Unit 6: IUPAC Naming of Inorganic Compounds
Using oxidation numbers for systematic and unambiguous chemical naming.
6.3 IUPAC Naming Rules (Binary, Polyatomic)
The International Union of Pure and Applied Chemistry (IUPAC) provides a systematic way of naming compounds to avoid ambiguity. For many inorganic compounds, especially those involving metals that can form multiple ions (like iron or copper), the oxidation number is a key part of the name.
Naming Binary Compounds (e.g., Ionic and Simple Covalent):
- The element with the positive oxidation number (the cation or more electropositive element) is named first.
- The element with the negative oxidation number (the anion or more electronegative element) is named second, with its ending changed to -ide. (e.g., oxygen becomes oxide, chlorine becomes chloride).
- If the first element can have multiple oxidation numbers, its oxidation number in the compound is written in Roman numerals in parentheses immediately after its name.
Examples:
- $FeCl_2$: Iron has an oxidation number of +2. The name is iron(II) chloride.
- $FeCl_3$: Iron has an oxidation number of +3. The name is iron(III) chloride.
- $CO_2$: Carbon has an oxidation number of +4. The name is carbon(IV) oxide (though it is commonly called carbon dioxide).
- $NaCl$: Sodium only forms a +1 ion. The Roman numeral is usually omitted. The name is sodium chloride.
Naming Compounds with Polyatomic Ions:
For polyatomic anions containing oxygen (oxoanions), the ending is changed to -ate. The oxidation number of the central atom is included in Roman numerals.
- $SO_4^{2-}$: Sulphur is +6. The ion is sulphate(VI). Therefore, $K_2SO_4$ is potassium sulphate(VI).
- $NO_3^{-}$: Nitrogen is +5. The ion is nitrate(V). Therefore, $Cu(NO_3)_2$ is copper(II) nitrate(V).
- $MnO_4^{-}$: Manganese is +7. The ion is manganate(VII). Therefore, $KMnO_4$ is potassium manganate(VII).
Solved Examples:
- What is the IUPAC name for $CuO$?
Solution: Oxygen is -2, so copper must be +2. The name is copper(II) oxide. - What is the IUPAC name for $Cu_2O$?
Solution: Oxygen is -2, so the two copper atoms must total +2. Each copper is +1. The name is copper(I) oxide. - Give the IUPAC name for $K_2SO_3$.
Solution: K is +1, O is -2. For sulphur (x): 2(+1) + x + 3(-2) = 0, so x = +4. The $SO_3^{2-}$ ion is the sulphite or sulphate(IV) ion. The name is potassium sulphate(IV). - What is the name of the compound $PbO_2$?
Solution: Oxygen is -2. For lead (x): x + 2(-2) = 0, so x = +4. The name is lead(IV) oxide. - Name the compound $Na_2SO_4$.
Solution: S is +6. The name is sodium sulphate(VI). Since sodium only has one oxidation state, it is often just called sodium sulphate. - What is the IUPAC name for $KClO_3$?
Solution: K is +1, O is -2. For chlorine (x): 1 + x + 3(-2) = 0, so x = +5. The name is potassium chlorate(V). - Give the formula for tin(IV) chloride.
Solution: Tin(IV) means $Sn^{4+}$. Chloride is $Cl^-$. The formula is $SnCl_4$. - Give the formula for manganese(II) nitrate(V).
Solution: Manganese(II) is $Mn^{2+}$. Nitrate(V) is $NO_3^-$. The formula is $Mn(NO_3)_2$. - What is the name of the compound $Fe_2(SO_4)_3$?
Solution: The sulphate ion ($SO_4^{2-}$) has a 2- charge. Three of them give a total charge of 6-. Therefore, the two iron ions must have a total charge of 6+, meaning each is $Fe^{3+}$. The name is iron(III) sulphate(VI). - Name the compound $NH_3$ systematically.
Solution: H is +1. For nitrogen (x): x + 3(+1) = 0, so x = -3. The systematic name is nitrogen(III) hydride, although it is universally known as ammonia.
Knowledge Check (20 Questions)
Answer: Iron(III) chloride.
Answer: The oxidation number of lead, which is +4.
Answer: $Cu_2S$.
Answer: Sulphate(VI).
Answer: Sulphur(IV) oxide.
Answer: -ide.
Answer: Potassium manganate(VII).
Answer: $Fe_3(PO_4)_2$. (Iron(II) is $Fe^{2+}$, phosphate(V) is $PO_4^{3-}$).
Answer: Because calcium is in Group 2 and only forms a +2 ion. Its oxidation number is not variable.
Answer: +5.
Answer: Phosphorus(V) chloride.
Answer: Nitrate(III) (commonly known as nitrite).
Answer: $Cr_2O_3$.
Answer: The one with the positive oxidation number (the more electropositive element).
Answer: Sodium oxide (the Roman numeral (I) is omitted).
Answer: +6.
Answer: -ate.
Answer: $PbI_2$.
Answer: Nitrogen(I) oxide.
Answer: Chlorate(I) (commonly known as hypochlorite).