Why do these names need prefixes?
Carbon and oxygen form both CO and CO₂, and nitrogen and oxygen form N₂O, NO, NO₂, N₂O₄, and N₂O₅. Nonmetal atoms bond by sharing electrons (lesson 2.4.2), with no fixed ion charges, so you can’t work out the numbers of atoms from charges the way you can for ionic compounds. A molecular compound’s name has to state the numbers directly: English uses prefixes, and Chinese uses number words. Lesson 1.5.4 introduced mono- through penta-; this lesson covers 1 to 10 and the details that are easy to get wrong.
Step 1: Prefixes or not?
First decide what kind of compound it is:
- Contains a metal (or NH₄⁺) → ionic: no prefixes; name it as in lessons 2.5.1 and 2.5.2. CaCl₂ is calcium chloride.
- Only nonmetals → molecular: use prefixes. CCl₄ is carbon tetrachloride.
The prefixes and the rules
The ten prefixes are 1 mono-, 2 di-, 3 tri-, 4 tetra-, 5 penta-, 6 hexa-, 7 hepta-, 8 octa-, 9 nona-, and 10 deca-.
- The first element keeps its element name and gets a prefix only if there is more than one: CO₂ is carbon dioxide, not monocarbon dioxide.
- The second element takes its -ide name and always gets a prefix, including mono-: CO is carbon monoxide.
- Drop a vowel: when a prefix ending in a or o comes right before oxide, drop the prefix’s final vowel: monoxide, tetroxide, pentoxide, heptoxide, decoxide. The i of di- and tri- stays: dioxide, trioxide.
- Order of elements: the element farther left on the periodic table (or lower in the same group), the less electronegative one, usually comes first. So oxygen usually comes second, except with fluorine: OF₂ is oxygen difluoride.
- Common names: some molecular compounds always go by common names, such as H₂O water, NH₃ ammonia, and CH₄ methane. N₂O is also called nitrous oxide (laughing gas).
Chinese names read back to front and say the numbers: CO 一氧化碳, N₂O₄ 四氧化二氮, P₄O₁₀ 十氧化四磷. As in English, a single first atom gets no “one” (一), but a single second atom does.
Choose names for formulas, or formulas for names. Each answer splits the name into prefixes and element names:
Example 1: From formulas to names
Name N₂O₄ and P₄O₁₀ in English and Chinese.
- Identify the type first. Both contain only nonmetals, so they are molecular and their names use prefixes for the numbers of atoms.
- N₂O₄. Two N: dinitrogen. Four O: tetra- + oxide, dropping the a before the o of oxide: tetroxide. The name is dinitrogen tetroxide (四氧化二氮).
- P₄O₁₀. Four P: tetraphosphorus. Ten O: deca- + oxide → decoxide. The name is tetraphosphorus decoxide (十氧化四磷).
- English versus Chinese order. English names the first element first; Chinese reads from the back (“四氧化二氮” says oxygen first), but both state every number.
- Think about it. P₄O₁₀ is often written by its simplest ratio, P₂O₅, and called diphosphorus pentoxide (五氧化二磷, the form Chinese textbooks use). The real molecule is P₄O₁₀.
Example 2: Check the type before using prefixes
Write formulas for sulfur hexafluoride, dichlorine heptoxide, and calcium fluoride.
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Sulfur hexafluoride. Two nonmetals, so it’s molecular and the prefixes give the numbers: one S (mono- is dropped on the first element) and hexa- for six F: SF₆.
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Dichlorine heptoxide. di- for two Cl and hepta- for seven O: Cl₂O₇.
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Calcium fluoride. Calcium is a metal, so this is ionic: no prefixes in the name, and the numbers come from the charges, Ca²⁺ and F⁻:
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Compare. A molecular compound’s name states its numbers; an ionic compound’s numbers are hidden in its charges. A common mistake is writing CaF, assuming no prefix means one.
- CO₂
Carbon and oxygen are both nonmetals: carbon dioxide.
- CaCl₂
Calcium is a metal: calcium chloride, never calcium dichloride.
- N₂O₅
Two nonmetals: dinitrogen pentoxide.
- Fe₂O₃
Iron is a metal: iron(III) oxide.
- SF₆
Two nonmetals: sulfur hexafluoride.
- Na₂O
Sodium is a metal: sodium oxide.
- PCl₃
Two nonmetals: phosphorus trichloride.
- NH₄Cl
It contains only nonmetals, but NH₄⁺ makes it ionic: ammonium chloride.
- SiO₂
Two nonmetals: silicon dioxide.
Common mistakes
Mistake 1: “The first element takes mono- too.” CO is carbon monoxide, not monocarbon monoxide. Mono- is used only on the second element.
Mistake 2: “Just join the prefix and oxide: pentaoxide.” When a prefix ending in a or o meets oxide, drop the prefix’s final vowel: pentoxide, monoxide, tetroxide.
Mistake 3: “English names read back to front, like Chinese ones.” The Chinese name of N₂O₄, 四氧化二氮, mentions oxygen first, but the English name, dinitrogen tetroxide, mentions nitrogen first. English names follow the formula from front to back.
Quick check
What is the name of P₄O₁₀?
Molecular compounds use prefixes: four P is tetraphosphorus, and ten O is deca- + oxide, dropping the a to give decoxide.
What is the formula of dinitrogen monoxide?
Di- means two N and mono- means one O: N₂O, also known as nitrous oxide or laughing gas.
Summary
- Decide the type first: compounds with a metal or NH₄⁺ are ionic and use no prefixes; compounds of only nonmetals are molecular and use prefixes.
- The prefixes for 1 to 10 are mono, di, tri, tetra, penta, hexa, hepta, octa, nona, and deca. The first element never takes mono-; the second takes its -ide name and always a prefix; a prefix ending in a or o drops that vowel before oxide.
- Chinese names read back to front and say the numbers: 四氧化二氮, 十氧化四磷.
On to AP (for later)
Molecular compounds that come up often on the AP exam, such as SO₂, SO₃, NO₂, N₂O₄, P₄O₁₀, PCl₅, and SF₆, are all named with prefixes as in this lesson. Also know the common names water, ammonia, and methane, which AP questions use directly.