Inorganic Acid Names and Formulae Introductory Chemistry Tutorial

Key Concepts

IUPAC nomenclature⁽¹⁾ provides a systematic framework for the naming of inorganic (non-carbon containing) compounds and organic (carbon containing) compounds based on their composition or structure.

Some inorganic acids have been given non-systematic names containing the word "acid".
These "acid" names describe a property of the compound, "acidity", instead of the compound's composition or structure.

These "acid" names apply to the following two different groups of acids:
⚛ binary acids: do not contain oxygen (particularly hydrohalic acids)

⚛ oxoacids (oxyacids)⁽²⁾: contain oxygen

Binary inorganic acids refer to aqueous solutions of compounds made up of hydrogen and one other element.

Hydrohalic acids are binary inorganic acids in which the other element is a halogen (Group 17) element.
⚛ Names of hydrohalic acids always have the prefix "hydro" followed by the name of the halogen with final "ine" of the name substituted with "ic", and this is then followed by the word "acid".

⚛ Molecular formula of a hydrohalic acid is always HX_(aq), where H is the symbol for hydrogen, X is the symbol for the halogen (Group 17) element, and (aq) refers to an aqueous solution.

Oxoacids (or oxyacids) are inorganic compounds made up of oxygen, hydrogen and one other element (called the central atom or central element).
⚛ Oxoacids are named with the name of the central element first using a modified ending (suffix) to indicated the relative amount of oxygen present, followed by the word "acid".

⚛ The "ic" suffix indicates more oxygen is present in the compound than for the "ous" suffix.

⚛ Molecular formula of an oxoacid is of the form H_xEO_y, in which there are x hydrogen atoms (H), a central atom (E) and y oxygen atoms (O).

Oxoacids (oxyacids) in which the central atom is a halogen (Group 17) element (X) can be one of 4 different compounds so the prefixes "per" and "hypo" are used in addition to the suffixes "ic" and "ous" to denote relative amounts of oxygen in the compound:⁽³⁾
⚛ perhalic acid has the most oxygen of all

general molecular formula HXO₄

⚛ halic acid has less oxygen than perhalic acid

general molecular formula HXO₃

⚛ halous acid has less oxygen than halic acid but more oxygen than hypohalous acid

general molecular formula HXO₂

⚛ hypohalous acid has the least oxygen of all

general molecular formula HXO

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Binary Acids (hydrohalic acids)

Hydrohalic acids are aqueous solutions of binary inorganic compounds in which hydrogen, H, is combined with a halogen (Group 17) element.

	Names and Symbols of Group 17 Elements
Name of halogen (Group 17 element)	fluorine	chlorine	iodine	bromine	astatine
Chemical Symbol	F	Cl	I	Br	At

The general molecular formula for all hydrohalic acids is HX_(aq), where H is the symbol for hydrogen, X represents the symbol for one of the halogen elements and (aq) indicates that this is an aqueous solution.
Note that the chemical symbol for hydrogen, H, is written before the chemical symbol of the Group 17 element.
The formula for each hydrohalic acid you are likely to encounter is given in the table below:

	Hydrohalic acids⁽⁴⁾
Molecular Formula	HF_(aq)	HCl_(aq)	HI_(aq)	HBr_(aq)

The name of hydrohalic acids always have the prefix "hydro", representiong the acidic hydrogen, followed by the name of the halogen with final "ine" of the name substituted with "ic", then followed by the separate word "acid".

molecular formula	prefix	+	modified name of element	+ acid =	"acid" name
HF_(aq)	hydro	+	fluor~~ine~~ + ic	+ acid =	hydrofluoric acid
HCl_(aq)	hydro	+	chlor~~ine~~ + ic	+ acid =	hydrochloric acid
HBr_(aq)	hydro	+	brom~~ine~~ + ic	+ acid =	hydrobromic acid
HI_(aq)	hydro	+	iod~~ine~~+ ic	+ acid =	hydroiodic acid

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Oxoacids (Oxyacids)

