Ionic Bonding

“Students know atoms combine by exchanging electrons to form ionic compounds, and that salt crystals, such as NaCl, are repeating patterns of positive and negative ions held together by electrostatic action.”

-State Standards 2a and 2c.

How ionic compounds form

Positive ions are attracted to negative ions. (Cations and anions attract).

Attracted ions are held close together by electrostatic forces (like a balloon rubbed on hair is attracted to the white board)

The repeating pattern of positive and negative ions forms a crystal lattice structure.

NaCl Example The cation Na+ forms

when Na gives up an electron to Cl. Cl- then attracts the cation and the two oppositely charged ions are held tightly in an ionic bond.

Repeating units of NaCl bonds forms a salt crystal.

Ionic Compounds are NOT Molecules Molecules are made

by sharing electrons Molecules involve

atoms, not ions The atoms in a

molecule are touching each other, the electron orbitals are shared

Ionic compounds (sometimes called formula units) are made by oppositely charged ions that attract

Ions don’t actually touch…they are held close together by electrostatic forces.

Ionic Compounds are typically made by a metal and a nonmetal

Metals tend to form positive ions (cations) Nonmetals tend to form negative ions

(anions) Group 1 metals and Group 17 nonmetals

attract readily (+1 and -1) Group 2 metals and Group 16 nonmetals

attract readily (+2 and -2)

Ionic bonds are very strong The ions in a crystal

are actually surrounded on all sides by oppositely charged ions.

The attraction of the ions gives the crystal a very rigid, brittle structure.

Properties of Ionic Compounds Very high melting and boiling points Solid at room temperature Low electrical conductivity in the solid state, but

good electrical conductivity in solution or in the liquid state

Hard crystal structure Brittle (can be broken into smaller crystal pieces

by force)

What makes a crystal brittle? Opposite charges attract,

but like charges repel In a crystal, the ions are

arranged so that every positive ion is completely surrounded by negative ions

With enough force, you can knock the ions out of alignment, so that ions of the same charge are next to one another. They repel, and the structure fractures.