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Covalent molecules and their properties
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Covalent bonds hold atoms together inside molecules. Weak forces between molecules explain why simple molecular substances melt easily.
Covalent bonds
It is the strong electrostatic attraction between the shared pair of electrons and the nuclei of the two atoms.
In dot-and-cross diagrams, shared pairs sit where the shells overlap.
Molecules to know
Properties of molecular substances
Melting and boiling points generally increase with relative molecular mass, because larger molecules have stronger forces between them.
Giant covalent substances (diamond, graphite) have very high melting points because many strong covalent bonds must be broken.
C60 fullerene is a simple molecule (a ball of 60 carbon atoms), so it is much softer and melts or turns to gas at a much lower temperature than diamond or graphite.
Covalent compounds do not usually conduct electricity.
Why does water boil at a low temperature even though O-H covalent bonds are strong?
- Boiling separates molecules from each other.
- The forces between molecules are weak.
- The covalent bonds inside the molecules are not broken.
Answer: Only the weak forces between molecules are overcome, not the covalent bonds.
A covalent bond is a shared pair of electrons attracted to both nuclei. O2 has a double bond and N2 a triple bond. Simple molecules have weak forces between them, so low melting and boiling points that rise with M_r. Giant covalent structures have very high melting points.
The interactive lesson includes the diagrams for this topic.
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