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CIE iGCSE Co-ordinated Sciences-C2.4 Ions and ionic bonds- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-C2.4 Ions and ionic bonds – Study Notes

CIE iGCSE Co-ordinated Sciences-C2.4 Ions and ionic bonds – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Formation of Ions

Atoms form ions by gaining or losing electrons to achieve a stable electronic configuration (a full outer shell, like the noble gases).

Cations (positive ions):
Formed when atoms lose electrons. They have more protons than electrons, giving them a positive charge.

Anions (negative ions):
Formed when atoms gain electrons. They have more electrons than protons, giving them a negative charge.

Example 

Explain how a sodium atom forms a sodium ion.

▶️Answer/Explanation

A sodium atom has the electronic configuration \( 2,8,1 \). It loses one electron from its outer shell to form a stable configuration \( 2,8 \).
Equation: \( \text{Na} \rightarrow \text{Na}^+ + e^- \)

Example 

Explain how a chlorine atom forms a chloride ion.

▶️Answer/Explanation

A chlorine atom has the electronic configuration \( 2,8,7 \). It gains one electron to achieve a stable configuration \( 2,8,8 \).
Equation: \( \text{Cl} + e^- \rightarrow \text{Cl}^- \)

Ionic Bond

An ionic bond is a strong electrostatic attraction between oppositely charged ions (positive cations and negative anions). It is formed when electrons are transferred from a metal atom (which forms cations) to a non-metal atom (which forms anions).

Example 

Define an ionic bond in your own words.

▶️Answer/Explanation

An ionic bond is the electrostatic force of attraction between a positively charged ion (cation) and a negatively charged ion (anion). For example, the bond between \( \text{Na}^+ \) and \( \text{Cl}^- \) in sodium chloride.

Example 

Explain why magnesium and oxygen form an ionic bond.

▶️Answer/Explanation

Magnesium (\( 2,8,2 \)) loses two electrons to form \( \text{Mg}^{2+} \).
Oxygen (\( 2,6 \)) gains two electrons to form \( \text{O}^{2-} \).
The strong electrostatic attraction between \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \) ions forms an ionic bond in magnesium oxide (\( \text{MgO} \)).

Formation of Ionic Bonds (Group I and Group VII)

When a Group I element (metal) reacts with a Group VII element (non-metal), electrons are transferred:

  • Group I metals (e.g. sodium, potassium) each have 1 electron in their outer shell. They lose this electron to form a \( +1 \) cation.
  • Group VII non-metals (e.g. chlorine, fluorine) each have 7 electrons in their outer shell. They gain 1 electron to form a \( -1 \) anion.
  • The resulting oppositely charged ions are held together by strong electrostatic attraction, forming an ionic bond.

Example 

Show the formation of sodium chloride using a dot-and-cross diagram.

▶️Answer/Explanation

Sodium (\( 2,8,1 \)) loses its 1 outer electron → \( \text{Na}^+ \)
Chlorine (\( 2,8,7 \)) gains this electron → \( \text{Cl}^- \)
Equation: \( \text{Na} + \text{Cl} \rightarrow \text{Na}^+ + \text{Cl}^- \)

Dot-and-cross diagram:

  • Sodium’s outer electron is shown as a cross (×).
  • Chlorine’s electrons are shown as dots (•).

\( \text{Na} \, (•×) \;\; \rightarrow \;\; [\text{Na}]^+ \;\; [\text{Cl} (•×)]^- \)

Example 

Explain the formation of potassium fluoride using a dot-and-cross diagram.

▶️Answer/Explanation

Potassium (\( 2,8,8,1 \)) loses 1 electron → \( \text{K}^+ \)
Fluorine (\( 2,7 \)) gains 1 electron → \( \text{F}^- \)
Equation: \( \text{K} + \text{F} \rightarrow \text{K}^+ + \text{F}^- \)

Dot-and-cross diagram:

  • Potassium’s outer electron is shown as a cross (×).
  • Fluorine’s electrons are shown as dots (•).

\( \text{K} \, (•×) \;\; \rightarrow \;\; [\text{K}]^+ \;\; [\text{F} (•×)]^- \)

Formation of Ionic Bonds (Metals and Non-Metals)

Ionic bonds are formed between metallic elements (which lose electrons) and non-metallic elements (which gain electrons). This electron transfer gives rise to positively charged cations (from metals) and negatively charged anions (from non-metals). The strong electrostatic attraction between these oppositely charged ions forms the ionic bond.

  • Metals have few electrons in their outer shell → they lose electrons to achieve noble gas configuration.
  • Non-metals have nearly full outer shells → they gain electrons to complete their octet.
  • The resulting ions arrange into a regular lattice structure in ionic compounds.

Example 

Describe the formation of magnesium oxide using a dot-and-cross diagram.

