Ions and ionic bonds- CIE iGCSE Chemistry Notes - New Syllabus
Ions and ionic bonds for iGCSE Chemistry Notes
Core Syllabus
- Describe the formation of positive ions, known as cations, and negative ions, known as anions
- State that an ionic bond is a strong electrostatic attraction between oppositely charged ions
- Describe the formation of ionic bonds between elements from Group I and Group VII, including the use of dot-and-cross diagrams
- Describe the properties of ionic compounds:
(a) high melting points and boiling points
(b) good electrical conductivity when aqueous or molten and poor when solid
Supplement Syllabus
- Describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions
- Describe the formation of ionic bonds between ions of metallic and non-metallic elements, including the use of dot-and-cross diagrams
- Explain in terms of structure and bonding the properties of ionic compounds:
(a) high melting points and boiling points
(b) good electrical conductivity when aqueous or molten and poor when solid
Formation of Ions
Formation of Ions
An ion is an atom or a group of atoms that has gained or lost electrons, resulting in a net electrical charge.
- Cations are positively charged ions formed when atoms lose electrons.
- Anions are negatively charged ions formed when atoms gain electrons.
This happens so that atoms can achieve a full outer shell of electrons (a stable noble gas configuration).
Formation of Cations (positive ions):
- Usually formed by metal atoms (Group I, II, etc.).
- Metals have few electrons in their outer shell and tend to lose them easily.
- Example: A sodium atom (Na) has the electronic configuration 2,8,1.
It loses 1 electron to form a Na+ ion with configuration 2,8.
Formation of Anions (negative ions):
- Usually formed by nonmetal atoms (Group VI, VII, etc.).
- Nonmetals have nearly full outer shells and tend to gain electrons.
- Example: A chlorine atom (Cl) has the electronic configuration 2,8,7.
It gains 1 electron to form a Cl− ion with configur>ation 2,8,8.
Ionic Bond
Ionic Bond
An ionic bond is a type of chemical bond that forms between two atoms when one atom loses electrons to become a positively charged ion (cation) and another atom gains those electrons to become a negatively charged ion (anion). When a metallic element reacts with a non-metallic element, an ionic bond is formed through the transfer of electrons from the metal atom to the non-metal atom.
This transfer of electrons results in the formation of oppositely charged ions, which are held together by a strong electrostatic force of attraction.
Key characteristics of ionic bonding:
- Occurs between a metal and a non-metal.
- The metal atom (from Groups I, II, etc.) have few electrons in their outer shell. They loses electrons to form a positive ion (cation).
- The non-metal atom gains electrons (from Groups VI, VII, etc.) have nearly full outer shells. They gain electrons to form negatively charged ions (anions).
- The resulting ions have full outer electron shells (noble gas configuration), making them more stable.
- The strong attraction between the oppositely charged ions is what constitutes the ionic bond.
Example
Explain how an ionic bond is formed between magnesium and chlorine.
▶️Answer/Explanation
Magnesium is a metal with the electronic configuration 2,8,2. It has two electrons in its outer shell and readily loses them to achieve a stable configuration, forming a Mg2+ ion.
Chlorine is a non-metal with the electronic configuration 2,8,7. It needs one more electron to complete its outer shell. It gains one electron to form a Cl– ion.
Since magnesium loses 2 electrons, it can transfer one electron to each of two chlorine atoms:
\( \text{Mg} + 2\text{Cl} \rightarrow \text{Mg}^{2+} + 2\text{Cl}^- \)
The Mg2+ and Cl– ions are held together by strong electrostatic forces, forming the ionic compound magnesium chloride (MgCl2).
Formation of Ionic Bonds Between Group I and Group VII Elements
Ionic bonding occurs when atoms transfer electrons to form oppositely charged ions that are held together by strong electrostatic forces. This commonly happens between:
- Metals in Group I (alkali metals) – which tend to lose one electron to form positive ions (cations)
- Non-metals in Group VII (halogens) – which tend to gain one electron to form negative ions (anions)
Group I metals have one electron in their outermost shell. By losing this electron, they achieve the stable electronic configuration of a noble gas (full outer shell).
Group VII non-metals have seven electrons in their outermost shell. By gaining one electron, they also achieve a stable noble gas configuration.
For example:
- Sodium (Na) has an electronic configuration of 2,8,1. It loses 1 electron to become Na+ with configuration 2,8.
- Chlorine (Cl) has an electronic configuration of 2,8,7. It gains 1 electron to become Cl– with configuration 2,8,8.
Once these ions are formed, the strong electrostatic attraction between the oppositely charged Na+ and Cl– holds them together in an ionic bond, forming the compound sodium chloride (NaCl).
Dot-and-Cross Diagram:
These diagrams are used to show how electrons are transferred from metal atoms to non-metal atoms during the formation of ionic bonds. Electrons from the metal are shown as one symbol (e.g. dots), and electrons from the non-metal as another (e.g. crosses).
Example – Formation of Magnesium Chloride (MgCl2):
- Magnesium (Mg) has 2 electrons in its outer shell → it loses 2 electrons → forms Mg²⁺
- Each chlorine (Cl) atom has 7 electrons in its outer shell → gains 1 electron → forms Cl⁻
- Two Cl atoms are needed to accept the 2 electrons lost by one Mg atom
→ 
→ 
→ 
This forms a stable ionic compound with the formula MgCl2.
Example
Describe how an ionic bond forms between sodium and chlorine. Also, draw the dot-and-cross diagram.
