Why is Bi considered a metal, but As and Sb metalloids?

I’ve done quite a bit of home chemistry experimentation over the past year or so, but I got a knock on the door from some friendly FBI agents who wanted to know what I was doing with all the chemicals and glassware. Turns out Big Brother gets a little suspicious when people make repeated ebay chemical purchases. Luckily they didn’t press charges, but they did call the fire department, which forced me to get rid of everything interesting. Luckily my element collection and radioactives survived, though.

So it’s back to basics for me. I’ve been playing around with some element samples from my collection, and I have a really basic classification question. Arsenic, antimony, and bismuth all seem to behave roughly the same to me: they’re all soft, crumbly and extremely brittle, they all appear metallic, and all of them conduct electricity (poorly, but better than e.g. graphite, and far better than semiconductors). Electrical conductivity even decreases as you go down: As is a better conductor than Sb which is in turn better than Bi. Melting points are fairly low for all of them but lowest for bismuth.

thnx!

I didn’t find the right solution from the Internet.
References: http://www.chemicalforums.com/index.php?PHPSESSID=675ci5e30c25tre73i366tmod7&topic=90826.0

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