Naming Molecular Compounds and Binary Acids — Key Points
Naming Molecular Compounds and Binary Acids — Key Points
Overall idea: Naming molecular (covalent) compounds follows simple rules because these compounds contain no charges and are not ionic. They are built from nonmetals (and sometimes metalloids) rather than ions.
Core rule: For molecular compounds, prefixes indicate how many atoms of each element are present. Do not use charges in the name. Do not rely on ionic names.
Example: PCl5 is named phosphorus pentachloride, not monophosphorus pentachloride.
This illustrates that the prefix for the first element is typically not used as “mono-” in the first element.
Formula: → name: phosphorus pentachloride.
First element vs second element:
If the first element is nitrogen in N2O, use the prefixes to indicate the number of atoms: N2O = dinitrogen monoxide.
For the second element, the ending is -ide (e.g., chlorine becomes chloride, fluorine becomes fluoride).
Example: OF2 = oxygen difluoride (first element oxygen appears as one, so you don’t say monoxygen; you say “oxygen”).
The “a” drop rule for oxides with prefixes (subtle):
When naming oxides with a numeric prefix, there is a stylistic change where the ending of the oxide part may appear as tetroxide instead of tetraa-oxide in some conventions.
The instructor notes that you drop the extra ‘a’ in the oxide ending after the prefix (i.e., prefix + oxide form becomes something like “tetroxide” rather than “tetraoxide”).
Example discussion: instead of calling a compound “tetraoxide,” the common form used is tetroxide in some teaching contexts, reflecting a naming simplification. The key idea is: use the prefix to indicate the number of oxygens and end the second part with oxide; be mindful there are historical/common-name exceptions.
Exceptions: many simple compounds have common names that do not follow the strict systematic naming rules. Examples mentioned:
Water is not called dihydrogen oxide or dihydrogen monoxide in routine usage.
Hydrogen oxide is not commonly used for water.
Diborane can be used instead of the fully systematic name diboron hexahydride.
These examples illustrate that not everything follows the nice systematic scheme because the common name is entrenched.
Frequent and educational examples you’ll see often:
Methane (CH$_4$) is common and often introduced early in this topic.
OF2 (oxygen difluoride) is a practical example to practice the prefix rules.
PCl5 (phosphorus pentachloride) is a key example of using prefixes but omitting mono- for the first element.
Practice patterns (conversions both ways):
Given a formula, name it by identifying the number of atoms for each element from subscripts and applying the suffix -ide to the second element.
Given a name, construct the formula by translating prefixes to subscripts for each element and placing the appropriate element symbols.
Example flow:
Name: tetrachloride → Cl has 4 atoms; first element is phosphorus implied by context → formula: (one P, four Cl).
Name: tetraphosphorus decoxide (or tetraphosphorus decaoxide) → first element: P with prefix tetra-/tetra; second element: oxygen with prefix deca-/deco- and suffix -oxide → formula: .
Note on the suffix: second element name ends with -ide (chloride, fluoride, oxide, etc.) and the number is given by the prefix (di-, tri-, tetra-, penta-, deca-, etc.). For oxygen with a prefix, you’ll often see dioxide, trioxide, etc., with variations such as “tetroxide” depending on the naming tradition.
Acids: binary acids vs oxyacids
Binary acids (two elements total, hydrogen + another element): name them with the hydro- stem, the nonmetal stem, then -ic, then acid.
Example: HCl is hydrochloric acid (hydro + chlor + ic + acid).
Formula: -> name: hydrochloric acid.
Example: HF is hydrofluoric acid (hydro + fluor + ic + acid).
Formula: -> name: hydrofluoric acid.
Rule demonstrated: when there is no prefix, assume there is 1 atom of that element (e.g., HCl is not “monohydro chloride” in common practice).
Spelling/terminology tip: avoid confusing “fluorine” with “flour.” The element name is fluorine; the anion ending is fluoride.
The instructor notes that oxyacids (acids with oxygen in the anion) exist, but you will not be tested on the full oxyacid naming system yet. Binary acids (two-element acids) are the focus for now.
The acids naming system is described as somewhat complex; the plan is to cover a simpler portion (binary acids) first, and then move to other examples later.
Additional notes and student tips:
When given a name, if a prefix is not stated, you should assume the count is 1 for that element.
Always spell element names correctly (e.g., fluoride, not flour). Spelling mistakes are a common error in homework and exams.
In practice, some compounds have historically entrenched names (common names) that do not follow the strict rules; be aware of these exceptions.
Understand that molecular/naming rules apply to nonmetals or metalloids forming covalent bonds, not to ionic compounds.
Always practice both directions: from formula to name and from name to formula to become fluent.
Quick reference recap (rules at a glance):
Molecular compounds: nonmetals/metalloids; no charges; prefixes indicate numbers; second element ends in -ide.
Do not use the prefix mono- for the first element (e.g., no monophosphorus pentachloride).
For oxides with prefixes, the ending form around the oxide may vary (tetroxide vs tetraa-oxide) depending on convention; the key is to convey the prefix and oxide correctly.
Common exceptions exist due to historical naming; rely on the systematic rules most of the time, but be aware of well-known common names.
Representative problems discussed (summary):
Name the molecule from formula: → oxygen difluoride.
Name from formula: → phosphorus pentachloride.
Name from formula: → methane (frequently encountered; default example).
Name from formula: → hydrochloric acid (binary acid naming).
Name from formula: → hydrofluoric acid (binary acid naming).
Name from formula: → normally referred to as diborane (example of common vs systematic naming).
Final reminder: Use the system as your default, but recognize some standard names are entrenched and exceptions exist. For acids, focus on binary acids first before tackling oxyacids.