What does the prefix per- indicate in the names of chemical compounds?
Let’s look at hydrogen peroxide compared to dihydrogen monoxide (water). Mono- indicates, quite simply, “one” of something – in this case one oxygen atom. The prefix di- similarly indicates “two”.
So what does per- mean? In this case, what is the chemical notation for hydrogen peroxide (cf. H[sub]2[/sub]O = water)? Or the notation for potassium permanganate?
Per meaning, well, per. Each. AFAIK, anyway.
Hydrogen per-oxide (not spelled like that, of course):
H2O2
Potassium per manganate
KMnO4
It approximately means “more”.
Hydrogen peroxide is H[sub]2[/sub]O[sub]2[/sub], which is unstable as it tends to revert to oxygen and water.
The permanganate, or manganate (VII) ion MnO[sub]4[/sub][sup]-[/sup], has an oxidation number one greater than ordinary manganate (VI) ion (MnO[sub]4[/sub][sup]2-[/sup]).
Potassium permanganate, or potassium manganate (VII) is KMnO[sub]4[/sub], while potassium manganate (VI) is K[sub]2[/sub]MnO[sub]4[/sub].
It approximately means “more”.
Hydrogen peroxide is H[sub]2[/sub]O[sub]2[/sub], which is unstable as it tends to revert to oxygen and water.
The permanganate, or manganate (VII) ion MnO[sub]4[/sub][sup]-[/sup], has an oxidation number one greater than ordinary manganate (VI) ion (MnO[sub]4[/sub][sup]2-[/sup]).
Potassium permanganate, or potassium manganate (VII) is KMnO[sub]4[/sub], while potassium manganate (VI) is K[sub]2[/sub]MnO[sub]4[/sub].
Actually, the prefix [n]per**, as in peroxy-, or perhydro-, means “saturated with.” Or at least it did a bazillion years ago when I took o-chem. 
What erislover said. More concisely, PER = FOR EACH.
i.e., H2O2 = One Hydrogen FOR EACH Oxygen = Hydrogen [PER]oxide.
KMnO4 = One Potassium (K) FOR EACH Manganese (Mn) = Potassium [PER]manganate.
http://www.dictionary.com/search?q=per
Thans, erislover.
BTW, I’m not sure that “dihydrogen monoxide” is correct. Maybe it should be “hydrogen monoxide” despte the fact that two hydrogen atoms are present. I’m not sure.
It seems as though the second element only is typically marked by number somehow in most molecular formulae – e.g. carbon dioxide, not monocarbon dioxide. I’m sure there are a zillion exceptions.
Yep, it basically means “one more than normal”
The most common molecule is -ate (i.e. sulfate, nitrate, etc)
one atom less is -ite (sulfite, nitrite, etc)
one more is per- -ate (perchlorate, permangante)
two less is hypo–ite (hypochlorite)
Not the most intuitive system, but there you go.
I believe that erislover and kaylasdad99 are barking up the wrong tree. I don’t believe that the prefix “per” has anything to do with “each.”
As others have mentioned, it refers to the various oxidation states of the central atom. The following four ions, for example:
perchlorate ion (ClO[sub]4[/sub][sup]-[/sup])
chlorate ion (ClO[sub]3[/sub][sup]-[/sup])
chlorite ion (ClO[sub]2[/sub][sup]-[/sup])
hypochlorite ion (ClO[sup]-[/sup])
Have the following oxidation states for chlorine:
+7
+5
+3
+1
Thus, the “per” prefix refers to the highest oxidation state for chlorine.
Back to the example of the OP, the oxidation state for oxygen in hydrogen peroxide is -1, a higher oxidation state than oxygen in water, for which it is -2.
I am convinced, but have never seen it in print, that the “per” prefix was originally “hyper” and then shortened to “per” because of the similarity in sound with “hypo.” This would be akin to hyperthermia and hypothermia, which are two very different medical conditions. The former is now called “heat exhaustion,” I believe, to avoid such similar-sounding terms.
BTW, bordelond, prefixes are generally used for molecular compounds, such as carbon monoxide (CO) and carbon dioxide (CO[sub]2[/sub]), and not used for ionic compounds, such as magnesium oxide (MgO). The reason for this is because in molecular compounds, the elements making up the compound generally have multiple oxidation states. I would thus argue that water would be dihydrogen monoxide. A more systematic name for hydrogen peroxide (H[sub]2[/sub]O[sub]2[/sub]) would be dihydrogen dioxide, I suppose.
So the peroxide ion would then be…?
A peroxide ion is when the oxy has only one negative charge. Hydrogen monoxide is one O with a -2 charge. Hydrogen peroxide has two oxys, each having only a -1 charge.
And the rule with dihydrogen monoxide is that any chemical compound which does not contain a metal, according to IUPAC standards, is listed with the prefixes.
robby’s right. Or at least my junior high chemistry teacher said so. “Per” is a shortened form of “hyper”.
From the Glossary of Chemical Terms, C. A. Hampel & G. G. Hawley, 1976 (ISBN 0-442-23238-1):
From Ame.Her.Dic.: per- pref. 1. Thoroughly; completely; intensely: perfervid. 2. Containing an element in its highest oxidation state: perchloric acid. 3. Containing a large or the largest possible proportion of an element: peroxide. 4. Containing the peroxy group
sigh chemists at work didn’t know the naswer to this one, that’s where my “per” guess came from.
Hey, it made sense! Of course, that should have tipped me off that it couldn’t have been right. 
Don’t sweat it, erislover. It’s clear that the prefix has shades of meaning, from the specific, as in peroxide (RO-OR), to the general sense of a “saturated” oxidation state or ionic substitution. I even checked back to an old college textbook, and only found the “saturated” sense. Hmpf.
At least you didn’t screw up your coding, as I did. And looking back, my first answer also appears short and somewhat rude, which wasn’t intended.
RM Mentock, do you read threads before you post to them?
Just thought I would insert a word of caution. Dihydrogen Monoxide (DHMO)is a very dangerous chemical that should be handled with extreme caution. Among the dangers include health risks:
[ul]Accidental inhalation of DHMO, even in small quantities, may cause death
Prolonged exposure to solid DHMO causes severe tissue damage
Gaseous DHMO can cause severe burns
DMHO is found in biopsies of pre-cancerous tumors and lesions[/ul]
and environmental risks
[ul]DHMO is a major component of acid rain
DHMO leads to corrosion and oxidation of many metals.
DHMO contributes to soil erosion[/ul]
More info can be found here: http://www.dhmo.org/facts.html
Guess I should’ve seen that coming, Beeblebrox. But as funny as that site is, you should really throw in a smiley or something, lest a newbie see it and freak. Mission of the board, and all.