Naming Ions

All cations and anions have specific names. Here is a list of the most common ones:

Pb2+Lead (II)
Fe2+Iron (II)
Fe3+Iron (III)
Cu+Copper (I)
Cu2+Copper (II)
Hg+Mercury (I)
Hg2+Mercury (II)
Ag+Silver (I)
Cr³⁺Chromium (III)
Mn2+Manganese (II)
Co2+Cobalt (II)
Co3+Cobalt (III)
Ni2+Nickel (II)
Ni3+Nickel (III)

For anions, the naming convention typically involves replacing the ending of the element’s name with “-ide.” For example, a chlorine anion is called chloride.

Cations generally keep the name of their element. For elements that can form ions with different charges, the charge is indicated in Roman numerals in parentheses. For example, an Fe2+ ion is called iron (II), while an Fe3+ ion is called iron (III).

Naming Ionic Compounds

To name binary ionic compounds (those containing only two types of elements), you first write the name of the cation followed by the name of the anion from the list above. For example:

  • KCl, containing K+ and Cl ions, is known as potassium chloride.
  • NaCl, containing Na+ and Cl ions, is known as sodium chloride.
  • MgO, containing Mg2+ and O2- ions, is known as magnesium oxide.

Although these examples deal with compounds of two elements, the principle extends to compounds with more elements. However, the compound must always have a net charge of zero, meaning the total charge of anions equals the total charge of cations. For example:

Magnesium chloride consists of Mg2+ and Cl ions. The formula is not MgCl (which implies a +1 net charge), but instead MgCl2, ensuring the charges balance: one Mg2+ ion balances two Cl ions.

To determine any ionic compound’s formula from its name, follow these steps:

StepExample: Lithium OxideExample: Aluminium Nitride
1. Write the symbol and charge of the cation (positive ion) first and the anion (negative ion) second.Li+ and O2-Al3+ and N³⁻
2. Adjust the numbers of each ion so their charges balance.2 Li+ 2(+1) balances 1 O²⁻ 1(-2)1 Al3+ 1(+3) balances 1 N3- 1(-3)
3. Apply these numbers as subscripts to each ion in the formula.Li2O1Al1N1
4. Write the final formula, leaving out all charges and any subscript of 1.Li2O (not Li2O1)AlN (not Al1N1)

Polyatomic ions

Not all ions consist of a single atom from one element. Many include multiple atoms, sometimes from different elements, and these have their own specific names. Here are some important examples to remember:

  • Hydroxide: OH
  • Nitrate: NO3
  • Carbonate: CO­­32-
  • Sulfate: SO42-
  • Ammonium: NH4+

Another naming rule for these ions is that if an ion’s name ends with the suffix “-ate,” it typically contains oxygen.

These polyatomic ions, just like single-atom ions, form ionic bonds with ions of opposite but equivalent charges.

To determine the formula of an ionic compound containing polyatomic ions, the process is similar to that used for any ionic compound. When the formula requires multiple instances of a polyatomic ion, we put the entire ion in brackets, with the multiplicative number outside the brackets.

StepExample: Sodium CarbonateExample: Magnesium Hydroxide
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.Na+ and CO32-Mg2+ and OH
2. Use a multiplier to make the total charge on the cations and anions equal.2 Na+ → 2(1+) balances CO32- → 1(2-)Mg2+ → 1(2+) balances 2 OH⁻  2(1-)
3. Use the multipliers as a subscript for each ion.Na2CO3Mg(OH)2

You’ve used 9 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up