Compounds and Bonding
3. Ionic Bonding
Learning outcomes
- I can explain how ionic bonds form through the transfer of electrons.
- I can predict the ions formed by common metals and non-metals.
- I can use electron arrangements to explain ionic bond formation.
- I can draw dot-and-cross diagrams for simple ionic compounds.
- I can explain the properties of ionic compounds in terms of their structure and bonding.
Introduction
Many of the substances we use every day, such as table salt, ceramics, and some minerals, are made of ionic compounds. These compounds are formed when atoms of metals and non-metals combine through the transfer of electrons. This type of bonding is called ionic bonding.
Ionic bonding occurs because atoms tend to become more stable when their outer electron shell is full. Metals usually lose electrons, while non-metals usually gain electrons. The resulting positively and negatively charged ions attract one another, forming strong ionic bonds. Understanding ionic bonding helps explain the properties of many important materials and lays the foundation for studying chemical reactions.
What Is Ionic Bonding?
An ionic bond is the electrostatic force of attraction between positively charged ions and negatively charged ions.
Ionic bonding forms when:
- One or more electrons are transferred.
- A metal loses electrons.
- A non-metal gains those electrons.
This process produces oppositely charged ions that attract each other.
Figure 1. Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom.
Why Do Atoms Transfer Electrons?
Atoms become more stable when their outer electron shell is full.
Most atoms achieve this by obtaining a noble gas electron arrangement.
Metals
Metals usually have:
- 1, 2, or 3 valence electrons.
They tend to:
- Lose electrons.
- Form positive ions (cations).
Non-Metals
Non-metals usually have:
- 5, 6, or 7 valence electrons.
They tend to:
- Gain electrons.
- Form negative ions (anions).
Both atoms become more stable after the electron transfer.
Formation of Ions
When electrons are transferred, ions are formed.
Sodium
Electron arrangement:
2,8,1
Sodium loses one electron.
Na → Na⁺ + e⁻
New arrangement:
2,8
Chlorine
Electron arrangement:
2,8,7
Chlorine gains one electron.
Cl + e⁻ → Cl⁻
New arrangement:
2,8,8
The sodium ion and chloride ion are now attracted to each other.
Figure 2. Electron transfer gives both ions stable outer electron shells.
Predicting Common Ions
Many elements form predictable ions based on their group in the periodic table.
| Group | Common Ion |
|---|---|
| Group 1 | +1 |
| Group 2 | +2 |
| Group 13 | +3 |
| Group 15 | –3 |
| Group 16 | –2 |
| Group 17 | –1 |
| Group 18 | Usually do not form ions |
Examples:
| Element | Ion Formed |
|---|---|
| Sodium | Na⁺ |
| Magnesium | Mg²⁺ |
| Aluminium | Al³⁺ |
| Oxygen | O²⁻ |
| Chlorine | Cl⁻ |
| Nitrogen | N³⁻ |
These charges help determine the formulas of ionic compounds.
Using Electron Arrangements
Electron arrangements explain why atoms form ions.
Example:
Magnesium
Electron arrangement:
2,8,2
Loses two electrons.
Becomes:
Mg²⁺
Electron arrangement:
2,8
Oxygen
Electron arrangement:
2,6
Gains two electrons.
Becomes:
O²⁻
Electron arrangement:
2,8
Because both ions now have full outer shells, they are more stable.
Figure 3. Electron arrangements help explain why atoms lose or gain electrons to form ions.
Dot-and-Cross Diagrams
A dot-and-cross diagram shows:
- Which electrons belong to each atom.
- Which electrons are transferred.
- The completed outer electron shells.
Different symbols are used:
- Dots (•)
- Crosses (×)
This makes it easy to identify the transferred electrons.
Example:
Sodium chloride (NaCl)
- Sodium donates one electron.
- Chlorine accepts one electron.
- Both ions achieve full outer shells.
Dot-and-cross diagrams are also useful for compounds such as:
- Magnesium oxide (MgO)
- Calcium chloride (CaCl₂)
Figure 4. Dot-and-cross diagrams show electron transfer during ionic bond formation.
The Structure of Ionic Compounds
Ionic compounds do not exist as separate molecules.
Instead, the ions are arranged in a giant ionic lattice.
