20分!!化學問題T_T~~~識既入

Let consider N2 , O2 , Cl2 , are they compounds or elements ? They are element of the same type combined to form a molecule . If you have 2 of the same element , you donot have a compound . Just like pure iron , there are many iron element combined to form the iron ore . Or diamond there are pure carbons to form the hardest crystal .

It's dangerous to put the answer as Brian did, by mentioning a few examples.
You probably can never list out all examples; and there's always possibility that there's a counter-example.

2012-09-22 21:12:15 補充：
So the best way to answer this (kind) of question(s) is to base on the definition.
Definition of "compound" is that "a substance formed by two or more elements having chemically combined".
When there's only ONE element, a compound cannot be formed.

2012-09-22 21:13:10 補充：
Without knowing the definition, you'll never be able to tell if an unknown element X can form a strange compound X2 or X3 or X54 or etc.

i think bryan's answer is ok