Lesson 6: Nomenclature of Acids, Bases, & Salts

Here are some examples involving representative metals. Because each of these metals has only one possible oxidation state, which is equal to its group number in the periodic table, we don’t have to specify the metals positive charge in the name.

The next examples involve transition metals. These elements can have more than one positive ion, so the name must indicate which one it is. You can determine the positive charge by seeing how many hydroxide ions, each with a charge of minus one, it takes to make a neutral compound.

Another hydroxide compound that is basic is ammonium hydroxide. The ammonium ion, NH₄⁺ is the conjugate acid of ammonia, NH₃. But the ammonium ion is a very weak acid, while the hydroxide ion is a very strong base, so a solution of ammonium hydroxide is basic.

Ammonia itself, NH₃, is also a base. This molecule forms hydroxide ions when it dissolves in water by reacting with the water molecules.

In fact, solutions of ammonia in water and solutions of ammonium hydroxide in water are indistinguishable. Ammonia and ammonium hydroxide do, however, behave quite differently under other circumstances, as, for example, in their pure forms (ammonia is a gas and ammonium hydroxide is a solid) or when they are dissolved in solvents other than water.

Note that “binary” does not mean “only two atoms.” Binary acids can have more than two atoms, but they are atoms of only two different elements.

Similarly, ternary acids can have more than three atoms, but they are atoms of only three different elements. Many ternary acids are also called “oxy-acids” because one of the three elements is oxygen.

The names of the binary acids begin with the prefix “hydro,” followed by a root that is based on the name of the element the hydrogen is combined with, then the suffix “ic.” They end with the word “acid.” HCl, for example, would be hydrochloric acid.

Only the binary acids that involve the halogens (F, Cl, Br, and I) are commonly used as acids. These are hydrofluoric, hydrochloric, hydrobromic, and hydroiodic acids. H₂S is only weakly acidic in water, and so is usually referred to as hydrogen sulfide, while H₂O is not really an acid at all (though it can function as one when it reacts with a strong base), it’s just plain water. H₃N is actually a base not an acid. For this reason it is written instead as NH₃, which you should recognize as the formula of ammonia.

Ternary acids consist of a polyatomic ion combined with hydrogen. Their names of the polyatomic ions they contain. In general, the “ate” ending of the polyatomic ion is replaced with the “ic” ending and the word “acid” is added.

To name these acids, you must remember the name, the formula, and the charge of each of the common polyatomic oxy-ions you learned in Chemistry 104. They are acetate, carbonate, nitrate, phosphate, sulfate, and chlorate. The two additional polyatomic ions you studied were the ammonium ion and the hydroxide ion.

There are two exceptions to this general rule. The acid formed from the sulfate ion is called sulfuric acid, while the acid formed from the phosphate ion is called phosphoric acid. Note the addition of “ur” in the first name and “or” in the second.

Note also that in all of these ternary acid (oxy-acid) names, the prefix “hydro” is NOT used. Including the “hydro” prefix in the names of the ternary acids and leaving if off of the names of binary acids are common errors among beginning students of chemistry. The “hydro” prefix is used only with binary acids, never with ternary acids.

Most non-metal atoms that form polyatomic ions form more than one polyatomic ion. These ‘families’ of polyatomic ions all have the same charge, but different number of oxygen atoms. Nitrogen, sulfur and phosphorus each form two such polyatomic ions, while chlorine, bromine and iodine each form four.

Carbon is one of the few elements that forms only one inorganic oxy-anion: carbonate. There are also some transition metals atoms that form oxy-anions – chromium and manganese are the two most common – and each of these forms more than one polyatomic ion, but they do not follow the simple pattern illustrated here and you will not be expected to memorize their names.

Finally, both phosphorus and sulfur form several additional oxy-anions, some of them involving two or more phosphorus or sulfur atoms. We will not be concerned with them here.

To name a polyatomic ion with one oxygen atom fewer than an “ate” ion, change the “ate” ending to “ite.” For example, to name the SO₃^2- ion, which has one oxygen atom less than the sulfate ion, change the “ate” ending in sulfate to “ite.” The name becomes sulfite.

Each of these ions has one oxygen atom fewer then the corresponding “ate” ion, but the same negative charge.

