygen. The existence of numerous examples of this –
where a certain combination of elements can produce more than one compound – prompted
Dalton to define the Law of Multiple Proportions. We can understand this in modern terms by
showing that many combinations of non-metals can form different covalent-bonding arrangements
to achieve stability. The bottom line is that we need some way to represent these compounds as
different from each other by both name and formula.
2. Dalton’s Law of M.P. explained the existence of such families of compounds by proposing that
elements can combine in different ratios of atoms; we still believe that today. It turns out that the
two different combinations that you were investigating in that lab from Section Fire would be
given the formulas CO and CO2. What we need now is a way to name them differently.
3. At the outset, both compounds would be given the same base name: The elements in [almost all]
compounds are named in order from the least electronegative element to the most electronegative.
Additionally, the name of the second or most electronegative element is modified to end in –ide.
Put those two ideas together and the base name of both compounds from above would be . . .
4. The problem is that without some additional feature, these two different compounds would have
the same name. Since they differ in the number of oxygen atoms, the logical solution to this
problem would be to add numbering prefixes to emphasize this difference, so that . . .
a. The compound CO would become carbon monoxide (mono for 1 oxygen) and . . .
b. . . . the compound CO2 would become carbon dioxide (di for 2 oxygens).
5. Some things you should know about these numbering prefixes, used in covalent compounds:
a. They are derived from the familiar Latin system; you should know all of them from 1 to 10.
b. They are used any time there is more than one compound for a particular pair of elements.
c. The prefixes are used in front of the names of both elements as necessary. For example, a
compound of nitrogen and oxgyen that you have seen a couple of times in this course is
represented by the formula N2O4; its formal chemical name would be . . . dinitrogen tetroxide
B. Formulas for Binary Covalent Compounds
1. The most important thing to note right off the bat is the major difference between naming covalent
compounds and naming ionic compounds: Numbering prefixes our not used in the names of ionic compounds. For instance, the familiar compound Quik-Joe – with the formula CaCl2 – is
not named calcium dichloride. The reason for this is simple: There is only one way in which
calcium and chlorine can combine and so numbering prefixes are redundant. As pointed out in
the Introduction, these elements combine in such a way as to balance their charges (Ca is +2 and Cl is -1) and since they each have a certain (and consistent) charge, they must combine in a
specific, unchangeable ratio. (We will run across metals later on that can form more than one type
of ion, and we will invoke a different system for distinguishing between these different charges.)
2. All of that being said, the naming of ionic compounds is relatively simple: You identify the name
of the element represented by the first symbol in the formula (the less electronegative or metallic
element; you then identify the name of the element represented by the second symbol (the more
electronegative or non-metallic element), remembering to modify the ending to –ide. For
example, the compound from the Introduction, Na2O, would be given the name sodium oxide.
3. That would be the end of the story except for one complicating factor: Sometimes you will run
across ionic compounds that have more than two symbols in their formula. For example, the
familiar compound washing soda has the formula Na2CO3. As soon as you recognize this fact,
you have to look for the presence of a molecular or polyatomic ion in the formula. Since all of the
polyatomic ions we will encounter except one (ammonium ion, NH4
(and also contain oxygen), that search should center on the second half of the formula. (This is
where the Polyatomic Ion List you were given previously will prove very useful.) Hopefully, you
would recognize the formula for the carbonate ion [CO -2
correctly washing soda by the chemical name sodium carbonate.
Part III, Section Beneath . . . Worksheet
A. Write the correct formula for each of the following binary ionic compounds:
B. Provide the correct name for each of the following binary compounds:
7. Ba(HSO3)2 ____________________________
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