Slide presentation
Naming Hydroxides
General Chemistry • Chemical Compounds
Topic 1 · Polyatomic ion naming
Naming Hydroxides
Hydroxides are ionic compounds that contain a metal cation and the hydroxide ion, OH−. Their names are built from the metal name and the word hydroxide.
Learning target
Identify hydroxide compounds, balance metal charge with OH−, and name fixed-charge or variable-charge metal hydroxides correctly.
iron(III) hydroxide
Why it matters
Hydroxide names connect formulas to bases and ion charges.
Many common bases are metal hydroxides. Naming them correctly helps students interpret formulas, laboratory labels, and neutral ionic charge relationships.
Bases in chemistry
NaOH and KOH are common strong bases, and their formulas show the hydroxide ion.
Charge reasoning
Ca(OH)2 has two hydroxide ions because calcium is Ca2+.
Roman numerals
Fe(OH)2 and Fe(OH)3 must have different names because iron has different charges.
Core concept
Hydroxide acts as one unit: OH−.
Even though OH− contains oxygen and hydrogen, it is treated as one polyatomic ion with a −1 charge. When more than one hydroxide ion is needed, parentheses are used.
1. Identify metal
The metal cation is named first.
2. Identify OH−
Hydroxide is the anion and keeps its name.
3. Balance charge
Use enough OH− ions to neutralize the metal charge.
4. Add Roman numeral
Use one only for variable-charge metals.
Pattern: metal name + optional Roman numeral + hydroxide.
Vocabulary
Hydroxide formulas combine metal charge with OH−.
The number of hydroxide ions is controlled by the positive charge of the metal cation.
| Term | Meaning | Example | Naming or formula rule |
|---|---|---|---|
| Hydroxide ion | Polyatomic ion with one oxygen and one hydrogen | OH− | Always named hydroxide. |
| Fixed-charge metal | Metal with one common charge | Na+, Ca2+, Al3+ | No Roman numeral is used. |
| Variable-charge metal | Metal that can form different cations | Fe2+, Fe3+ | Use Roman numeral to show charge. |
| Parentheses | Show multiple copies of a polyatomic ion | Mg(OH)2 | Use when more than one OH− is needed. |
| Base | Substance that can produce hydroxide ions in water | NaOH | Many metal hydroxides are bases. |
Main relationship
Charge balance determines the number of hydroxide ions.
Because each OH− has a −1 charge, a metal with a \(2+\) charge needs two hydroxide ions, and a metal with a \(3+\) charge needs three hydroxide ions.
For Al3+, \(n=3\), so the formula is Al(OH)3. The name is aluminum hydroxide.
One hydroxide ion
Na+ balances one OH−, so the formula is NaOH.
Multiple hydroxide ions
Fe3+ balances three OH− ions, so the formula is Fe(OH)3.
Interactive simulation
Select a metal and build the hydroxide formula.
The builder uses the metal charge to decide how many hydroxide ions are needed, whether parentheses are required, and whether a Roman numeral appears in the name.
Hydroxide name builder
Formula and name
Ca(OH)₂ — calcium hydroxide
Ca2+ needs two OH− ions, so parentheses are used.
Static fallback model
Ca2+ needs two OH− ions. The formula is Ca(OH)2, and the name is calcium hydroxide.
Cation
Ca2+
Hydroxide ions
2 OH−
Dynamic relationship
The metal charge controls hydroxide count.
The graph compares total positive charge and total negative charge after adding the correct number of OH− ions.
This visual uses the selected metal from the interactive builder.
Worked example
Name Fe(OH)3.
The compound contains iron and hydroxide. Since iron can have multiple charges, the formula must be used to determine the Roman numeral.
Identify hydroxide
OH− is the hydroxide ion. The formula has three hydroxide ions.
Find total negative charge
Three OH− ions give a total charge of −3.
Find iron charge
To make the compound neutral, iron must be Fe3+.
Name the compound
Fe3+ is iron(III), so Fe(OH)3 is iron(III) hydroxide.
Final answer: Fe(OH)3 is iron(III) hydroxide.
Common mistake
Do not split OH− into separate oxide and hydride pieces.
Hydroxide is one polyatomic ion. The formula Ca(OH)2 means two hydroxide ions, not two oxygen atoms plus two separate hydrogen atoms named independently.
Incorrect reasoning
“Ca(OH)2 should be named calcium oxygen hydrogen because the formula contains O and H.”
This ignores the polyatomic ion identity.
Correct reasoning
OH− is the hydroxide ion. The correct name is calcium hydroxide.
Practice check
Name Cu(OH)2 and write the formula for aluminum hydroxide.
Question: What is the correct name of Cu(OH)2, and what formula represents aluminum hydroxide?
Show answer
Name Cu(OH)2
Two OH− ions give −2 total charge, so copper must be Cu2+. The name is copper(II) hydroxide.
Write aluminum hydroxide
Aluminum is Al3+. Three OH− ions are needed, so the formula is Al(OH)3.
Reasonableness check
The number outside parentheses applies to the whole hydroxide ion: Al(OH)3 contains three OH− ions.
Apply the topic
Use hydroxide as a single ion in formulas and names.
Hydroxide naming becomes reliable when you identify the metal charge, keep OH− together, and apply Roman numerals only when the metal is variable-charge.
Open the calculator
Practice naming metal hydroxides and writing formulas from hydroxide names.
Try related questions
Check your understanding of OH−, charge balance, parentheses, and Roman numerals.
Find metal
Name it first.
Find charge
Determine metal cation charge.
Balance OH−
Use enough hydroxide ions.
Name
Metal + hydroxide, with Roman numeral if needed.
Final summary
Hydroxide names are built from metal cations and OH−.
OH− is one ion.
Treat hydroxide as a polyatomic ion with a −1 charge.
Charge balance controls formula.
A 2+ metal needs two hydroxide ions; a 3+ metal needs three.
Parentheses matter.
Use parentheses when more than one OH− ion is present.
Roman numerals are selective.
Use them only for variable-charge metals such as iron and copper.
Key idea: Naming hydroxides is ion accounting: metal charge tells how many OH− ions are needed and whether a Roman numeral is required.