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:
Core
- 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
(c) generally soluble in water
Supplement
- 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 - Describe the giant lattice structure of ionic compounds as a regular arrangement of alternating positive and negative ions, exemplified by sodium chloride
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Formation of Positive and Negative 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.
Positive Ions (Cations)
Cations are ions with a positive charge. They are formed when an atom loses one or more electrons. This usually occurs in metals because they have few electrons in their outer shell.
Key points:
- Metals in Group 1 lose 1 electron to form +1 cations.
- Metals in Group 2 lose 2 electrons to form +2 cations.
- Losing electrons leaves the atom with more protons than electrons, giving it an overall positive charge.
- The resulting ion has a stable electronic configuration, like the nearest noble gas.
Negative Ions (Anions)
Anions are ions with a negative charge. They are formed when an atom gains one or more electrons. This usually occurs in non-metals because they have nearly full outer shells.
Key points:
- Non-metals in Group 7 gain 1 electron to form -1 anions.
- Non-metals in Group 6 gain 2 electrons to form -2 anions.
- Gaining electrons gives the atom more electrons than protons, producing an overall negative charge.
- The resulting ion has a full outer shell, like the nearest noble gas.
Example
How does a sodium atom form a sodium ion?
▶️Answer/Explanation
A sodium atom has the electronic configuration 2,8,1. It loses 1 electron to achieve 2,8. The result is a sodium ion with a +1 charge.
\( \text{Na} \rightarrow \text{Na}^+ + e^- \)
Example
How does a chlorine atom form a chloride ion?
▶️Answer/Explanation
A chlorine atom has the electronic configuration 2,8,7. It gains 1 electron to achieve 2,8,8. The result is a chloride ion with a -1 charge.
\( \text{Cl} + e^- \rightarrow \text{Cl}^- \)
Ionic Bond
An ionic bond is a strong electrostatic attraction between oppositely charged ions. It forms when metals transfer electrons to non-metals, producing positive ions (cations) and negative ions (anions). The attraction between these ions holds them together in a fixed structure called an ionic lattice.
How ionic bonds form:
- Metals lose electrons to form positively charged ions (cations).
- Non-metals gain electrons to form negatively charged ions (anions).
- The oppositely charged ions are held together by strong electrostatic forces.
- This attraction is very strong and extends in all directions throughout the structure, forming a giant ionic lattice.
Key features of ionic bonding:
- Ionic bonds are very strong, requiring a lot of energy to break.
- The resulting compound is usually a crystalline solid with a regular arrangement of ions.
- Ionic compounds have high melting and boiling points due to the strong forces of attraction.
- They conduct electricity when molten or dissolved in water, because the ions are free to move and carry charge.
- In solid state, ionic compounds do not conduct electricity because the ions are fixed in place.
Example
Explain how an ionic bond forms in sodium chloride (\( \text{NaCl} \)).
▶️Answer/Explanation
A sodium atom has the electronic configuration 2,8,1. It loses its outer electron to form a sodium ion (\( \text{Na}^+ \)) with configuration 2,8.
A chlorine atom has the electronic configuration 2,8,7. It gains this electron to form a chloride ion (\( \text{Cl}^- \)) with configuration 2,8,8.
The oppositely charged ions (\( \text{Na}^+ \) and \( \text{Cl}^- \)) attract strongly to form an ionic bond. Millions of these ions are arranged in a giant 3D ionic lattice.
\( \text{Na} \rightarrow \text{Na}^+ + e^- \)
\( \text{Cl} + e^- \rightarrow \text{Cl}^- \)
Example
Why does magnesium oxide (\( \text{MgO} \)) have a higher melting point than sodium chloride (\( \text{NaCl} \))?
▶️Answer/Explanation
Magnesium loses 2 electrons to form \( \text{Mg}^{2+} \), and oxygen gains 2 electrons to form \( \text{O}^{2-} \). The electrostatic attraction between these ions is stronger than that in sodium chloride, which only involves +1 and -1 ions.
Therefore, more energy is required to break the bonds in \( \text{MgO} \), giving it a higher melting point than \( \text{NaCl} \).
Formation of Ionic Bonds between Group I and Group VII Elements
Group I elements (alkali metals) have 1 electron in their outer shell, while Group VII elements (halogens) have 7 electrons in their outer shell. Ionic bonds form between them because:
- The Group I metal atom loses its 1 outer electron to form a positive ion (cation).
- The Group VII non-metal atom gains this electron to form a negative ion (anion).
- The oppositely charged ions are held together by a strong electrostatic attraction, which is the ionic bond.
- This transfer of electrons can be represented using dot-and-cross diagrams, which clearly show the movement of electrons.
Steps in ionic bond formation:
- Draw the outermost electrons of each atom (dots for one atom, crosses for the other).
- Show the transfer of the electron from the Group I atom to the Group VII atom.
- Write the resulting ions with their charges and show that each has a full outer shell.
Example
Describe the formation of an ionic bond in sodium chloride using a dot-and-cross diagram.
