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Originally Posted by jfribrg
One question he needs to answer is where gold is in the periodic table and why.
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The facetious answer, of course, is that it has 79 protons, so it needs to go between numbers 78 and 80.
The real question you want answered, I think, is "why is the periodic table shaped the way it is?"
To expand upon what Swift said, "When you get to gold you have this ground state configuration [Xe].4f14.5d10.6s1," silver has the configuration [Kr].4d10.5s1, and copper is [Ar].3d10.4s1 . Because the outer electrons (called valence electrons) are similar, silver and copper will be very similar to gold in terms of physical properties. In fact, this is true for almost all columns of the periodic table: they have similar valence electrons, and it turns out that that means they have similar properties.
And that's the real reason why the periodic table is set up like that. Elements in the same column share the same electron configuration. Gold is under copper because they both have (n)d10 (n-2)s1.
Interestingly, the elements just to the left of gold, silver and copper have a s2d8 configuration: they fill the s orbital and leave a d orbital empty.
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We did a little research and found that gold is one of the metals where more than one electron shell is not complete.
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You are talking about how there are no electrons in the 6s orbital and the 5p orbitals (so the n=5 and n=6 shells are incomplete). That's true, but it's a bit misleading. Those p orbitals almost never come into play in most chemistry (sometimes, in biological systems, they are important). For transition metals, only the s and d orbitals are usually important (and f orbitals for lanthanides and actinides).
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The question is whether this is a characteristic of all the elements in the same column as gold (Copper and Silver), or is this characteristic of metals in general (in which case, how can Hydrogen under pressure be metallic), or of some subset.
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It is true of all metals except for those in the p block (last six columns) of the periodic table and Zn, Cd, and Hg. Lead, for instance, is only missing p electrons: all of its s and d orbitals are filled. Those in the first column have three not-completely-filled orbitals: the (n+1)s, (n+1)p, and nd orbitals. Lanthanides and actinides may also have at least three incompletely filled: f, p, and d (and sometimes s, too, making three incomplete shells).
You have to be careful using the word "metal" around astronomers, by the way. Some (many) of them regard any element that is not hydrogen as a metal, which is probably not what you mean at all! Hydrogen acting metallic is due in part to something really strange (called degeneracy pressure), but you can see that it's reasonable to assume that it might: it shares an electron configuration with the metals lithium, sodium, and potassium.