[Rutherford]

Is Se a non-metal or a metalloid?
Is Po a metalloid or a metal?
I read in two differents books two different statements so I am a bit confused.

[fledarmus]

This isn't exactly a useful question. There is a continuum between metallic and non-metallic materials based on their properties, and the dividing point is sort of arbitrary. You are much better off knowing what the properties of metals and non-metals are, and how these elements fall in terms of those properties, than trying to remember a rather arbitrary distinction. It's sort of like the distinction between tall and short for people - do you really think that men that are 5'9" tall are short, while people that are 5'9-1/2" tall are tall?

Of course, if you need to know the answer for a test, you are asking in the wrong place. Find out from the people that administer the test what their definitions are, and use those to answer their questions.

[Rutherford]

I don't need it for a test. I was just wondering because of the two different statements in two different books.

[Arkcon]

That's the problem.  That zig-zag line at the border of non-metals and metals is, in some ways arbitrary.  So different textbooks will give different responses.  This is why fledarmus: mentions that you have to rely on your textbook class notes -- if this was a question on the exam, you would be marked wrong for failing to memorize the correct textbook response.

FWIW, I believe the consensus is that polonium is a metal, I don't think many people play with its compounds trying to understand the extent of its non-metalic character, because its radioactive.  Selenium is most likely considered a non-metal.  Yes, it may have some metallic characteristics, but really, more non-metallic.  The best example of a metalloid would be the element germanium, that one really shares properties of both.

I always remember when some old textbook, highlighting the zig-zag line, mentioned tin as a metal, but "grey tin", or "tin pest" was a distinctly non-metallic allotrope.  That one struck me as "Oh, come on.  How many 'buts' and 'ifs' and 'howevers' are they going to layer on this topic." http://en.wikipedia.org/wiki/Tin_pest

[Rutherford]

Ok, thanks.

[Arkcon]

Wow.  I never thought to look at Wikipedia for this topic, because as I recall, it was just a brief blub years ago.  But look at it now ... hundreds of references, to all the diversity of explanations:  http://en.wikipedia.org/wiki/Metaloid  The Wikipedia page really teaches you something ... and that is:  Don't be an obsessive geek, it gets annoying after a while.  Seriously, whole books written on this topic, all just to show one person's point of view.  Someday an official group will try to make a singular set of rules.  I suspect on that day, there will be blood spilled.

[PIQgoogleme]

Polonium is considered a metalloid by many. Selenium, however, rarely is. Relative to Polonium, Selenium has a smaller atomic radius due to it's valence shell being in a lower energy level. It thus has a tighter hold on these valence electrons and fewer metallic characteristics.