The other “ite” ions would therefore be:

phosphite = PO₃^3-
nitrite = NO₂^-
bromite = BrO₂^-
iodite = IO₂^-

 

To name a polyatomic ion with one oxygen atom more than an “ate” ion, add the prefix “per” to the name. For example, the ClO₄^- ion has one more oxygen atom than the chlorate ion, and so would be called perchlorate.

The “per” prefix comes from the Latin and means “thoroughly,” “utterly,” or “very.” It occurs in words like perfect, pervert, pervade and is similar to the prefix “hyper,” as in hyperactive.

The other “per” ion would therefore be:

periodate = IO₄^-

(pronounced "per - eye - oh - date")

Finally, to name a polyatomic ion with one oxygen less than an “ite” ion, add the prefix “hypo” to the name. ClO^- has one oxygen less than the chlorite ion, and so is called hypochlorite.

The “hypo” prefix also comes from Latin and means “beneath.” You probably recognize it from the word “hypo-dermic,” as in “hypodermic needle,” a needle used to deliver beneath the skin.

The other "hypo" ion would therefore be:

hypobromite = BrO^-

The complete series of ions based on chlorine would be:

perchlorate	ClO4-
chlorate	ClO3-
chlorite	ClO2-
hypochlorite	ClO-

(The series for the other two halogens will follow the same pattern.)

The acids that form from all of these polyatomic ions are named simply by changing the “ate” ending to “ic” or the “ite” ending to “ous” and adding the work “acid.”

HClO₄, the acid made from the perchlorate ion, is perchloric acid.

HClO, the acid made from the hypochlorite ion, is hypochlorous acid.

As before, in the case of sulfur and phosphorus, we add a “ur” or an “or” so that the sulfite ion forms sulfurous acid and the phosphite ion forms phosphorous acid.

Polyatomic ions that have a negative charge greater than one can, under the right circumstances, associate with only one positive hydrogen ion. Carbonate, for example, can associate with a single hydrogen ion to form a new polyatomic ion with the formula HCO₃^-. This ion is called the hydrogen carbonate ion.

Note that the charge on these ions is one less than the charge on the corresponding ions without the hydrogen. This is due to the 1+ charge on the hydrogen ion.

Another common name for these ions is formed by adding the prefix “bi” to the original name:

HCO₃^- would be bicarbonate, and HSO₃^- would be bisulfite.

As we saw in the last lesson, the other product from an acid-base neutralization is usually water. All salts are ionic compounds, but not all ionic compounds are salts. Salts are those ionic compounds that can be formed from an acid-base neutralization reaction.

Salts are named simply by giving the names of the positive and negative ions. This is the same way we named the metal hydroxide bases to begin this lesson, except that now the negative ion is not hydroxide. Also remember, for transition metals, to include the Roman numeral that indicates the charge on the metal.

3.

potassium hydroxide

KOH

potassium nitrate

KNO₃

calcium hydroxide

Ca(OH)₂

sodium nitrite

NaNO₂

ammonia

NH₃

sodium hypochlorite

NaClO

hydrobromic acid

HBr

sodium bisulfate

NaHSO₄

bromic acid

HBrO₃

sodium hydrogen carbonate

NaHCO₃

sulfuric acid

H₂SO₄

sodium bicarbonate

NaHCO₃

sulfurous acid

H₂SO₃

bicarbonate of soda

NaHCO₃

phosphoric acid

H₃PO₄

magnesium perchlorate

Mg(ClO₄)₂

periodic acid

HIO₄

aluminum sulfite

Al₂(SO₃)₃

4.

NaOH

sodium hydroxide

Li₂CO₃

lithium carbonate

Mg(OH)₂

magnesium hydroxide

Al(HCO₃)₃

aluminum bicarbonate

NH₄⁺

ammonium ion

NH₄Cl

ammonium chloride

OH^-

hydroxide ion

NaKSO₄

sodium potassium sulfate

NO₃^-

nitrate ion

KBr

potassium bromide

NO₂^-

nitrite ion

HC₂H₃O₂

acetic acid

HI

hydroiodic acid

NaC₂H₃O₂

sodium acetate

HIO₂

iodous acid

HCl

hydrochloric acid

HIO₄

periodic acid

HNO₂

nitrous acid