Metals and Non-metals · Lesson 2 of 13
Non-metals And Exceptions To Physical Properties
“The rules look simple until mercury, iodine, graphite and friends start breaking them.”
• Describe the general physical properties of non-metals. • Compare the physical behaviour of metals and non-metals. • Explain why physical properties alone cannot classify every element perfectly. • Describe important exceptions involving mercury, iodine, graphite, diamond and alkali metals. • Explain the meaning of allotrope using carbon as an example.
Non-metals often show physical behaviour very different from metals, but the important idea is that these are general trends rather than absolute rules. Several familiar elements break the expected pattern.
Non-metals
Examples include carbon, sulphur, iodine, oxygen and hydrogen. Non-metals may be solids or gases at room temperature, while bromine is a liquid. Solid non-metals are generally not malleable or ductile and usually do not conduct heat or electricity well.
| Feature | General Metal Trend | General Non-metal Trend |
|---|---|---|
| Surface | Usually lustrous when clean | Usually dull |
| Malleability | Generally malleable | Generally non-malleable |
| Ductility | Generally ductile | Generally non-ductile |
| Electrical conduction | Generally good | Generally poor |
| Sonority | Often sonorous | Usually non-sonorous |
Important Exceptions
Physical properties alone cannot classify every element perfectly because several exceptions exist.
Mercury
Mercury is a metal but is liquid at room temperature, unlike most metals.
Gallium And Caesium
Gallium and caesium are metals with unusually low melting points and can melt when warmed by the hand.
Iodine
Iodine is a non-metal but is lustrous. A shiny surface alone is therefore not enough to identify a metal.
Allotropes Of Carbon
An Allotrope is one of the different structural forms in which the same element can exist.
Diamond and graphite are allotropes of carbon. Diamond is extremely hard and has very high melting and boiling points. Graphite is a non-metal that conducts electricity.
Alkali Metals
Lithium, sodium and potassium are soft metals that can be cut with a knife. They also have low densities and low melting points compared with the usual metallic trend.
Do not identify an element as a metal or non-metal from only one physical observation. Iodine is lustrous, graphite conducts electricity and mercury is liquid, yet their classifications do not change.
Quiz
Which non-metal is liquid at room temperature?
Which non-metal conducts electricity?
Which statement about iodine is correct?
What are diamond and graphite?
Why can physical properties alone be unreliable for classification?
Practice Problems
- Compare the general physical properties of metals and non-metals.
- Explain why iodine should not be classified as a metal merely because it is lustrous.
- Explain why graphite is an important exception to the usual electrical behaviour of non-metals.
- Define allotrope and compare diamond with graphite.
- List the exceptions involving mercury, gallium, caesium and alkali metals and state which general trend each one breaks.
- Explain why several physical observations should be considered together rather than relying on one property.
Key Takeaways
• Non-metals generally show physical properties opposite to metals, but these are broad trends. • Bromine is a liquid non-metal, while mercury is a liquid metal. • Iodine is lustrous and graphite conducts electricity even though both are non-metals. • Diamond and graphite are allotropes of carbon with very different properties. • Chemical properties become especially important when physical exceptions make simple classification unreliable.