▶️Answer/Explanation
A sodium atom (\( \text{Na} \)) has the electronic configuration 2,8,1. A chlorine atom (\( \text{Cl} \)) has the configuration 2,8,7.
Sodium loses 1 electron to form a sodium ion (\( \text{Na}^+ \)) with a full outer shell. Chlorine gains this electron to form a chloride ion (\( \text{Cl}^- \)) with a full outer shell.
The oppositely charged ions are held together by a strong electrostatic force of attraction, forming an ionic bond.
Equation: \( \text{Na} \rightarrow \text{Na}^+ + e^- \)
\( \text{Cl} + e^- \rightarrow \text{Cl}^- \)
Example
Explain how an ionic bond forms in potassium bromide using a dot-and-cross diagram.
▶️Answer/Explanation
Potassium (\( \text{K} \)) has the electronic configuration 2,8,8,1. Bromine (\( \text{Br} \)) has the configuration 2,8,18,7.
Potassium loses its 1 outer electron to form \( \text{K}^+ \). Bromine gains this electron to form \( \text{Br}^- \).
Both ions achieve a stable noble gas configuration. The strong electrostatic attraction between \( \text{K}^+ \) and \( \text{Br}^- \) forms the ionic bond.
Potassium (cross lost) → \( \text{K}^+ \)
Bromine (dots + 1 cross) → \( \text{Br}^- \)
Each ion has a full outer shell.
Equation: \( \text{K} \rightarrow \text{K}^+ + e^- \)
\( \text{Br} + e^- \rightarrow \text{Br}^- \)
Properties of Ionic Compounds
(a) High melting points and boiling points
Ionic compounds have high melting and boiling points because the ions are held together by strong electrostatic forces of attraction in a giant ionic lattice. A large amount of energy is required to break these bonds, so they remain solid at room temperature and only melt at high temperatures.
(b) Electrical conductivity
Ionic compounds conduct electricity only when molten (liquid) or dissolved in water (aqueous solution). In these states, the ions are free to move and carry charge. In the solid state, the ions are fixed in the lattice and cannot move, so they do not conduct electricity
(c) Solubility in water
Ionic compounds are generally soluble in water because water molecules are polar. The slightly positive hydrogen atoms in water attract negative ions, while the slightly negative oxygen atoms attract positive ions. This interaction pulls the ions apart, dissolving the compound.
Example
Why does magnesium oxide (\( \text{MgO} \)) have a higher melting point than sodium chloride (\( \text{NaCl} \))?
▶️Answer/Explanation
\( \text{MgO} \) has \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \) ions, which have stronger electrostatic attraction than the +1 and -1 ions in \( \text{NaCl} \). More energy is needed to overcome these stronger forces, giving \( \text{MgO} \) a higher melting point.
Example
Why does solid sodium chloride not conduct electricity, but molten sodium chloride does?
▶️Answer/Explanation
In solid sodium chloride, the ions are fixed in position and cannot move, so no current flows. When molten, the ions are free to move, allowing them to carry charge and conduct electricity.
Example
Why does sodium chloride dissolve in water, but not in kerosene?
▶️Answer/Explanation
Water is a polar solvent, so it can attract and separate the \( \text{Na}^+ \) and \( \text{Cl}^- \) ions, dissolving the salt. Kerosene is non-polar, so it cannot break the strong ionic bonds, and sodium chloride does not dissolve.
Formation of Ionic Bonds between Metallic and Non-Metallic Elements
Ionic bonds are formed when electrons are transferred from atoms of metallic elements to atoms of non-metallic elements. This transfer creates positively charged ions (cations) and negatively charged ions (anions), which are held together by strong electrostatic forces of attraction.
Steps in ionic bond formation:
- Metal atoms (on the left side of the Periodic Table) lose electrons to form positive ions (cations).
- Non-metal atoms (on the right side of the Periodic Table) gain electrons to form negative ions (anions).
- The resulting oppositely charged ions attract each other strongly, forming an ionic bond.
- This is usually represented with dot-and-cross diagrams, which clearly show how electrons are transferred.
Features of dot-and-cross diagrams:
- Electrons from the metal are shown using crosses (×).
- Electrons from the non-metal are shown using dots (•).
- The diagram shows the transfer of electrons and the resulting full outer shells of both ions.
- Charges are added to indicate the ions formed.
Example
Explain how an ionic bond forms between sodium and chlorine.
▶️Answer/Explanation
A sodium atom has the electronic configuration 2,8,1. A chlorine atom has the configuration 2,8,7.
Sodium loses its 1 outer electron to form a sodium ion (\( \text{Na}^+ \)), while chlorine gains this electron to form a chloride ion (\( \text{Cl}^- \)).
The oppositely charged ions attract each other strongly, forming an ionic bond. Millions of such bonds form a giant ionic lattice in sodium chloride.
• Sodium shows one outer electron (cross) which is transferred.
• Chlorine shows seven outer electrons (dots) plus one transferred cross, giving a full outer shell.
