2.3C Ionic Bond Strength and Electrostatic Forces- Pre AP Chemistry Study Notes - New Syllabus.

2.3.C.1 — Strength of Attractions in Ionic Solids

Ionic solids are composed of cations and anions arranged in a regular, repeating crystal lattice. The relative strength of attraction between these ions depends on two key factors: the charges of the ions and the distance between them.

These attractions are electrostatic in nature and are responsible for many macroscopic properties of ionic compounds, such as high melting points and brittleness.

Electrostatic Nature of Ionic Bonding

Ionic bonding results from the attraction between oppositely charged ions:

• Cations have a positive charge
• Anions have a negative charge
• Opposite charges attract due to electrostatic forces

The strength of this attraction determines how strongly the ions are held together in the solid lattice.

Overall Electrical Neutrality

In an ionic solid, the ratio of cations to anions is such that the total positive charge equals the total negative charge.

• This ensures overall electrical neutrality
• The ratio depends on the charges of the ions, not on equal numbers of ions

For example:

• One \( \mathrm{Mg^{2+}} \) ion balances two \( \mathrm{Cl^-} \) ions
• Two \( \mathrm{Na^+} \) ions balance one \( \mathrm{O^{2-}} \) ion

Effect of Ion Charge on Attraction Strength

As the magnitude of ionic charge increases, the strength of electrostatic attraction also increases.

• Ions with larger charges exert stronger forces on each other
• Greater charge → stronger attraction → more stable lattice

For example, the attraction between \( \mathrm{Al^{3+}} \) and \( \mathrm{Cl^{-}} \) is stronger than between \( \mathrm{Na^+} \) and \( \mathrm{Cl^-} \).

Effect of Distance Between Ions

The distance between ions also affects the strength of attraction.

• Shorter distance → stronger attraction
• Greater distance → weaker attraction

Smaller ions can get closer together, increasing electrostatic attraction, while larger ions are held farther apart.

Combining Charge and Distance Effects

The strongest ionic attractions occur when:

• The ions have large charges
• The ions are small, allowing close approach

Ionic compounds with strong attractions typically have higher melting points and greater lattice stability.

Summary of Factors Affecting Ionic Attraction

Creating and Evaluating Claims

A correct claim about ionic attraction strength must:

• Reference the charges of the ions
• Discuss the distance between ions
• Connect these factors to electrostatic attraction

Claims that mention only charge or only size are incomplete.