2.2.3 Ionic Bonding

Formation of Ions

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
• This loss or gain of electrons takes place to obtain a full outer shell of electrons
• The electronic structure of ions of elements in groups 1, 2, 3, 5, 6 and 7 will be the same as that of a noble gas - such as helium, neon, and argon
• Negative ions are called anions and form when atoms gain electrons, meaning they have more electrons than protons
• Positive ions are called cations and form when atoms lose electrons, meaning they have fewer electrons than protons
• All metals lose electrons to other atoms to become positively charged ions
• All non-metals gain electrons from other atoms to become negatively charged ions

Diagram showing the formation of the sodium ion

Diagram showing the formation of the chloride ion

Exam Tip

Representing Ionic Bonds

• Positively and negatively charged ions are held together by the strong electrostatic forces of attraction between the oppositely charged ions - this is what an ionic bond is
• Ionic bonds can be represented diagrammatically using dot and cross diagrams
• These are a simple and quick way to show the formation of an ionic compound
• The electrons from each atom should be represented by using solid dots and crosses
  
  • If there are more than two atoms, then hollow circles or other symbols / colours may be used to make it clear
• The large square brackets should encompass each atom and the charge should be in superscript and on the right-hand side, outside the brackets
• For larger atoms with more electron shells, only the valence shell (outer shell) needs to be drawn

Diagram representing the formation of the ionic bond in sodium chloride

Example: The Formation of Sodium Chloride

• Sodium is a Group 1 metal with only one electron in the outer shell
  • It needs to lose this one outer electron to another atom, leaving the next shell down as the full outer shell of electrons
  • A positive sodium ion with the charge 1+ is formed
• Chlorine is a Group 7 non-metal with seven electrons in the outer shell
  • It needs to gain one electron to have a full outer shell of electrons and be stable
  • The chlorine atom will gain an electron to form a negatively charged chloride ion with a charge of 1- 
• Therefore one electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom, giving a sodium ion with a +1 charge and a chloride ion with a 1- charge
  
  • The ions are then attracted to one another and held together by strong electrostatic forces
  • The formula of the ionic compound is thus NaCl, as the ratio of sodium ions to chloride ions is 1:1

Dot-and-cross diagram of sodium chloride

Example: The Formation of Magnesium Oxide

• Magnesium is a Group 2 metal with two electrons in the outer shell
  • It needs to lose two outer electrons to another atom to have a full outer shell of electrons
  • A positive magnesium ion with the charge 2+ is formed
• Oxygen is a Group 6 non-metal with six electrons in the outer shell
  • It needs to gain two electrons to have a full outer shell of electrons
  • The oxygen atom will gain two electrons to form a negative oxide ion with charge 2-
• Therefore two electrons are transferred from the outer shell of the magnesium atom to the outer shell of the oxygen atom, giving a magnesium ion with a 2+ charge and an oxide ion with a 2- charge
• The ions are then attracted to one another and held together by strong electrostatic forces
• The formula of the ionic compound is thus MgO, as the ration of magnesium ions to oxide ions is 1:1

Dot-and-cross diagram of magnesium oxide

Limitations of Dot and Cross Diagrams

• Advantages:
  • Useful for illustrating the transfer of electrons
  • Indicates from which atom the bonding electrons come from
• Disadvantages:
  • Fails to illustrate the 3D arrangements of the atoms and electron shells
  • Doesn’t indicate the relative sizes of the atoms

Exam Tip