• Resulting ions: \( \text{Na}^+ \) and \( \text{Cl}^- \), both with full outer shells.
Equation: \( \text{Na} \rightarrow \text{Na}^+ + e^- \)
\( \text{Cl} + e^- \rightarrow \text{Cl}^- \)
Example
Explain how an ionic bond forms between magnesium and oxygen.
▶️Answer/Explanation
A magnesium atom has the electronic configuration 2,8,2. An oxygen atom has the configuration 2,6.
Magnesium loses 2 electrons to form a \( \text{Mg}^{2+} \) ion. Oxygen gains 2 electrons to form an \( \text{O}^{2-} \) ion.
The strong electrostatic attraction between \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \) forms the ionic bond. In solid magnesium oxide, these ions are arranged in a giant ionic lattice.
• Magnesium’s two outer crosses are transferred.
• Oxygen’s six outer dots plus two transferred crosses give a complete outer shell of 8 electrons.
• Resulting ions: \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \).
Equation: \( \text{Mg} \rightarrow \text{Mg}^{2+} + 2e^- \)
\( \text{O} + 2e^- \rightarrow \text{O}^{2-} \)
Properties of Ionic Compounds Explained by Structure and Bonding
(a) High melting points and boiling points
Ionic compounds have very high melting and boiling points because:
- They exist as giant ionic lattices with millions of ions arranged in a regular 3D structure.
- Each positive ion is surrounded by negative ions, and each negative ion is surrounded by positive ions.
- These ions are held together by very strong electrostatic forces of attraction (ionic bonds).
- A large amount of heat energy is needed to overcome these forces, which gives them high melting and boiling points.
- The strength of the ionic bonds increases with higher charges (e.g. MgO has a higher melting point than NaCl because Mg2+ and O2- attract more strongly).
Example
Why does magnesium oxide have a higher melting point than sodium chloride?
▶️Answer/Explanation
Magnesium oxide contains Mg2+ and O2- ions, which have higher charges than Na+ and Cl–. The stronger electrostatic attraction between these ions requires more energy to break, so MgO has a much higher melting point than NaCl.
(b) Electrical conductivity
The ability of ionic compounds to conduct electricity depends on the mobility of ions:
- In the solid state, ions are held in fixed positions in the lattice and cannot move, so solids do not conduct electricity.
- When molten, the lattice breaks down, and ions are free to move and carry charge, so molten compounds conduct electricity.
- When dissolved in water, the ionic lattice separates into individual ions (dissociation). These ions move freely in the solution and carry current, so aqueous ionic compounds conduct electricity.
- Therefore, ionic compounds are good conductors when molten or aqueous but poor conductors when solid.
Example
Why does sodium chloride not conduct electricity as a solid but does when dissolved in water?
▶️Answer/Explanation
In solid sodium chloride, the Na+ and Cl– ions are locked in a rigid lattice and cannot move, so no charge is carried. In aqueous solution, water molecules separate the ions, making them free to move and conduct electricity.
(c) Solubility in water
Ionic compounds are generally soluble in water because:
- Water is a polar solvent, with slightly positive hydrogen atoms and slightly negative oxygen atoms.
- When an ionic compound is added to water, water molecules surround the ions.
- The partial charges on water attract the oppositely charged ions (e.g. δ+ H attracts Cl–, δ– O attracts Na+).
- This breaks apart the ionic lattice, and the ions become surrounded by water molecules (hydrated ions).
- This process is called dissociation and explains why many ionic compounds dissolve in water.
Example
Why does sodium chloride dissolve in water?
▶️Answer/Explanation
The Na+ ions are attracted to the partially negative oxygen atoms of water molecules, and the Cl– ions are attracted to the partially positive hydrogen atoms. This attraction breaks the ionic lattice, and the ions disperse in water as hydrated ions, allowing NaCl to dissolve.
Giant Ionic Lattice Structure
Ionic compounds form a giant lattice structure, which is a three-dimensional, repeating arrangement of ions. In this lattice:
- Positive ions (cations) and negative ions (anions) are arranged alternately in a regular pattern.
- Each ion is surrounded by ions of opposite charge, maximizing electrostatic attraction and minimizing repulsion.
- This structure extends in all directions, forming a giant ionic lattice.
Example: Sodium Chloride (\( \text{NaCl} \))
- Sodium chloride consists of Na+ and Cl– ions.
- Each Na+ ion is surrounded by six Cl– ions, and each Cl– ion is surrounded by six Na+ ions.
- The ions are held together by strong electrostatic forces in all directions.
- This repeating pattern gives NaCl its cubic crystalline structure and contributes to its high melting point and hardness.
Example
Describe the lattice structure of sodium chloride and its significance.
▶️Answer/Explanation
Sodium chloride forms a cubic lattice where Na+ and Cl– ions alternate in a regular 3D arrangement. Each ion is surrounded by six ions of opposite charge.
This regular arrangement creates a strong, stable structure, giving NaCl a high melting point, brittleness, and the ability to conduct electricity when molten or dissolved in water.