Ionic Bonding

• Metal atoms with low ionization energies and non-metal atoms with high electron affinity form cations (+) and anions (-)

• Oppositely charged ions attract to a crystalline structure

Ionic Bonding

Coulomb’s Law

The energy of interaction between the pair of ions can be calculated

Where Q1 and Q2 are the charges of the ions and r the distance between the ion’s centers

Since one charge is always + and the other -, the energy will be a negative number

The ion pair has lower energy than separate ions

1 2QQEnergy k

r

1 2QQEnergy k

r

Lattice Energy

Quantity of energy required for 1 mole of the solid ionic substance to be separated into its ions

Higher the negative value (exothermic) the more stable the compound

The higher the lattice energy of an ionic compound the harder, more brittle, and the higher the melting, boiling and vaporization points

Crystal Lattice Strength

Based on Coulomb’s Law, we expect that ionic solids formed by smaller atoms or by atoms that have greater charge, will be stronger

Example:

Why is the lattice energy for MgO about four times greater than the lattice energy form NaF?

• Which substance would have a higher melting point : NaF or CsI?

• Which would have a higher lattice energy: FeO or Fe2O3?

Ionic Structures

Can have many types of arrangements Arrangement depends on the charge and size

of the ions involved

Forming The Ions

The ions are formed when electrons are transferred from the atom with low electronegativity to an atom with high electronegativity

Predict the compound formed between aluminum and fluorine.

Sizes of Ions

Cations are smaller than their parents Anions are larger than their parents Ions of the same charge, size increases going

down a group List the following in order of decreasing size

Mg2+, Ca2+, Ca

Isoelectric Series

Comparitive sizes of ions all possessing the same number of electrons

As nuclear charge increases, the radius decreases

O2- > F- > Na+ > Mg2+ > Al3+

Example

Arrange the ions S2- , K+ , Ca2+, and Cl- in order of decreasing size.