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"...as my eyes grew accustomed to the light, details of the room
within emerged slowly from the mist, strange animals, statues, and gold -
everywhere the glint of gold. For the moment - an eternity it must have
seemed to the others standing by - I was struck dumb with amazement,
and when Lord Carnarvon, unable to stand the suspense any longer,
inquired anxiously, 'Can you see anything?' it was all I could do to get
out the words, 'Yes, wonderful things."
― Howard Carter, Tomb of Tutankhamen
King Tut
What British archaeologist Howard Carter was gazing upon when he uttered those now famous words was
the intact Tomb of Tutankhamen, 11th pharaoh of the 18th dynasty of ancient
Egypt. The tomb consisted of the mummified remains of a 17 year old boy-king surrounded
by priceless artifacts, most of which were made of gold.
In 1922 when this discovery was made, it quickly became a worldwide
sensation. People couldn't get enough of the boy pharaoh and his
magnificent treasure. How was it that the tomb had escaped grave robbers
for 33 centuries? Who was Tutankhamen? Why did he die so young? Of
course the one thing no one asked was "Why was the pharaoh surrounded by
gold?". Everyone understood that.
It's simply understood that gold was/is valuable. Of course someone
powerful like a Pharaoh would be surrounded by gold. But that raises an
interesting question. Why do we take for granted that gold is valuable?
Why has it always (as far as we can tell) been a symbol of wealth? Is it
because it's easy to work with? Is it because it shines and is rare? Is
it arbitrary?
I think ultimately the answer has several parts. It doesn't hurt that
it's relatively scarce on the Earth's surface. It doesn't tarnish or
corrode, so in an impermanent world, it lasts. It can be combined easily
(melt it). It's malleable, and so can be worked to produce jewelry
(humans have been adorning themselves with jewelry for 100,000 years). It's shiny (high luster). In short, it has a lot of properties humans find appealing.
So where do these properties come from? And why haven't metals with similar properties been embraced like gold?
Noble Metals
The electron configuration of Gold is [Xe]4f145d106s1. Let's ignore the [Xe] and concentrate on the last part. 4f14 means that there are 14 electrons in the f subshell of the 4th electron shell. 5d10 means there are 10 electrons in the d subshell of the 5th electron shell. Finally 6s1
means there is 1 electron in the s subshell of the 6th electron shell.
The subshells s,p,d, and f can respectively hold 2, 6, 10, and 14
electrons. So if there are 14 electrons in an f subshell, it is
considered "full". We'll come back to that in a minute, but first let's
look at that [Xe] in the front of the configuration and try to understand what it means.
Xe is the symbol for the noble gas Xenon. [Xe] is a way of truncating
the notation for an electron configuration. It basically means that the
orbitals are filled up completely to Xenon's electron configuration
(see on left). That symbol [Xe] represents the inner electrons and
everything after it (in the configuration) are the valence electrons.
In chemistry the inner electrons are far less important than the
outermost (valence) electrons. The six elements displayed on the left
have completely filled subshells and are known as the noble gases.
Noble gases don't react much at all except under extreme conditions.
This is because they are perfectly happy with their filled shells and
have nothing to gain through a reaction. You see, reactions happen
because an atom is trying to fill a valence subshell.
Think of it this way. Imagine you're spinning and wobbling and you
see someone else spinning and wobbling. You two reach out and clasp
hands and suddenly neither wobbles anymore! Now imagine you are not
wobbling and any time you reached out and clasped hands you started
wobbling. Obviously you wouldn't want to clasp hands but rather keep to
yourself (the goal here is to minimize wobbling). That's a huge
simplification of why the outermost shells of an atom want to be
filled*, but hopefully it gives you a little intuition. To put it in
more scientific terms, a filled valence subshell, even if it means the
atom has to share it's valence electrons with another atom, is a lower
net energy arrangement for the system of atoms then just standing around
with unfilled electron subshells next to each other. Since nature loves
to minimize the energy of a system (when it can), atoms with unfilled
valence subshells will react to fill them. Atoms with filled valence
subshells will not react.
