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Silicon-based life | Properties of carbon | Biology | Khan Academy

We’ve already spent a lot of time thinking about how awesome carbon is for life, that really life as we know is carbon based. And this all comes out of carbon’s ability to bond with other carbons and form all sorts of neat structures, bonds with oxygen, hydrogens, and other things. In fact, we have a whole
class of molecules called organic molecules that just are
molecules that involve carbon and carbons bonding capabilities really
come out of its electron structure. We talked about this in the previous video but if we think about carbon, it has six
protons and a neutral carbon will have six electrons. Two of them are sitting at its innermost
shell and then the other four are on its outermost shell, and those four are the ones
that tend to react and we call them the valence electrons. And so these
valence electrons tend to form four covalent bonds, for example if you wanted a bond with four hydrogens,
well each hydrogen would contribute each hydrogen would contribute an
electron to one of these pairs, which are essentially going to be a covalent bond and then the hydrogens, can feel that their outer shell is
complete because hydrogen is just trying to get to two to fill its outer shell to try to be a
little bit more like helium and carbon is trying to get to eight in its
outer shell to be a little bit more like neon. Neon has two in its inner shell and eight in its outer shell. You might remember
the octet rule that, that atoms try to get to eight or pretend like they have eight electrons in the outer shell to
feel stable. And here carbon is sharing eight
electrons. It contributed four and the four hydgrogens contributed four and it forms, it forms methane. Now those of you who are quite astute might have said: okay you know what, I know of
other elements that tend to form four covalent bonds, that
have four valence electrons. And in particular there’s one that’s
awfully close to carbon on the periodic table and that’s right
over here. This is, this is silicon. Silicon has 14
protons and a neutral silicon would have 14 electrons. So
let me draw that. So, if we have silicon right over here. I’m just gonna focus on the electrons. You would have two
electron sitting in its innermost, in its innermost shell, so they are
jumping here They’re not these neat, well-defined orbits. They are just jumping around in that, in that lowest energy in that, in
that, in the innermost shell. Then you have
another eight in its second shell. So, one… I’ll just draw them three, four, five, six, seven, eight, all jumping around all jumping around in the second shell. And it has a total of fourteen, so we already used up ten so it is going to have four in its
outermost shell. So, one, two, three, four and these four in its outermost
shell, these are the ones that tend to do the reacting. We call them valence
electrons. So if we want to just draw the valence
electrons, we could do silicon has four valence electrons, four has four. One, two, three, four valence electrons, just like carbon. It has a
different number of electrons. These four valence electrons are actually one shell further
out but it has four that tend to react. And it does tend to form four covalent bonds. So, you might be saying, well, can’t we have
life that is dependent, that a silicon-based as opposed to
carbon-based and if you thought that, you would not be
the first person to think about that. Science fiction or authors have have have theorized that,
and this right actually This is a, this is a screenshot from “The Devil In The Dark ” episode 1967, from
the original Star Trek, where they said maybe they encounter, Kirk encounters
that, the enterprise encounters these creatures that are silicon-based. They are called Hortas and they have all sorts of interesting properties. So, there’s
you know, there’s some folks who like to think maybe maybe silicon. But when you actually look at
the chemistry, things start to break down a little bit.
For example, one of the neat things about carbon is it readily forms bonds with
other carbons and that these bonds can actually be quite long. You can form all
sorts of these hydrocarbons and all sorts of other carbon-carbon structures, long chains
of carbon. So you can do that with carbon but it turns out that silicon-silicon
bonds are not that strong and silicon is not going to form, is not going to form really long chains.
So silicon is not going to do that. Even if you look at a simple, a
simple molecule like methane there is, there is a
molecule called silane but this does not readily form, it doesn’t have the
same characteristics. So this is also not going to be as good
as methane and even if you think about something
like carbon dioxide which is carbon which is carbon bonded, having two double
bonds so this is carbon having two double
bonds to oxygens so two, so one double bond to one oxygen, another
double bond to another oxygen And we’re used to thinking about carbon
dioxide in our everyday life as a gas. Plants are using these to fix into their structure, to
actually grow, they are taking the carbon out of it in their carbon fixation. We are, we exhale this carbon dioxide, it’s
essential to life and you might think well what about, what
about silicon oxide. And silicon dioxide, is actually a fairly
common molecule, fairly common compound but it
does not exist at a gaseous state at the temperatures that we are normally, that
we normally associate with life. Most of silicon oxide is
in the form of quartz in the ground. So that is quartz over there. So when you look at the
actual ways that silicon forms bonds it actually does not seem actually close
to as good as carbon. Something like silicon when silicon bonds with oxygen it’s a much —
not only is it in the solid form at normal temperatures where we normally associate life but
these are very, very, very strong bonds, much stronger than the bonds that carbon
is forming with oxygen. They’re so strong, they make it hard for
them to be manipulated with the types of chemical reactions that we’re used to seeing inside of
organisms. So, you know, it’s fun to theorize
about this and I’d be the last person to rule anything out. I think the
universe will continue to surprise us with things that
right now we cannot even imagine. But based on the chemistry we know and
based on life as we know it even though silicon can form these four
covalent bonds and does have four valence electrons, it’s still nowhere near as good as carbon in doing the types of processes
that we think are necessary for life.

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