2.2.5Bonds, Structure and Properties of Matter
Properties of Small Molecules
5 min read 6 parts Lesson 11 of 20 1 quiz
Section 1 of 6Simple molecular structures
Covalent molecules
- Covalent molecules Small molecules are made of a few non-metal atoms joined by strong covalent bonds,
- as in water (H2O), carbon dioxide (CO2), methane (CH4) and chlorine (Cl2).
Two kinds of force
- Two kinds of force Within each molecule the covalent bonds are strong.
- Between separate molecules there are only weak intermolecular forces.
What is actually overcome
- What is actually overcome When a simple molecular substance melts or boils, the intermolecular forces are overcome.
- The covalent bonds stay intact.
Section 2 of 6Properties and reasons
| Property | Observation | Explanation |
|---|---|---|
| Melting and boiling points | Low; many are gases or liquids at room temperature | Weak intermolecular forces need little energy to overcome |
| Electrical conductivity | Do not conduct, in any state | Molecules have no overall charge and there are no free ions or delocalised electrons |
| Solubility in water | Many do not dissolve well | The molecules are not charged, so water molecules do not pull them out |
| State at room temperature | Usually gas, liquid or soft solid | Because the forces between molecules are weak |
Section 3 of 6Size and boiling point
1Larger molecule
- Larger molecule A bigger molecule has more electrons and more atoms.
2Stronger intermolecular forces
- Stronger intermolecular forces The attraction between larger molecules is greater.
3More energy needed
- More energy needed More energy is needed to separate the molecules.
4Higher boiling point
- Higher boiling point As molecular size increases, melting and boiling points increase, so large molecules are more often liquids or solids.
Section 4 of 6Answering a typical question
1
Identify the substance as a simple molecule made of non-metal atoms.
2
Say that it has weak intermolecular forces between the molecules.
3
State that little energy is needed to overcome these forces, so the melting or boiling point is low.
4
State that it does not conduct because there are no charged particles free to move.
5
Do not refer to breaking covalent bonds.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Simple molecule | A small group of atoms held together by covalent bonds. |
| Covalent bond | A strong bond formed when two atoms share a pair of electrons. |
| Intermolecular force | A weak attraction between separate molecules. |
| Volatile | Easily turns into a gas because of low boiling point. |
| Conductor | A substance that allows electric charge to flow through it. |
Section 6 of 6Exam technique
1
Saying covalent bonds break when a simple molecular substance melts or boils. Only intermolecular forces are overcome.
2
Describing the covalent bonds as weak. They are strong; the forces between molecules are weak.
3
Explaining non-conduction by saying there are 'no electrons'. The real reason is no free charged particles.
4
Assuming all covalent substances have low melting points. Giant covalent structures such as diamond have very high ones.
5
Not mentioning that larger molecules have stronger intermolecular forces and so higher boiling points.