Oxoacids (or oxyacids) are inorganic compounds made up of oxygen (O), hydrogen (H) and one other element (E) called the central atom or central element.
Examples of molecular formula and their corresponding possible structures showing the relative positions of hydrogen (H), oxygen (O) and the central element (E) are shown below:⁽⁵⁾

possible
structure

HO-

	O \|\|
HO-	E	-O

	O \|\|
HO-	E	-OH

	O \|
HO-	E	-OH
	\| O

	O \|\|
HO-	E-OH
	\| OH

molecular
formula

HEO₂

HEO₃

H₂EO₃

H₂EO₄

H₃EO₄

Molecular formulae are given in the classical oxoacid format with the "acid" or "replaceable" hydrogen atoms (oxygen-bound hydrogen atoms) listed first, followed by the central atom (E), and then the oxygen atoms (O).

Oxoacids are named with the name of the central element first using a modified ending (suffix) to indicated the relative amount of oxygen present, followed by the word "acid".

The "ic" suffix indicates more oxygen is present in the compound than for the "ous" suffix

The table below includes compounds containing oxygen and hydrogen and one other element that is NOT a halogen (Group 17) element. ⁽⁶⁾

elements in acid oxygen, hydrogen and ...	most oxygen (ic) (central atom in highest oxidation state)	least oxygen (ous) (central atom in lowest oxidation state)
boron	boric acid H₃BO₃
carbon	carbonic acid H₂CO₃
silicon	silicic acid H₄SiO₄
nitrogen	nitric acid HNO₃	nitrous acid HNO₂
phosphorus	phosphoric acid H₃PO₄	phosphorous acid H₃PO₃
arsenic	arsenic acid H₃AsO₄	arsenous acid H₃AsO₃
antimony	antimonic acid H₃SbO₄	antimonous acid H₃SbO₃
sulfur	sulfuric acid H₂SO₄	sulfurous acid H₂SO₃
selenium	selenic acid H₂SeO₄	selenous acid H₂SeO₃
tellurium	telluric acid H₂TeO₄	tellurous acid H₂TeO₃

Oxoacids (oxyacids) in which the central atom is a halogen (Group 17) element (X) can be one of 4 different compounds so the prefixes "per" and "hypo" are used in addition to the suffixes "ic" and "ous" to denote relative amounts of oxygen in the compound:

⚛ perhalic acid has the most oxygen of all with the general molecular formula HXO₄

⚛ halic acid has less oxygen than perhalic acid and has the general molecular formula HXO₃

⚛halous acid has less oxygen than halic acid but more oxygen than hypohalous acid and has the general molecular formula HXO₂

⚛ hypohalous acid has the least oxygen of all and has the general molecular formula HXO

Name and Molecular Formula of Oxoacids with a Central Halogen Atom
elements in acid oxygen, hydrogen and...	most oxygen (central atom in highest oxidation state)	less oxygen	even less oxygen	least oxygen (central atom in lowest oxidation state)
chlorine	perchloric acid HClO₄	chloric acid HClO₃	chlorous acid HClO₂	hypochlorous acid HClO
bromine	perbromic acid HBrO₄	bromic acid HBrO₃	bromous acid HBrO₂	hypobromous acid HBrO
iodine	periodic acid HIO₄	iodic acid HIO₃	iodous acid HIO₂	hypoiodous acid HIO

And this allows us to end this page with a chemistry joke or two:

Question: What do you call an acid that only works some of the time?
Answer: periodic acid (mispronunciation of periodic acid)
Question: What acid is prepared by boiling up old periodic tables?
Answer: periodic acid (mispronunciation of periodic acid)

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Footnotes: reference "Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005" (Red Book)

(1) IUPAC is the acronym for the International Union of Pure and Applied Chemistry.
IUPAC reviews existing methods of naming compounds (called nomenclature) and makes recommendations on how compounds should be named in the future so that there is no ambiguity in the name (and formula) of any particular compound.

(2) IUPAC refers to these as oxoacids, but some syllabus documents use the term oxyacids.

