Edexcel IGCSE Chemistry · Spec 1.44-1.48
Covalent Bonding & Simple Molecules
How covalent bonds form and the properties of simple molecular substances.
Chemistry revision video
Covalent Bonding & Simple Molecules
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Explained
Sharing electrons, and what that gives you
Ionic bonding transfers electrons from one atom to another. Covalent bonding shares them, and it happens between non metal atoms, which would all need to gain electrons, so none of them has any to transfer.
A covalent bond is a shared pair of electrons. The bond exists because both nuclei are attracted to that shared pair, which holds the two atoms together.
Drawing dot and cross diagrams
Show the outer shell electrons only, using dots for one atom and crosses for the other so it is clear which came from where. Overlap the two outer shells and put the shared pair in the overlap.
Each atom needs to end up with a full outer shell counting the shared electrons: eight for most atoms, two for hydrogen.
Hydrogen gives one single bond, as in H2 and HCl. Oxygen forms two bonds, so water has two single bonds and oxygen gas has one double bond. Nitrogen forms three, so N2 has a triple bond. Carbon forms four, which is why methane has four.
Simple molecular substances
Substances such as water, methane, carbon dioxide and iodine exist as small separate molecules.
They have low melting and boiling points, so many are liquids or gases at room temperature. And they do not conduct electricity, because the molecules are neutral overall, so there are no free electrons and no ions to carry charge.
The point that decides the marks
Melting or boiling a simple molecular substance separates the molecules from each other. It does not break the covalent bonds inside them.
The forces between molecules, the intermolecular forces, are weak, which is why little energy is needed and the melting point is low. The covalent bonds within each molecule are strong and survive the process intact.
Longer molecules have more contact between them, so the intermolecular forces are stronger and the boiling point is higher. That is why the fractions in crude oil separate by chain length.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme explains why hexane boils at a higher temperature than butane in three points: hexane is a larger molecule with a longer chain, there are stronger intermolecular forces between the molecules, and so more energy is needed to overcome those forces.
Its note then withholds the second and third marks entirely if there is any mention of breaking covalent bonds. The examiner report on that paper records that most candidates did exactly that, and lost two of the three marks as a result.
The forces being overcome are between the molecules, not within them. Reaching for covalent bonds shows the wrong thing is being separated.
The same mark scheme is equally strict in the other direction. On a giant covalent structure it says any mention of intermolecular forces scores zero out of three, because there are no molecules in a giant structure to have forces between.
On a dot and cross diagram it wants the shared pairs between the correct atoms first, then the rest of the molecule correct, with the second mark depending on the first. And defining a covalent bond, it wants the attraction between a shared pair of electrons and two nuclei, specifying the plural: nucleus alone is not allowed, because a bond needs two.
Simple molecular or giant covalent
Both are held together by covalent bonds, and they behave completely differently.
A simple molecular substance has separate small molecules, so it melts easily. A giant covalent structure such as diamond is one continuous network, so melting it means breaking covalent bonds and its melting point is enormous.
Deciding which you are dealing with, before you write anything about forces, prevents the single most costly error in this part of the course.
Spec 1.44-1.48
What you need to know
- Explain how a covalent bond forms
- Draw dot-and-cross diagrams for simple molecules
- Describe the properties of simple molecular substances
Active recall
Quick check
Answer each question before opening the answer.
What is a covalent bond?
A shared pair of electrons between two non-metal atoms.
Why do simple molecular substances have low melting and boiling points?
They have weak intermolecular forces between molecules, which need little energy to overcome.
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