▶️Answer/Explanation
Sodium (Na) has the electronic configuration 2,8,1. It loses one electron to become a sodium ion:
\( \text{Na} \rightarrow \text{Na}^+ + e^- \)
Chlorine (Cl) has the electronic configuration 2,8,7. It gains one electron to become a chloride ion:
\( \text{Cl} + e^- \rightarrow \text{Cl}^- \)
The oppositely charged ions are attracted to each other and form an ionic bond.
Balanced symbol equation:
\( 2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl} \)
Dot-and-Cross Diagram Explanation:
- Use dots (•) for electrons from sodium, and crosses (×) for electrons from chlorine.
- Show the transfer of one electron from Na to Cl.
- Include square brackets around the ions to show charge.
→ 
Example
Draw a dot-and-cross diagram to show the formation of magnesium oxide (MgO).
▶️Answer/Explanation
Step 1 – Electronic configurations:
Magnesium (Mg): 2,8,2
Oxygen (O): 2,6
Magnesium loses 2 electrons to form Mg2+
Oxygen gains 2 electrons to form O2-
Dot-and-Cross Diagram:
(• = electron from Mg, × = electron from O)
Magnesium transfers 2 electrons (••) to the oxygen atom.
This results in a strong ionic bond between Mg2+ and O2-.
Lattice Structure of Ionic Compounds
Lattice Structure of Ionic Compounds
In ionic compounds, the ions are arranged in a giant ionic lattice structure. This is a highly ordered and regular three-dimensional arrangement of alternating positive and negative ions.
Key Features:
- Repeating Pattern: The structure extends in all directions, forming a continuous and repeating pattern of oppositely charged ions.
- Strong Electrostatic Forces: Each ion is held in place by strong electrostatic attractions between oppositely charged ions (ionic bonds). These forces are very strong and act in all directions around each ion.
- Rigid and Stable: Because of the strength of the ionic bonds and the way the ions are packed together, the lattice is very stable and forms hard crystalline solids.
- No Discrete Molecules: Unlike covalent compounds, ionic compounds do not consist of separate molecules but of a giant network of ions bonded together.
Diagrammatic Example:
In sodium chloride (NaCl), each Na⁺ ion is surrounded by 6 Cl⁻ ions, and each Cl⁻ ion is surrounded by 6 Na⁺ ions. This regular cubic arrangement is repeated throughout the entire crystal.
Properties Arising from Giant Ionic Lattices:
- High melting and boiling points
- Brittleness
- Electrical conductivity
This giant ionic structure is a key reason why ionic compounds have characteristic physical properties that differ significantly from covalent compounds.
Example
Explain how the giant lattice structure of calcium fluoride (CaF2) affects its melting point and electrical conductivity.
▶️Answer/Explanation
Calcium fluoride (CaF2) has a giant ionic lattice made of Ca²⁺ and F⁻ ions. The Ca²⁺ ions have a +2 charge and each is surrounded by F⁻ ions, while each F⁻ ion is surrounded by Ca²⁺ ions. The strong electrostatic forces between the multiply charged ions make the lattice very stable and require a lot of energy to break, giving CaF2 a high melting point (around 1418°C). In the solid state, the ions are fixed and cannot move, so it does not conduct electricity. However, in molten or aqueous state, the ions are free to move and the compound conducts electricity well.
Properties of Ionic Compounds
Properties of Ionic Compounds (Structure and Bonding)
The unique structure and bonding in ionic compounds explain their characteristic physical properties as follows:
- High Melting and Boiling Points:
- Ionic compounds have very strong electrostatic forces between the positively and negatively charged ions in their lattice.
- A large amount of energy is needed to overcome these forces in order to melt or boil the substance.
- Therefore, ionic compounds usually have high melting points and boiling points.
- Example: Sodium chloride (NaCl) melts at about 801°C.
- Electrical Conductivity:
- When solid: Ionic compounds do not conduct electricity because the ions are held in fixed positions within the lattice and cannot move.
- When molten or aqueous (dissolved in water): Ionic compounds do conduct electricity because the ions are free to move and carry charge.
- This movement of ions allows the ionic solution or molten compound to conduct an electric current.
- Example: Molten sodium chloride or aqueous NaCl solution can conduct electricity.
- Solubility in Water:
- Many ionic compounds are soluble in water.
- Water molecules are polar and can surround the individual ions in the lattice, weakening the electrostatic forces and pulling the ions apart into solution.
- Brittleness:
- Although ionic compounds are hard, they are brittle.
- When a force is applied that shifts the layers of ions, ions of the same charge may come close together, resulting in repulsion. This causes the crystal to shatter.
All these properties can be directly traced to the giant lattice structure and the nature of ionic bonding, which involves strong attractions between ions arranged in a regular and repeating pattern.
Example
Explain in terms of structure and bonding why magnesium oxide (MgO) has a higher melting point than sodium chloride (NaCl).
▶️Answer/Explanation
Both MgO and NaCl are ionic compounds with giant lattice structures. However, MgO contains Mg²⁺ and O²⁻ ions, while NaCl contains Na⁺ and Cl⁻ ions.
The electrostatic forces between Mg²⁺ and O²⁻ ions are stronger than those between Na⁺ and Cl⁻ ions because the ions in MgO have higher charges. Stronger forces mean more energy is required to break the bonds in MgO, resulting in a higher melting point compared to NaCl.
Example
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
▶️Answer/Explanation
In solid sodium chloride, the ions are locked in a rigid lattice and cannot move freely. Since the flow of electric current requires free-moving charged particles, solid NaCl cannot conduct electricity.
However, when sodium chloride is melted, the ionic lattice breaks apart, and the ions become free to move. These free ions can carry electric current, so molten sodium chloride can conduct electricity.