S2.1.2 Ionic bonds and binary compounds

The Nature of the Ionic Bond and Naming Ionic Compounds

Formation of the Ionic Bond: Electrostatic Attraction Between Opposites

Why Do Atoms Form Ions?

Definition

Octet rule

Atoms are driven by their quest for stability, often achieved by attaining a noble gas electron configuration—commonly referred to as the octet rule.

Metals, with only a few electrons in their outermost shells, tend to lose electrons, forming positively charged ions called cations. In contrast, non-metals gain electrons to fill their outer shells, forming negatively charged ions, or anions.

How Ionic Bonds Form

When a metal and a non-metal interact, the metal atom transfers one or more electrons to the non-metal atom.

This transfer creates oppositely charged ions, which are held together by electrostatic forces—the attraction between positive and negative charges.

This force is the essence of an ionic bond.

Definition

Ionic bond

An ionic bond is the electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion.

Example

Formation of Sodium Chloride ( $NaCl$ )

A sodium atom ( $Na$ ) loses one electron, becoming a sodium ion ( ${Na}^{+}$ ).
A chlorine atom ( $Cl$ ) gains that electron, becoming a chloride ion ( ${Cl}^{-}$ ).
The ${Na}^{+}$ and ${Cl}^{-}$ ions are then attracted to each other, forming the ionic compound sodium chloride.

Tip

The strength of an ionic bond depends on the charges of the ions and the distance between them. Higher charges and smaller ionic radii result in stronger bonds.

Formation of ionic bond between sodium and chlorine.

Naming Binary Ionic Compounds: Cation First, Anion Second

When naming ionic compounds, the cation (positive ion) is always named first, followed by the anion (negative ion). The name of the cation is the same as the name of the metal, while the name of the anion is derived from the non-metal with the suffix -ide.

Example

$NaCl$ : Sodium chloride
$MgO$ : Magnesium oxide
$AlN$ : Aluminum nitride

Common Mistake

Do not reverse the naming order. Always name the cation first, even though it is written to the left of the anion in the chemical formula.

Self review

What is the name of the compound $CaS$ ?

Polyatomic Ions: Groups of Atoms Acting as a Single Ion

Not all ions are single atoms. Some ions consist of multiple atoms bonded together, carrying an overall charge. These are called polyatomic ions, and they behave as a single unit in chemical reactions.

Common Polyatomic Ions You Should Know:

Sulfate: $S O_{4}^{2 -}$ (found in gypsum and detergents)
Nitrate: $N O_{3}^{-}$ (common in fertilizers and explosives)
Carbonate: $C O_{3}^{2 -}$ (present in limestone and baking soda)
Hydroxide: $O H^{-}$ (important in bases like sodium hydroxide)
Ammonium: $N H_{4}^{+}$ (found in ammonium salts and fertilizers)
Phosphate: $P O_{4}^{3 -}$ (essential in biological molecules like DNA and ATP)
Chlorate: $C l O_{3}^{-}$ (used in disinfectants and bleaching agents)
Acetate: $C H_{3} C O O^{-}$ (present in vinegar and food preservatives)
Permanganate: $M n O_{4}^{-}$ (used as a disinfectant and oxidizing agent)
Bicarbonate: $H C O_{3}^{-}$ (found in baking soda and blood buffer systems)

From left to right: ammonium, carbonate, nitrate, phosphate.

Naming Ionic Compounds with Polyatomic Ions

The rules for naming ionic compounds remain the same: the cation is named first, followed by the anion. If the compound contains a polyatomic ion, use the name of the ion as it appears in the table above.

Example

${NaNO}_{3}$ : Sodium nitrate
${CaCO}_{3}$ : Calcium carbonate
${NH}_{4} Cl$ : Ammonium chloride

Tip

When writing formulas with polyatomic ions, enclose the ion in parentheses if more than one is needed. For example, ${Mg(NO}_{3})_{2}$ indicates two nitrate ions for every magnesium ion.

Self review

What is the formula for potassium sulfate?

Writing Formulas for Ionic Compounds: Balancing Charges

The formula of an ionic compound reflects the ratio of cations to anions necessary to achieve electrical neutrality. The total positive charge must equal the total negative charge.

Steps to Determine the Formula:

Identify the charges of the cation and anion.
Balance the charges by determining the smallest whole-number ratio of ions.
Write the formula, placing the cation first and the anion second.

Example question

Deduce the formula for magnesium chloride.

Solution

Magnesium ( $Mg$ ) forms ${Mg}^{2 +}$ , and chlorine ( $Cl$ ) forms ${Cl}^{-}$ .
To balance the charges, two ${Cl}^{-}$ ions are needed for every ${Mg}^{2 +}$ ion.
The formula is ${MgCl}_{2}$ .

Example question

Deduce the formula for aluminum sulfate.

Solution

Aluminum ( $Al$ ) forms ${Al}^{3 +}$ , and sulfate ( ${SO}_{4}^{2 -}$ ) forms ${SO}_{4}^{2 -}$ .
To balance the charges, two ${Al}^{3 +}$ ions (total charge $+ 6$ ) combine with three ${SO}_{4}^{2 -}$ ions (total charge $- 6$ ).
The formula is ${Al}_{2} ({SO}_{4})_{3}$ .

Example question

Deduce the formula for calcium nitrate.

Solution

Calcium forms ${Ca}^{2 +}$ , and nitrate forms ${NO}_{3}^{-}$ .
Two nitrate ions are needed to balance the charge of one calcium ion.
The formula is ${Ca(NO}_{3})_{2}$ .

Common Mistake

Do not forget to balance the charges. Incorrect balancing leads to incorrect formulas.

Self review

What is the formula for barium phosphate?

Reflection

Theory of Knowledge

How does the understanding of ionic bonding connect to the broader question of how humans utilize natural resources? Consider the ethical implications of mining for ionic compounds like lithium for batteries.

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S2.1.2 Ionic bonds and binary compounds

The Nature of the Ionic Bond and Naming Ionic Compounds

Formation of the Ionic Bond: Electrostatic Attraction Between Opposites

Why Do Atoms Form Ions?

How Ionic Bonds Form

Formation of Sodium Chloride (NaCl)

Naming Binary Ionic Compounds: Cation First, Anion Second

Polyatomic Ions: Groups of Atoms Acting as a Single Ion

Common Polyatomic Ions You Should Know:

Naming Ionic Compounds with Polyatomic Ions

Writing Formulas for Ionic Compounds: Balancing Charges

Steps to Determine the Formula:

Reflection

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Atoms and Ions

Formation of Sodium Chloride ( $NaCl$ )