Covalent bonding (AQA GCSE Chemistry Combined Science): Revision Notes
Covalent bonding
What is covalent bonding?
Covalent bonding happens when non-metal atoms join together by sharing electrons. This is one of the three main types of strong chemical bonds you need to know about.
When atoms form a covalent bond, they share a pair of electrons between them. This sharing helps both atoms get a full outer shell of electrons, which makes them stable.
Unlike ionic bonding where electrons are transferred from one atom to another, covalent bonding involves the sharing of electrons between atoms. This sharing allows both atoms to achieve stability without completely losing or gaining electrons.
How covalent bonds form
Non-metal atoms have spaces in their outer electron shells. To become stable, they need to fill these spaces. Instead of giving away or taking electrons (like in ionic bonding), they share electrons with other atoms.
Each covalent bond involves two electrons - one from each atom that's bonding.
The key difference between covalent and ionic bonding is that covalent bonds involve sharing electrons, while ionic bonds involve transferring electrons from one atom to another.
Dot-and-cross diagrams
Scientists use special diagrams called dot-and-cross diagrams to show how electrons are shared in covalent bonds.
Understanding Dot-and-Cross Diagrams:
In these diagrams:
- Dots (•) show electrons from one atom
- Crosses (×) show electrons from the other atom
- Only outer shell electrons are shown
- The shared pair of electrons appears where the atoms overlap
These diagrams are essential tools for visualising electron sharing in covalent compounds.
Types of covalent bonds
Single bonds
When atoms share one pair of electrons, this makes a single covalent bond.
Examples: H-H in hydrogen gas, H-Cl in hydrogen chloride
Double bonds
When atoms share two pairs of electrons, this makes a double covalent bond.
Example: O=O in oxygen gas
Triple bonds
When atoms share three pairs of electrons, this makes a triple covalent bond.
Example: N≡N in nitrogen gas
Worked Example: Identifying Bond Types
Step 1: Count the shared electron pairs
- Single bond: 1 shared pair (represented as -)
- Double bond: 2 shared pairs (represented as =)
- Triple bond: 3 shared pairs (represented as ≡)
Step 2: Apply to molecules
- H₂: Each H needs 1 electron → share 1 pair → single bond
- O₂: Each O needs 2 electrons → share 2 pairs → double bond
- N₂: Each N needs 3 electrons → share 3 pairs → triple bond
Lone pairs
Not all electrons in the outer shell are used for bonding. Lone pairs are pairs of electrons that belong to one atom but are not shared in a bond.
Lone pairs are crucial because they affect the shape of molecules. Even though they don't participate in bonding, they take up space around the central atom and influence molecular geometry.
Common examples
Simple molecules
Worked Example: Common Covalent Molecules
- Hydrogen (H₂): Two hydrogen atoms share one pair of electrons
- Chlorine (Cl₂): Two chlorine atoms share one pair of electrons
- Water (H₂O): Oxygen shares electrons with two hydrogen atoms
- Methane (CH₄): Carbon shares electrons with four hydrogen atoms
Finding the formula
To work out molecular formulas, remember that atoms need full outer shells:
- Hydrogen needs 1 more electron
- Carbon needs 4 more electrons
- Nitrogen needs 3 more electrons
- Oxygen needs 2 more electrons
Formula Prediction Tip: The number of electrons an atom needs equals the number of covalent bonds it can form. This helps predict molecular formulas - for example, carbon can form 4 bonds, so in CH₄, it bonds with 4 hydrogen atoms.
Key properties of covalent compounds
Substances with covalent bonds often form small molecules. These have different properties compared to ionic compounds or metals.
Key Points to Remember:
- Covalent bonds form when non-metal atoms share pairs of electrons
- Dot-and-cross diagrams show outer shell electrons only
- Lone pairs are electron pairs not used in bonding
- Single bonds share 1 pair, double bonds share 2 pairs, triple bonds share 3 pairs
- Common examples include H₂, O₂, H₂O, CH₄, and NH₃