About the defination of metal.
Metal is something that have high electricity conductivity, heat conductivity, ductile, and so on. I know this. However, if there is a clear defination that WHAT is metal? Elements that can form metalic structure?
If there is no clear defination, then how did the scientists divide the periodic table into metal, semi-metal and non metal if thre is no a clear defination?

Scientists did this by
Scientists did this by making generalisations when comparing lots of different properties (like the properties you list above).
What else can you do? Nature is very complex and messy... scientists try to tidy it up the best they can :)
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So, are you meaning that
So, are you meaning that unlike those words 'acid' or 'alkali', the word 'metal' means a group of properties insteed of only one property like 'release H+ ions in water'?
More or less, yes! And some
More or less, yes!
And some metals are more metallic than others.
However, one thing which helps tie all these properties together is the underlying atomic properties: the way "metals" are all electropositive.
This tendancy to shed electrons is what gives rise to "metallic bonding" (which results in most of their characteristic shiny ductile refractive properties etc etc) and their chemical behaviour.
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