Exceptions
Physical properties alone cannot classify elements.
Is the rule always true?
Metals are shining, hard solids that conduct well. Non-metals are mostly the opposite.
Can we sort every element using physical properties alone?
What this lesson covers
The idea
Elements cannot be classified by physical properties alone because of exceptions: mercury is a liquid metal, gallium and caesium have very low melting points, alkali metals are soft, iodine is lustrous and graphite conducts electricity.
Is the rule always true?
Metals are shining, hard solids that conduct well. Non-metals are mostly the opposite.
Can we sort every element using physical properties alone?
- No, because there are exceptions
- Yes, without any problem
- Yes, if we use two properties
Five exceptions
Open every tab.
Why we need more than looks
Mercury is a liquid metal. Gallium and caesium melt easily. Alkali metals are soft. Iodine, a non-metal, is lustrous. Graphite conducts electricity. Because of such exceptions, physical properties alone cannot classify elements.
Elements cannot be classified by physical properties alone because of exceptions: mercury is a liquid metal, gallium and caesium have very low melting points, alkali metals are soft, iodine is lustrous and graphite conducts electricity.
Notes
Because of exceptions (liquid mercury, low-melting gallium and caesium, soft alkali metals, lustrous iodine, conducting graphite), physical properties alone cannot classify elements.
Check yourself
Which metal is a liquid at room temperature?
Which non-metal is lustrous?
Which of these is a soft metal that can be cut with a knife?
Non-metals are bad conductors of electricity, but one form of carbon is an exception. It is ...
- Mercury — correct. Yes!
- Sodium. Sodium is a solid; all metals except mercury exist as solids at room temperature.
- Gold. Gold is a solid; all metals except mercury exist as solids at room temperature.
- Sulphur. The book names iodine as the lustrous non-metal.
- Iodine — correct. Yes!
- Hydrogen. The book names iodine as the lustrous non-metal.
- Gallium. Gallium is a metal with a very low melting point; the soft metals the book says can be cut with a knife are lithium, sodium and potassium.
- Mercury. Mercury is a liquid metal; the soft metals the book names are lithium, sodium and potassium.
- Sodium — correct. Yes!
- graphite — correct. Yes!
- a bad conductor like the rest. The book says graphite conducts electricity, so it is an exception.
- bromine. Bromine is a liquid non-metal; the form of carbon that conducts is graphite.