IGCSE Chemistry 0620 · Common mistakes
Common mistakes in IGCSE Chemistry bonding and structure
Updated · by the Fahmazing team
Bonding answers are marked on key words. Say ions when you mean ions, electrons when you mean electrons, and always name the force that holds the particles together.
Getting ion charges backwards
Electrons are negative. An atom that loses electrons becomes positive; one that gains electrons becomes negative.
What ion does a sodium atom form?
✗ Wrong: Na⁻, because it loses an electron
✓ Right: Na⁺, because it loses one electron
Free lesson: Ions and ionic bonds
Saying ionic compounds share electrons
Sharing is covalent bonding, between non-metals. In ionic bonding electrons are transferred from a metal to a non-metal.
Describe the bonding in sodium chloride.
✗ Wrong: Sodium and chlorine share electrons.
✓ Right: Sodium transfers one electron to chlorine, forming Na⁺ and Cl⁻ ions held together by strong electrostatic attraction between opposite charges.
Free lesson: Ions and ionic bonds
Leaving out the attraction
Forming the ions is only part of the answer. The final mark is usually for the attraction between opposite charges.
Why does magnesium oxide have a high melting point?
✗ Wrong: It is made of Mg²⁺ and O²⁻ ions.
✓ Right: It has a giant lattice of Mg²⁺ and O²⁻ ions with strong electrostatic attraction between them, which needs a lot of energy to overcome.
Free lesson: Ions and ionic bonds
Saying covalent bonds break when a simple molecule boils
Covalent bonds inside a molecule are strong and stay intact. Boiling only overcomes the weak intermolecular forces between molecules.
Why does chlorine have a low boiling point?
✗ Wrong: Because the covalent bonds are weak and break easily.
✓ Right: Because the intermolecular forces between Cl₂ molecules are weak, so little energy is needed to separate the molecules.
Free lesson: Simple molecules and covalent bonds
Counting electrons instead of shared pairs
A single covalent bond is one shared pair of electrons. A double bond is two pairs.
How many covalent bonds does a nitrogen atom form in N₂?
✗ Wrong: 6, one for each shared electron
✓ Right: 3. It shares three pairs, making a triple bond.
Free lesson: Simple molecules and covalent bonds
Saying graphite conducts with ions
Graphite conducts because each carbon atom has one delocalised electron that can move along the layers. Diamond uses all four outer electrons in bonds, so it does not conduct.
Why does graphite conduct electricity?
✗ Wrong: It contains ions that can move.
✓ Right: Each carbon uses three electrons in bonds; the fourth is delocalised and free to move.
Free lesson: Giant covalent structures
Giving metals negative ions
A metal is a lattice of positive ions in a sea of delocalised electrons. There are no negative ions. In a solid metal the ions stay put; only the electrons flow.
Describe metallic bonding.
✗ Wrong: Positive and negative ions attract each other.
✓ Right: Positive metal ions are held in a lattice by strong attraction to a sea of delocalised electrons.
Free lesson: Metallic bonding
Questions students ask
What key words do examiners want for ionic bonding?
Transfer of electrons, positive and negative ions, giant lattice, and strong electrostatic attraction between oppositely charged ions.
Are covalent bonds weak?
No. Covalent bonds are strong. Simple molecular substances have low melting points because the forces between molecules are weak.
Why do ionic compounds conduct only when molten or dissolved?
Their ions are fixed in the solid lattice. When the compound is melted or dissolved the ions are free to move and carry the charge.