Edexcel IGCSE Chemistry · Spec 1.45-1.48
Covalent Bonds, Dot-and-Cross Diagrams and Boiling Points
Covers explain the electrostatic attraction in a covalent bond, Draw the required range of simple molecules and Link relative molecular mass to melting and boiling point.
Chemistry revision video
Covalent Bonds, Dot-and-Cross Diagrams and Boiling Points
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Explained
What holds a covalent bond together, and what decides a boiling point
A covalent bond is a shared pair of electrons between two atoms. That is the definition, but it is not the explanation, and questions here ask for the explanation.
The shared electrons are negatively charged. Both nuclei are positively charged. The bond exists because of the electrostatic attraction between that shared pair and both nuclei, pulling the two atoms together.
Note the plural. There are two nuclei, and the attraction to both is what holds the atoms together at all. An answer describing attraction to one nucleus has described something that would not be a bond.
Drawing the diagrams
Show only the outer shell electrons. Use dots for one atom's electrons and crosses for the other's, so it is clear where each came from, and put the shared pair in the overlap between the two shells.
Every atom must finish with a full outer shell when the shared electrons are counted: two for hydrogen, eight for the rest.
The range worth being able to draw: hydrogen, chlorine, hydrogen chloride, water, methane, ammonia, oxygen with its double bond, nitrogen with its triple bond, carbon dioxide with two double bonds, and ethane and ethene.
Molecular mass and boiling point
Simple molecular substances have low melting and boiling points, and the trend within a family is consistent: the larger the relative molecular mass, the higher the boiling point.
The reason is not the covalent bonds. Boiling separates whole molecules from each other and leaves every covalent bond intact.
What is overcome is the intermolecular forces between molecules. A larger molecule has more surface in contact with its neighbours, so those forces are stronger, so more energy is needed to separate them and the boiling point is higher.
That is why the fractions in crude oil separate by chain length, and why the halogens go from gas to liquid to solid down Group 7.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme accepts, for the bond itself, a strong attraction between shared pairs of electrons and nuclei.
On the energy needed to overcome forces, its notes contain a short instruction that costs a surprising number of marks: do not allow more energy.
More energy is a comparison without a reference point, and the question is asking why the value is high in absolute terms. A large amount of energy is what earns the mark. The same paper elsewhere underlines the point by writing not just heat beside a similar answer.
The same notes then say a mark is lost if intermolecular forces or ionic bonding are mentioned in a giant covalent context, and a companion question withholds marks in the opposite direction, where an answer about a simple molecular substance mentions breaking covalent bonds.
Between them those two rulings define the whole distinction. In a simple molecular substance you overcome intermolecular forces and the covalent bonds survive. In a giant covalent structure there are no molecules, so intermolecular forces cannot exist and the covalent bonds themselves must break. Deciding which structure you are describing, before writing anything about forces, is what keeps the two apart.
Why they do not conduct
A simple molecular substance has no free electrons and no ions, because all the outer electrons are held in bonds and the molecules are neutral overall. With no charged particle free to move, there is nothing to carry a current, whether solid, liquid or dissolved.
Spec 1.45-1.48
What you need to know
- Explain the electrostatic attraction in a covalent bond
- Draw the required range of simple molecules
- Link relative molecular mass to melting and boiling point
Active recall
Quick check
Answer each question before opening the answer.
What is a covalent bond?
A shared pair of electrons
Which bonds are broken when a simple molecular substance boils?
Intermolecular attractions are overcome; covalent bonds are not broken
What general trend occurs as Mr increases?
Melting and boiling points generally increase
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