Chemical Bonding
Section: Chemistry | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
Why Do Atoms Bond?
- Atoms bond together to become more stable by achieving a full outer shell of electrons
- They can do this by either sharing or transferring electrons with other atoms
- The first shell can hold up to 2 electrons
- The second and third shells can hold up to 8 electrons
- Atoms bond by either sharing or transferring electrons
- Chemical bond: a force of attraction that holds atoms together in a compound
Ionic Bonding
- Ionic bonding occurs when electrons are transferred from one atom to another
- This creates oppositely charged ions that attract each other
- Ion: an atom that has gained or lost electrons and now has an electrical charge
- Ionic bond: the electrostatic force of attraction between oppositely charged ions
How Ionic Bonding Works
- Metal atoms lose electrons → become positive ions (cations)
- Non-metal atoms gain electrons → become negative ions (anions)
- Opposite charges attract → forms an ionic bond
- Like charges repel: an ionic bond can only form between two oppositely charged ions - two ions with the same charge (e.g. two negative iodide ions, I⁻) cannot form an ionic bond with each other, because they repel instead of attracting
Dot-and-Cross Diagrams for Ions
- A dot-and-cross diagram shows the outer shell electrons of each atom before and after electron transfer, using dots for the electrons of one atom and crosses for the electrons of the other
- Only the outer shell electrons are shown
- Square brackets are drawn around each ion, with the charge written outside the top right corner
Dot-and-cross diagram for the formation of sodium chloride (NaCl)
Example: Magnesium Oxide (MgO)
- Magnesium (Mg) has 2 electrons in its outer shell
- Magnesium loses these electrons → becomes Mg²⁺
- Oxygen (O) has 6 electrons in its outer shell
- Oxygen gains 2 electrons → becomes O²⁻
- Mg²⁺ and O²⁻ attract each other → forms MgO (magnesium oxide)
Dot-and-cross diagram for the formation of magnesium oxide (MgO)
| Atom Type | Action | Becomes | Example |
|---|---|---|---|
| Metal | Loses electrons | Positive ion (cation) | Mg → Mg²⁺ |
| Non-metal | Gains electrons | Negative ion (anion) | O → O²⁻ |
Properties of Ionic Compounds
- High melting and boiling points - strong forces between ions
- Conduct electricity when melted or dissolved in water - ions are free to move
- Do NOT conduct electricity when solid - ions are fixed in place
- Often soluble in water
- Form crystalline structures - regular arrangement of ions
Common Mistakes
- Ionic compounds only conduct electricity when their ions can move (molten or dissolved), NOT when solid
Covalent Bonding
- Covalent bonding occurs when electrons are shared between atoms (usually non-metals)
- Each shared pair of electrons forms a covalent bond
- Covalent bond: a bond formed when two atoms share a pair of electrons
What is a Molecule?
- Molecule: two or more atoms joined together by covalent bonds
- When atoms are joined together by covalent bonding, the particle that is formed is called a molecule
- Examples of molecules: H₂ (two hydrogen atoms), H₂O (water), CO₂ (carbon dioxide), NH₃ (ammonia)
How Covalent Bonding Works
- Two non-metal atoms share pairs of electrons
- The shared electrons count toward the outer shell of both atoms
- This allows both atoms to achieve a full outer shell
Dot-and-Cross Diagrams for Covalent Bonds
- A dot-and-cross diagram shows a covalent bond as a shared pair of electrons between two atoms, using dots for the electrons of one atom and crosses for the electrons of the other
- The shared pair is drawn in the overlap between the two atoms' outer shells
- At this level, only single covalent bonds (one shared pair) need to be shown
Dot-and-cross diagram for a hydrogen molecule (H₂), showing a single covalent bond
Example: Water (H₂O)
- Oxygen needs 2 more electrons to fill its outer shell
- Each hydrogen needs 1 electron
- Oxygen shares 1 electron with each hydrogen atom
- Result: H₂O molecule with 2 covalent bonds
Dot-and-cross diagram for a water molecule (H₂O)
Types of Covalent Bonds
| Bond Type | Electrons Shared | Example |
|---|---|---|
| Single Bond | 2 electrons (1 pair) | H₂, Cl₂, H₂O |
| Double Bond | 4 electrons (2 pairs) | O₂, CO₂ |
| Triple Bond | 6 electrons (3 pairs) | N₂ |
- At this level, dot-and-cross diagrams are limited to single bonds only
Worked Example: Counting Atoms in a Formula
Tapiwa is given the molecular formula for ethane, C₂H₆, and asked how many atoms are bonded together in total in one molecule.
- Step 1: read the subscript after each element symbol - this shows how many atoms of that element are present
- Step 2: C₂H₆ contains 2 carbon atoms and 6 hydrogen atoms
- Step 3: add the atoms of each element together: 2 + 6 = 8 atoms in total
Properties of Simple Covalent Molecules
- Low melting and boiling points - weak forces between molecules
- Do NOT conduct electricity - no charged particles that can move
- Can be gases, liquids, or soft solids at room temperature
Comparing Ionic and Covalent Bonding
- Understanding the differences between ionic and covalent bonding helps predict compound properties
| Property | Ionic Bonding | Covalent Bonding |
|---|---|---|
| Occurs between | Metal + Non-metal | Non-metal + Non-metal |
| Electron behaviour | Transfer (lose/gain) | Share |
| Particles formed | Ions | Molecules |
| Melting point | High | Low (for simple molecules) |
| Conducts electricity? | Yes (when molten/dissolved) | No |
- Metals + Non-metals = Ionic bonding; Non-metals + Non-metals = Covalent bonding
Interactive revision notes, videos and practice questions load below.