Name the following ionic compounds:
a) KOH
b) FeCl2
c) MgI_(2)
d) AlCl
The correct answer and explanation is:
Correct answers:
a) Potassium hydroxide
b) Iron(II) chloride
c) Magnesium iodide
d) Aluminum chloride
An ionic compound is named by combining the name of its cation (the positively charged ion) with the name of its anion (the negatively charged ion). The cation is always named first. For compounds with metals that can form more than one positive charge, the charge is specified in Roman numerals in parentheses after the metal’s name.
For example, potassium hydroxide (KOH) contains potassium ions (K⁺) and hydroxide ions (OH⁻). Potassium has only one possible positive charge so the name is straightforward. Hydroxide is a common polyatomic ion. Putting them together gives potassium hydroxide.
Iron(II) chloride (FeCl₂) demonstrates the need for Roman numerals. Iron can form Fe²⁺ or Fe³⁺ ions. In FeCl₂, each chloride ion has a negative charge (Cl⁻). Since there are two chlorides, the total negative charge is -2. To balance this, iron must be +2, so the compound is iron(II) chloride. If it were FeCl₃, it would be iron(III) chloride. The Roman numeral shows the oxidation state of the metal.
Magnesium iodide (MgI₂) follows the simple rule because magnesium always has a +2 charge. Each iodide ion has a -1 charge, so two iodides balance the +2 from magnesium. No Roman numerals are needed because magnesium does not have variable charges in simple ionic compounds.
Aluminum chloride (AlCl₃) is also straightforward. Aluminum usually forms a +3 ion. Chloride is Cl⁻. Three chloride ions balance the +3 from aluminum. Since aluminum does not commonly form other charges, Roman numerals are unnecessary here too.
Understanding how to name these compounds helps in writing and reading chemical formulas correctly. Ionic compounds have predictable ratios based on the charges of the ions involved. By ensuring the total positive and negative charges cancel out, the compound remains neutral overall.