▶️Answer/Explanation

Magnesium (\( 2,8,2 \)) loses 2 electrons → \( \text{Mg}^{2+} \)
Oxygen (\( 2,6 \)) gains 2 electrons → \( \text{O}^{2-} \)
Equation: \( \text{Mg} + \tfrac{1}{2}\text{O}_2 \rightarrow \text{MgO} \)

Dot-and-cross diagram:

  • Magnesium’s 2 outer electrons are shown as crosses (×).
  • Oxygen’s 6 outer electrons are shown as dots (•), and it gains 2 crosses to complete its octet.

Result: \( [\text{Mg}]^{2+} \;\; [\text{O}(••••××)]^{2-} \)

Example 

Explain the formation of calcium chloride using a dot-and-cross diagram.

▶️Answer/Explanation

Calcium (\( 2,8,8,2 \)) loses 2 electrons → \( \text{Ca}^{2+} \)
Each chlorine atom (\( 2,8,7 \)) gains 1 electron → \( \text{Cl}^- \)
Two chloride ions are needed to balance the \( 2+ \) charge of calcium.
Equation: \( \text{Ca} + \text{Cl}_2 \rightarrow \text{CaCl}_2 \)

Dot-and-cross diagram:

  • Calcium’s 2 outer electrons shown as crosses (×).
  • Each chlorine atom gains one cross to complete its octet of 8 electrons.

Result: \( [\text{Ca}]^{2+} \;\; 2[\text{Cl}(•••••••×)]^- \)

Properties of Ionic Compounds (Structure and Bonding)

Ionic compounds are formed when metals transfer electrons to non-metals, producing positively charged cations and negatively charged anions. These ions arrange themselves in a giant lattice structure, held together by strong electrostatic forces of attraction (ionic bonds). The structure and bonding explain their characteristic properties.

(a) High melting points and boiling points:

  • The giant ionic lattice is held together by strong electrostatic attractions between all the positive and negative ions in the structure.
  • A large amount of energy is required to overcome these forces in order to melt or boil the compound, resulting in high melting and boiling points.

Example

Explain why sodium chloride has a high melting point.

▶️Answer/Explanation

Sodium chloride consists of \( \text{Na}^+ \) and \( \text{Cl}^- \) ions held together by strong electrostatic forces in a giant lattice. A large amount of energy is required to overcome these forces, giving NaCl a high melting point (~801°C).

Example

Explain why magnesium oxide has a very high melting point.

▶️Answer/Explanation

Magnesium oxide contains \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \) ions in a giant lattice. The electrostatic attraction between doubly charged ions is extremely strong, requiring a very large amount of energy to overcome. Therefore, MgO has a very high melting point (~2852°C).

(b) Electrical conductivity when aqueous or molten, poor when solid:

  • In solid ionic compounds, ions are fixed in the lattice and cannot move, so the solid does not conduct electricity.
  • When the compound is melted or dissolved in water, the lattice breaks down and the ions are free to move.
  • Moving ions carry charge, allowing the compound to conduct electricity.

Example

Explain why molten sodium chloride conducts electricity but solid sodium chloride does not.

▶️Answer/Explanation

In solid NaCl, the ions are fixed in the lattice and cannot move, so it does not conduct electricity. When melted, the ions are free to move, allowing the flow of electric current. Similarly, dissolved NaCl conducts electricity because ions are free in solution.

(c) Solubility in water:

  • Many ionic compounds are soluble in water because the polar water molecules can attract and separate the ions from the lattice.
  • The partial positive charge of the hydrogen in water molecules is attracted to negative ions, while the partial negative charge on oxygen atoms is attracted to positive ions.
  • This interaction weakens and breaks the ionic lattice, allowing the ions to disperse in solution.

Example

Explain why potassium chloride dissolves in water.

▶️Answer/Explanation

Water molecules are polar and attract the \( \text{K}^+ \) and \( \text{Cl}^- \) ions. This interaction overcomes the lattice forces and separates the ions, allowing KCl to dissolve in water.

Giant Lattice Structure of Ionic Compounds

Ionic compounds form a giant lattice, which is a regular, repeating three-dimensional arrangement of oppositely charged ions. In the lattice:

  • Each positive ion is surrounded by negative ions, and each negative ion is surrounded by positive ions.
  • The strong electrostatic attraction in all directions gives the compound its stability.
  • Sodium chloride (NaCl) is a typical example of a giant ionic lattice.

Example 

Describe the structure of sodium chloride in terms of its lattice.

▶️Answer/Explanation

Sodium chloride consists of \( \text{Na}^+ \) and \( \text{Cl}^- \) ions arranged in a cubic lattice.

Each \( \text{Na}^+ \) ion is surrounded by six \( \text{Cl}^- \) ions, and each \( \text{Cl}^- \) ion is surrounded by six \( \text{Na}^+ \) ions.

The strong electrostatic forces between the oppositely charged ions hold the lattice together, giving NaCl high melting and boiling points.

Example 

Explain why the giant lattice structure makes sodium chloride brittle.

▶️Answer/Explanation

When a force is applied to the lattice, ions of the same charge can be forced next to each other.

Like charges repel, causing layers of ions to shift and the lattice to break. This explains why NaCl crystals are brittle.

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