Back To Gold's Configuration
So there is gold, [Xe]4f145d106s1,
filled f-subshell, filled d-subshell,...but wait! What about its
s-subshell? That's one short of being full! Why doesn't gold react with
other atoms to try to get rid of that electron so that it only has
filled valence subshells, like an Alkali metal
would? The answer is simple (and very complicated). Basically it is
harder to remove an 6s electron from gold than it is a 5d electron due
to screening effects. So all any other atom trying to react with gold
has to work with are the 5d electrons, and that subshell is full (and
thus wants to be left alone). As far as other atoms are concerned, gold
looks chemically like a noble gas!
What is screening, you may ask? I'll try to explain electron
screening in a basic way (avoiding the quantum rabbit hole). Just
picture an atom as a positive nucleus surrounded by shells (1,2,3,4) of
electrons that orbit it. These shells aren't really nice, well defined
circular shells, but more like average orbital distances for the
electrons which get really close to the nucleus and really far away
too.**
The shape of the orbits that the electrons follow depend on their
subshell type. For instance s-subshells get closer to the nucleus than
p-subshells. The closer an electron gets to the nucleus, the less
electrons there are in between that electron and the nucleus. Since
electrons are negatively charged, the more electrons between an electron
its nucleus, the less strongly charged the nucleus appears to that
electron (the negatively charged electrons in between are "screening"
the positive charge of the nucleus). Now without screening it's simple.
The closer the electron is, on average, to the nucleus, the harder it is
to remove the electron from the atom. So all other things being equal,
pulling an electron from the 6 shell should be easier than pulling an
electron from the 5 shell. HOWEVER, since 6s electrons get closer to the
nucleus than 5d electrons, they experience less screening and thus
"feel" the positive charge more, making them harder to remove even
though they are slightly farther (on average) from the nucleus. This
screening versus average radial distance competition for electron
binding to an atom is why a 6s electron in Gold is harder to remove than
a 5d electron. In fact, the larger the noble metal, the more powerful
the screening effect, the less reactive it is!
Silver has an electron configuration of [Kr]4d105s1
and is also a noble metal. Once again, atoms from the outside see only a
nice full 4d shell and move on thinking they've met a noble gas.
Copper, [Ar] 3d10 4s1, is another noble metal.
It's important to remember that all noble metals are not created
equal. Since electron screening is the key to making that single s
subshell electron difficult to remove, the larger the atom, the more
dramatic the screening effects (more electrons to screen the nucleus),
the more noble the noble metal. Gold has the most electrons and thus the
most screening effects on its valence electrons. It therefore is the
most resistant to reaction (among the noble metals).
The Value of Gold
The ancient Egyptians of course didn't know all this chemistry. They
simply noticed that over time gold tended not to react. Also gold tended
to be more scarce, not being able to bind with lighter elements to keep
it floating on the Earths crust, most of the gold (and precious metals
in general) were pulled down to the core by gravity while the Earth was
forming. In fact, there is a school of thought that believes that most
of the gold and other precious metals found on Earth's surface were brought here by meteorites after the crust cooled.
However it may be that Gold made its way onto the Earths surface,
once found, gold was prized by humans. For thousands of years it has
been coveted and fought over. Gold and Silver discoveries have shaped
the fate of nations, from ancient Athens to age of discovery Spain to the modern Unite States. This is not likely to change anytime soon. The value of gold is in its chemistry
Notes
*Filled subshells are spherically symmetric and have a total angular momentum of zero.
**If you're having trouble visualizing how something with an average
orbital distance further away from the nucleus can sometimes be closer
to the nucleus, picture the classical orbits of a comet and a planet. A
comet has a very elliptical orbit, so it gets very close to the Sun and
also very far from the Sun. A planet generally has a near circular
orbit and doesn't vary too far from it's average orbital distance. A
planet (Earth) can have an average orbital distance much closer to the
Sun than a comet (Halley's Comet) and yet be further away from the Sun
at times. Got it? Great! Now forget it, because Electron orbitals are
quantum mechanical.
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Re: Gold: The Noble Metal by Roger Pink
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