Ions and ionic bonds
Formation of Ions (LO1)
Atoms become ions to achieve a stable electronic configuration, typically matching the nearest noble gas. This involves gaining or losing electrons from the outer shell.
| Ion Type | Cation (Positive Ion) |
|---|---|
| Formation | Metal atoms lose electrons. |
| Result | More protons than electrons, resulting in a positive charge. |
| Ion Type | Anion (Negative Ion) |
|---|---|
| Formation | Non-metal atoms gain electrons. |
| Result | More electrons than protons, resulting in a negative charge. |
Why this happens: Metals (typically Group I and II) have few outer electrons. It is energetically easier to lose them to reveal a full inner shell than to gain many more. Non-metals (typically Group VI and VII) are close to a full outer shell, so they gain the few electrons needed to complete it.
Ionic Bond
An ionic bond is a strong electrostatic attraction between oppositely charged ions.
Key terms:
- Electrostatic: Relates to electric charges.
- Attraction: The force pulling opposite charges together.
- Oppositely charged: One ion is positive (cation), the other is negative (anion).
Formation of Ionic Bonds: Group I and Group VII (LO3 & LO7)
Ionic bonds form between metallic elements (which lose electrons) and non-metallic elements (which gain electrons). While this often involves Group I metals and Group VII non-metals, it applies to any metal reacting with a non-metal.
General Rule for Group I and Group VII:
- Group I Metal (e.g., Sodium, Na): Has 1 outer electron. It loses this 1 electron to form a +1 ion (Na^+).
- Group VII Non-metal (e.g., Chlorine, Cl): Has 7 outer electrons. It gains 1 electron to form a -1 ion (Cl^-).
- The resulting oppositely charged ions attract strongly, forming an ionic bond.
Dot-and-Cross Diagram Rules:
- Draw the outer shells of both atoms.
- Show the electron transfer: The metal's outer electron moves to the non-metal.
- Metal Ion: Show the new outer shell (which is the previous inner shell, now full) with no dots/crosses from the original valence. Add the positive charge.
- Non-metal Ion: Show the new full outer shell containing its original electrons plus the gained electron. Add the negative charge.
- Crucial: Ensure both ions have a full outer shell (usually 8 electrons, or 2 for Hydrogen/Helium).
Dot-and-Cross Diagram: Sodium Chloride (NaCl)
Step 1: Identify Electrons
- Sodium (Na): Group I, Period 3. Electronic structure 2,8,1. Loses 1 electron.
- Chlorine (Cl): Group VII, Period 3. Electronic structure 2,8,7. Gains 1 electron.
Step 2: Draw the Ions
- Na⁺ Ion: Draw a circle for the outer shell. It should have 8 electrons (from the previous inner shell, now exposed). Use one symbol (e.g., crosses) to represent all electrons in this ion to distinguish it from the other atom. Write + outside.
- Cl⁻ Ion: Draw a circle for the outer shell. It should have 8 electrons. Use two symbols (e.g., dots and crosses) to show that 7 came from Cl and 1 came from Na. Write – outside.
Common Error: Do not draw the inner shells of Sodium (2 electrons) in the final diagram unless specifically asked for the full structure. Cambridge usually requires only the outer shell of the ion, which must be full.
Giant Lattice Structure (LO6)
Ionic compounds do not exist as single molecules. Instead, they form a giant lattice structure.
Description:
- A regular, repeating 3D arrangement of alternating positive and negative ions.
- Each ion is surrounded by ions of the opposite charge.
- The entire crystal is held together by strong electrostatic forces in all directions.
Properties of Ionic Compounds (LO4 & LO5)
Physical Properties to Describe:
- High Melting and Boiling Points: They are solids at room temperature.
- Electrical Conductivity:
- Conducts when molten (liquid) or aqueous (dissolved in water).
- Does NOT conduct when solid.
Explaining Properties via Structure and Bonding (LO8 & LO9)
Why High Melting/Boiling Points? (LO8)
- Ionic compounds have a giant lattice structure.
- There are strong electrostatic forces of attraction between the oppositely charged ions.
- To melt or boil the compound, you must break these strong bonds.
- Breaking many strong bonds requires a large amount of heat energy.
Why Conductivity Differences? (LO9)
- Conductivity requires charged particles that are free to move.
- In Solid State: The ions are fixed in position within the lattice. They cannot move, so electricity cannot flow.
- In Molten/Aqueous State: The lattice breaks down. The ions become free to move (delocalized). These mobile charged particles carry the electric current.
⚠︎ Dot-and-Cross Diagram Errors
Mistake: Drawing the outer shell of the metal ion with only the electrons it originally had.
Correction: The metal ion's new outer shell is the previous inner shell, which is now full. For Sodium (Na), the Na^+ ion has 8 electrons in its outer shell (from the original second shell), not 1.
Correction: The metal ion's new outer shell is the previous inner shell, which is now full. For Sodium (Na), the Na^+ ion has 8 electrons in its outer shell (from the original second shell), not 1.
Mistake: Forgetting to show the charge on the ions.
Correction: Always include the correct charge (+ or -) outside the bracket or circle for each ion.
Correction: Always include the correct charge (+ or -) outside the bracket or circle for each ion.
Describing vs. Explaining Properties
Context: When asked to describe properties (LO4/5), simply state the facts: 'High melting point' and 'Conducts when molten.' Do not add reasons.
Reasoning: Examiners award marks for specific keywords. Adding explanations when not asked can lead to irrelevant information or errors.
Example:
- Describe: 'Ionic compounds have high melting points.' (Correct)
- Explain: 'Ionic compounds have high melting points because strong electrostatic forces between ions require large amounts of energy to break.' (Correct for 'explain' questions)
Conductivity in Solid State
Context: When explaining why ionic compounds do not conduct electricity as solids.
Reasoning: You must mention that the ions are fixed or cannot move. Saying 'there are no electrons' is incorrect because ionic bonding involves ions, not free electrons like in metals.
Example Phrase: 'In the solid state, the ions are held in fixed positions in the lattice and cannot move to carry charge.'
Past Paper Style Questions
Q:
Describe how a sodium atom becomes a sodium ion. [1]
A:
It loses one electron.
Q:
State the type of bonding in magnesium oxide. [1]
A:
Ionic bonding.
Q:
Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does. [2]
A:
- In solid NaCl, ions are fixed in place / cannot move. (1)
- In molten NaCl, ions are free to move / mobile. (1)
Q:
Explain why magnesium oxide has a high melting point. [2]
A:
- Strong electrostatic forces between oppositely charged ions. (1)
- Large amount of energy/heat required to break these bonds. (1)
Q:
Complete the dot-and-cross diagram for the formation of potassium fluoride (KF). Show outer electrons only. [3]
A:
- K ion with full outer shell (8 e⁻) and + charge. (1)
- F ion with full outer shell (8 e⁻, showing mixed dots/crosses) and – charge. (1)
- Correct transfer of one electron from K to F shown. (1)