An ionic lattice is:
- A repeating three-dimensional arrangement of positive and negative ions.
- Held together by strong electrostatic forces acting in all directions.
Every positive ion is surrounded by negative ions, and every negative ion is surrounded by positive ions.
This arrangement makes ionic compounds very stable.
Figure 5. Ionic compounds form giant crystal lattices made of alternating positive and negative ions.
Properties of Ionic Compounds
The structure of an ionic lattice explains the properties of ionic compounds.
High Melting and Boiling Points
Strong electrostatic attractions require a large amount of energy to overcome.
As a result, ionic compounds usually have high melting and boiling points.
Hard but Brittle
Ionic compounds are:
- Hard
- Brittle
If layers of ions are forced to slide, ions with the same charge are brought next to each other.
They repel strongly, causing the crystal to crack.
Conduct Electricity When Molten or Dissolved
Solid ionic compounds do not conduct electricity because the ions are fixed in place.
When melted or dissolved in water:
- The ions become free to move.
- They can carry electric current.
Crystalline Solids
Most ionic compounds form:
- Regular crystals.
- Ordered lattice structures.
Examples of Ionic Compounds
| Compound | Formula. | Ions Present |
|---|---|---|
| Sodium chloride | NaCl | Na⁺ and Cl⁻ |
| Magnesium oxide | MgO | Mg²⁺ and O²⁻ |
| Calcium chloride | CaCl₂ | Ca²⁺ and 2Cl⁻ |
| Potassium bromide. | KBr | K⁺ and Br⁻ |
| Lithium fluoride | LiF | Li⁺ and F⁻ |
These compounds all consist of positive and negative ions arranged in giant ionic lattices.
Why Ionic Bonding Is Important
Ionic compounds are found throughout everyday life.
They are used in:
- Food (table salt)
- Fertilisers
- Medicines
- Batteries
- Ceramics
- Building materials
Many naturally occurring minerals are also ionic compounds.
Understanding ionic bonding helps explain the properties and uses of these materials.
Worked Example
Question
Explain how magnesium oxide (MgO) forms.
Solution
- Magnesium has the electron arrangement 2,8,2 and loses two electrons to form Mg²⁺.
- Oxygen has the electron arrangement 2,6 and gains two electrons to form O²⁻.
- Both ions now have full outer electron shells.
- The oppositely charged ions attract each other, forming a strong ionic bond.
- Magnesium oxide forms a giant ionic lattice.
Real-World Connection
Table salt (NaCl) is one of the most familiar ionic compounds. It is essential for many biological processes, including nerve function and muscle contraction. Ionic compounds are also used in rechargeable batteries, where the movement of ions allows electrical energy to be stored and released. In medicine, many intravenous fluids contain dissolved ionic compounds that help maintain the body's electrolyte balance.
Did You Know?
A single grain of table salt contains billions upon billions of sodium and chloride ions arranged in a perfectly repeating crystal lattice. There are no individual "NaCl molecules" in solid table salt—only one enormous three-dimensional network of alternating positive and negative ions extending throughout the crystal.
Key Terms
Anion – A negatively charged ion formed when an atom gains electrons.
Cation – A positively charged ion formed when an atom loses electrons.
Dot-and-cross diagram – A diagram that uses dots and crosses to show the transfer of electrons during bond formation.
Electron transfer – The movement of one or more electrons from one atom to another.
Electrostatic attraction – The force of attraction between oppositely charged particles.
Ion – A charged particle formed when an atom gains or loses electrons.
Ionic bond – The electrostatic attraction between oppositely charged ions.
Ionic compound – A compound made of positive and negative ions held together by ionic bonds.
Ionic lattice – A giant three-dimensional arrangement of alternating positive and negative ions.
Valence electron – An electron in the outermost shell of an atom that participates in chemical bonding.
Key Takeaways
- Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom.
- Metals form positive ions (cations) by losing electrons, while non-metals form negative ions (anions) by gaining electrons.
- Atoms transfer electrons to achieve full outer electron shells, making them more stable.
- Dot-and-cross diagrams show how electrons are transferred during ionic bond formation.
- Ionic compounds form giant ionic lattices rather than individual molecules.
- The strong electrostatic attractions within the lattice explain why ionic compounds have high melting points, are hard but brittle, and conduct electricity when molten or dissolved in water.