Ionic Bonding
Formation of Ions
An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons.
- Cations are positively charged ions, formed when atoms lose electrons. Metals form cations.
- Anions are negatively charged ions, formed when atoms gain electrons. Non-metals form anions.
Ions form to achieve a full outer shell of electrons (the same electronic configuration as a noble gas). The number of electrons lost or gained equals the size of the charge on the ion.
Example: Sodium (2,8,1) loses 1 electron to form Na+ (2,8) with neon’s configuration.
Example: Chlorine (2,8,7) gains 1 electron to form Cl- (2,8,8) with argon’s configuration.
Ion Charges from Group Number
| Group | Charge | Examples |
|---|---|---|
| Group 1 metals | 1+ | Li+, Na+, K+ |
| Group 2 metals | 2+ | Mg2+, Ca2+ |
| Group 3 metals | 3+ | Al3+ |
| Group 5 non-metals | 3- | N3-, P3- |
| Group 6 non-metals | 2- | O2-, S2- |
| Group 7 non-metals | 1- | F-, Cl-, Br-, I- |
Ions to Learn
| Ion | Charge |
|---|---|
| Ag+ | Silver |
| Cu2+ | Copper(II) |
| Fe2+ | Iron(II) |
| Fe3+ | Iron(III) |
| Pb2+ | Lead(II) |
| Zn2+ | Zinc(II) |
| H+ | Hydrogen |
| OH- | Hydroxide |
| NH4+ | Ammonium |
| CO32- | Carbonate |
| NO3- | Nitrate |
| SO42- | Sulfate |
Exam Tip
The number of electrons that an atom gains or loses equals the magnitude of the charge. Magnesium loses 2 electrons to form Mg2+; nitrogen gains 3 electrons to form N3-.
Dot-and-Cross Diagrams for Ionic Compounds
Dot-and-cross diagrams show the transfer of electrons from metal to non-metal. Only the outer shell electrons need be shown. Square brackets surround each ion, with the charge written outside as a superscript.
Sodium chloride (NaCl):
- Na (2,8,1) loses 1 electron to Cl (2,8,7)
- Na+ and Cl- are formed
Magnesium oxide (MgO):
- Mg (2,8,2) loses 2 electrons to O (2,6)
- Mg2+ and O2- are formed
Exam Tip
For exam purposes, only show outer shell electrons in dot-and-cross diagrams. You must be able to draw diagrams for combinations of ions from Groups 1, 2, 3, 5, 6 and 7.
Ionic Bonding
Ionic bonding is the strong electrostatic attraction between oppositely charged ions. The force acts in all directions throughout the structure.
Ionic bonds form when a metal and a non-metal react together, transferring electrons from the metal to the non-metal.
Giant Ionic Lattice Structure
In an ionic compound, thousands of positive and negative ions are arranged in a regular, repeating pattern called a giant ionic lattice. Sodium chloride is a classic example: Na+ and Cl- ions are arranged alternately in a three-dimensional lattice.
The giant ionic lattice is held together by very strong electrostatic forces of attraction between the oppositely charged ions.
Properties of Ionic Compounds
High Melting and Boiling Points
Ionic compounds have high melting and boiling points because:
- They have giant ionic lattice structures
- There are strong electrostatic forces of attraction between oppositely charged ions, acting in all directions
- A large amount of energy is needed to overcome these forces
Compounds with higher-charged ions (e.g. MgO with Mg2+ and O2-) have higher melting points than those with lower-charged ions (e.g. NaCl with Na+ and Cl-).
Electrical Conductivity
Ionic compounds:
- Do not conduct electricity when solid: the ions are in fixed positions in the lattice and cannot move to carry a charge.
- Do conduct electricity when molten or in aqueous solution: the ions are free to move and can carry a charge.
Exam Tip
A very common exam error is to say that ionic compounds conduct because “electrons” move. This is wrong. In ionic compounds, it is the ions that move and carry the charge, not electrons.
Higher Tier
Anion: a negatively charged ion (e.g. Cl-, O2-). Anions migrate towards the anode (positive electrode) during electrolysis.
Cation: a positively charged ion (e.g. Na+, Mg2+). Cations migrate towards the cathode (negative electrode) during electrolysis.
Ionic compounds conduct because when molten or dissolved, the ions are free to move and carry charge through the solution.
Exam Questions
Q: Explain how ions are formed when magnesium reacts with oxygen.
Magnesium atoms lose 2 electrons from their outer shell to form Mg2+ ions. Oxygen atoms gain 2 electrons into their outer shell to form O2- ions. Both ions achieve a full outer shell configuration similar to a noble gas.
Q: Explain why sodium chloride has a high melting point.
Sodium chloride has a giant ionic lattice structure. There are strong electrostatic forces of attraction between the oppositely charged Na+ and Cl- ions, acting in all directions. A large amount of energy is needed to overcome these forces, giving it a high melting point.
Q: Explain why sodium chloride does not conduct electricity as a solid but does conduct when molten.
As a solid, the Na+ and Cl- ions are in fixed positions in the lattice and cannot move to carry a charge, so it does not conduct. When molten, the ions are free to move and can carry charge through the melt, so it does conduct.
Q: Write the formula of the compound formed between iron(III) ions (Fe3+) and hydroxide ions (OH-).
Fe(OH)3 (swap charges: 3 OH- for every Fe3+; brackets needed around OH)
Exam Tip
When forming ionic compounds, the total positive charge must equal the total negative charge, giving an overall charge of zero. Use the swap-and-drop method for ions with different charge sizes, and always include brackets for polyatomic ions when there is more than one of them.