Nomenclature of Ionic Compounds

– In this subject, we will discuss the Nomenclature of Ionic Compounds.

– In addition to using formulas to show the composition of molecules and compounds, chemists have developed a system for naming substances based on their composition.

– we divided naming substances into three categories:

(1) Nomenclature of ionic compounds.

(2) Nomenclature of molecular compounds.

(3) Nomenclature of acids and bases.

– In this lesson, we will talk about the Nomenclature of Ionic Compounds

Nomenclature of Ionic Compounds

– In the lesson (Molecules and Ions) we learned that ionic compounds are made up of cations (positive ions) and anions (negative ions).

– With the important exception of the ammonium ion, NH₄⁺, all cations of interest to us are derived from metal atoms.

– Metal cations take their names from the elements. For example:

Binary ionic compounds

– Many ionic compounds are binary compounds or compounds formed from just two elements.

– For binary ionic compounds, the first element named is the metal cation, followed by the nonmetallic anion.

– Thus, NaCl is sodium chloride.

– The anion is named by taking the first part of the element name (chlorine) and adding “-ide.”

– Potassium bromide (KBr), zinc iodide (ZnI₂), and aluminum oxide (Al₂O₃) are also binary compounds.

– The table below shows the “-ide” nomenclature of some common monatomic anions according to their positions in the periodic table:

Ternary ionic compounds

– The “-ide” ending is also used for certain anion groups containing different elements, such as hydroxide (OH^–) and cyanide (CN^–).

– Thus, the compounds LiOH and KCN are named lithium hydroxide and potassium cyanide.

– These and several other ionic substances are called ternary compounds, meaning compounds consisting of three elements.

– The table below lists alphabetically the names of several common cations and anions:

Ionic compounds of transition metals

– Certain metals, especially transition metals, can form more than one type of cation.

– Take iron as an example. Iron can form two cations: Fe²⁺ and Fe³⁺.

– The accepted procedure for designating different cations of the same element is to use Roman numerals.

– The Roman numeral I is used for one positive charge, II for two positive charges, and so on. This is called the Stock system.

– In the Stock system, the Fe²⁺ and Fe³⁺ ions are called iron(II) and iron(III), and the compounds FeCl₂ (containing the Fe²⁺ ion) and FeCl₃ (containing the Fe³⁺ ion) are called iron-two chloride and iron-three chloride, respectively.

– As another example, manganese (Mn) atoms can assume several different positive charges:

– These compound names are pronounced “manganese-two oxide,” “manganese-three oxide,” and “manganese four oxide.”

– The Figure blow summarizes the steps for naming ionic and molecular compounds:

Solved problems

Ex (1): Name the following compounds: (a) Fe(NO₃)₂, (b) Na₂HPO₄, and (c) (NH₄)₂SO_3.

solution

(a) The nitrate ion (NO₃^–) bears one negative charge, so the iron ion must have two positive charges.

– Because iron forms both Fe²⁺ and Fe³⁺ ions, we need to use the Stock system and call the compound iron(II) nitrate.

(b) The cation is Na⁺ and the anion is HPO₄^2- (hydrogen phosphate).

– Because sodium only forms one type of ion (Na⁺), there is no need to use sodium(I) in the name.

– The compound is sodium hydrogen phosphate.

(c) The cation is NH4⁺ (ammonium ion) and the anion is SO₃^2- (sulfite ion).

– The compound is ammonium sulfite.

Ex (2): Write chemical formulas for the following compounds: (a) mercury(I) nitrate, (b) cesium oxide, and (c) strontium nitride.

solution

(a) The Roman numeral shows that the mercury ion bears a +1 charge.  

– the mercury(I) ion is diatomic (that is, Hg₂⁺² ) and the nitrateion is NO₃^–.

– Therefore, the formula is Hg₂(NO₃)₂.

(b) Each oxide ion bears two negative charges, and each cesium ion bears one positive charge (cesium is in Group 1A, as is sodium). Therefore, the formula is Cs₂O.

(c) Each strontium ion (Sr²⁺) bears two positive charges, and each nitride ion (N^3-) bears three negative charges.

– To make the sum of the charges equal zero, we must adjust the numbers of cations and anions:

3(+2) + 2(-3) = 0

– Thus, the formula is Sr₃N₂.

Reference: General Chemistry: The Essential Concepts / Raymond Chang, Jason Overby. ( sixth edition) .