(3) Does this sound like a form of systematic nomenclature to you?
It isn't really, mainly because the suffixes "ic", "ous" are not used to refer to a particular amount of oxygen (or a particluar oxidation state of the central atom) and because the prefixes "per" and "hypo" are not used in a systematic way.
Therefore these "acid names" are not truly based on either the composition or the structure of the compounds.

(4) What happened to astatine? Astatine is highly radioactive and has not been found occurring naturally on Earth so its chemistry is not well understood. Using the principles of periodicity, the trend in the properties of group 17 elements of the periodic table leads us to expect its properties to be similar to iodine.

(5) The structures here are for demonstration of the central element (E) and possible arrangements of oxygen (O) and hydrogen (H) atoms, they should NOT be used as general structures in which you can just replace the "E" with the symbol of an element.

(6) Note that structurally each of the oxoacids must contain at least one OH group and this effects the IUPAC systematic additive names as shown below:

Acceptable common name (systematic additive name)	Formula in oxoacid format [formula as for coordination entity]
boric acid (trihydroxidoboron)	H₃BO₃ [B(OH)₃]
carbonic acid (dihydroxidooxidocarbon)	H₂CO₃ [CO(OH)₂]
cyanic acid (hydroxidonitridocarbon)	HOCN [C(N)OH]
silicic acid (tetrahydroxidosilicon)	H₄SiO₄ [Si(OH)₄]
nitric acid (hydroxidodioxidonitrogen)	HNO₃ [NO₂(OH)]
nitrous acid (hydroxidooxidonitrogen)	HNO₂ [NO(OH)]
phosphoric acid (trihydroxidooxidophosphorus)	H₃PO₄ [PO(OH)₃]
phosphorous acid (trihydroxidophosphorus)	H₃PO₃ [P(OH)₃]
arsenic acid, arsoric acid (trihydroxidooxidoarsenic)	H₃AsO₄ [AsO(OH)₃]
arsenous acid, arsorous acid (trihydroxidoarsenic)	H₃AsO₃ [As(OH)₃]
antimonic acid, stiboric acid (trihydroxidooxidoantimony)	H₃SbO₄ [SbO(OH)₃]
antimonous acid, stiborous acid (trihydroxidoantimony)	H₃SbO₃ [Sb(OH)₃]
sulfuric acid (dihydroxidodioxidosulfur)	H₂SO₄ [SO₂(OH)₂]
sulfurous acid (dihydroxidooxidosulfur)	H₂SO₃ [SO(OH)₂]
selenic acid (dihydroxidodioxidoselenium)	H₂SeO₄ [SeO₂(OH)₂]
selenonic acid (hydridohydroxidodioxidoselenium)	H₂SeO₃ [SeO(OH)₂]
telluric acid (dihydroxidodioxidotellurium)	H₂TeO₄ [TeO₂(OH)₂]
tellurous acid (dihydroxidooxidotellurium)	H₂TeO₃ [TeO(OH)₂]
perchloric acid (hydroxidotrioxidochlorine)	HClO₄ [ClO₃(OH)]
chloric acid (hydroxidodioxidochlorine)	HClO₃ [ClO₂(OH)]
chlorous acid (hydroxidooxidochlorine)	HClO₂ [ClO(OH)]
hypochlorous acid (chloridohydridooxygen)	HClO [O(H)Cl]
perbromic acid (hydroxidotrioxidobromine)	HBrO₄ [BrO₃(OH)]
bromic acid (hydroxidodioxidobromine)	HBrO₃ [BrO₂(OH)]
bromous acid (hydroxidooxidobromine)	HBrO₂ [BrO(OH)]
hypobromous acid (bromidohydridooxygen)	HBrO [[O(H)Br]
periodic acid (hydroxidotrioxidoiodine)	HIO₄ [IO₃(OH)]
iodic acid (hydroxidotrioxidoiodine)	HIO₃ [IO₂(OH)]
iodous acid (hydroxidooxidoiodine)	HIO₂ [IO(OH)]
hypoiodous acid (hydridoiodidooxygen)	HIO [O(H)I]