Ionic bonding
Three types of bonding
Atoms bond together by transferring or sharing electrons:
| Bonding | Between | What the electrons do |
|---|---|---|
| A metal and a non-metal | Transferred from metal to non-metal | |
| Non-metals | Shared in pairs | |
| Metal atoms | Delocalised (free to move) |
In each case, the atoms end up with a more stable arrangement of electrons, like the full outer shell of a noble gas.
Key terms
Forming ions
An is a charged particle made when an atom loses or gains electrons.
- atoms lose their outer electrons to form .
- atoms gain electrons to form .
Both end up with the electronic structure of a (a full outer shell). You can work out the charge from the group number:
| Group | Electrons | Ion | Example |
|---|---|---|---|
| 1 | lose 1 | +1 | Na → Na⁺ + e⁻ |
| 2 | lose 2 | +2 | Mg → Mg²⁺ + 2e⁻ |
| 6 | gain 2 | −2 | O + 2e⁻ → O²⁻ |
| 7 | gain 1 | −1 | Cl + e⁻ → Cl⁻ |
Key terms
Dot and cross diagrams for ionic compounds
A shows electrons moving from the metal to the non-metal. Electrons from one atom are drawn as dots and from the other as crosses.
Sodium chloride (NaCl)
- Sodium (2,8,1) gives its one outer electron to chlorine (2,8,7).
- Draw Na⁺ in square brackets with its charge outside: [Na]⁺. Its old outer shell is now empty, leaving a full shell (2,8).
- Draw Cl⁻ with eight outer electrons (2,8,8) — seven dots and one cross — in brackets: [Cl]⁻.
Magnesium oxide (MgO): magnesium (2,8,2) gives two electrons to oxygen (2,6), forming Mg²⁺ and O²⁻.
You usually only draw the .
Key terms
Giant ionic lattices
An ionic compound is a of ions called a . Positive and negative ions alternate in a regular 3D pattern, held by strong acting in all directions.
The charges must cancel out, which gives the formula. Mg²⁺ and Cl⁻ make MgCl₂; Na⁺ and O²⁻ make Na₂O.
Diagrams all have limits: dot and cross diagrams don't show the 3D arrangement; ball and stick models show gaps between ions that aren't really there and sticks for forces that aren't bonds; 2D drawings show only one layer.
Key terms
Properties of ionic compounds
- A lot of energy is needed to overcome the many strong electrostatic forces between the ions.
- Do not conduct electricity when solid. The ions are held in fixed positions and cannot move.
- Conduct when melted or dissolved in water. The ions are now free to move and carry charge through the liquid.
This is why molten or dissolved ionic compounds can be split up by (topic C4).