Hank: Hello, I'm Hank.

I assume that you are here

because you are interested in biology.

If you are, that makes sense,

because like any good 50 Cent song,

Biology is just about sex and not dying,

and everyone watching this
should be interested in sex

and not dying, being that you
are, I assume, a human being.

I'm gonna teach this biology
course a little differently

than most courses you've ever experienced.

For example, I'm not going
to spend the first class

talking about how I'm
going to teach the class.

I'm just going to start
teaching the class.

Starting right after this next cut.

First, I just wanted to say
if I'm going to fast for you,

the great thing about me
being a video and not a person

is that you can always go back and
listen to what I've said again.

I promise I will not mind.

You are encouraged to do this often.

A great professor of mine once told me

that in order to understand any topic,

you only really need to
understand a bit of the level

of complexity just below that topic.

The level of complexity just
below biology is chemistry,

or if you're a biochemist,

you would probably argue
that it's biochemistry,

so we need to know a little
bit more about chemistry,

and that is where we're gonna start.

(lively intro music)

I'm a collection of organic
compounds called Hank Green.

An organic compound is
more or less any chemical

that contains carbon,

and carbon is awesome.

Why? Lots of reasons.

I'm gonna give you three.

First, carbon is small.

It doesn't have that many
protons and neutrons.

Almost always 12, rarely
it has some extra neutrons

making it C-13 or C-14.

Because of that, carbon does
not take up a lot of space

and can form itself into elegant shapes.

It can form rings.

It can form double or even triple bonds.

It can form spirals and sheets

and all kinds of really awesome things

that bigger molecules
would never manage to do.

Basically, carbon is
like an olympic gymnast.

It can only do the remarkable
and beautiful things it can do

because it's petite.

Second, carbon is kind.

It's not like other elements that

desperately want to gain
or lose or share electrons

to get the exact number they want.

No, carbon knows what
it's like to be lonely,

so it's not all, "I can't
live without your electrons."

Needy, like chlorine or sodium is.

This is why chlorine
tears apart your insides

if you breathe it in gaseous form,

and why sodium metal, if
ingested, will explode.

Carbon, though, eh.

It wants more electrons, but
it's not going to kill for them.

It's easy to work with.

It makes and breaks bonds
like a 13-year-old mall rat,

but it doesn't ever really hold a grudge.

Third, carbon loves to bond

because it needs 4 extra electrons,

so it will bond with whoever
happens to be nearby.

Usually, it will bond
with 2 or 3 or 4 of them

at the same time.

Carbon can bond with lots
of different elements.

Hydrogen, oxygen, phosphorus, nitrogen,

and other atoms of carbon.

It can do this in infinite configurations,

allowing it to be the core element
of the complicated structures

that make living things like ourselves.

Because carbon is small,
kind, and loves to bond,

life is pretty much built around it.

Carbon is the foundation of biology.

So fundamental that
scientists have a hard time

even conceiving of life
that is not carbon-based.

Silicon, which is analogous
to carbon in many ways,

is often cited as a potential element

for alien life to be based on,

but it's bulkier, so it doesn't form

the same elegant shapes as carbon.

It's also not found in any gases,

meaning that life would have to be formed

by eating solid silicon,

whereas life here on
earth is only possible

because carbon is constantly
floating around in the air

in the form of carbon dioxide.

Carbon, on its own, is
an atom with 6 protons,

6 electrons, and 6 neutrons.

Atoms have electron shells,

and they need or want to
have these shells filled,

in order to be happy, fulfilled atoms.

The first electron shell
called the S-orbital

needs 2 electrons to be full.

Then there's the 2nd
S-orbital, which also needs 2,

carbon has this filled as well.

Then we have the first P-orbital,

which needs 6 to be full.

Carbon only has 2 left
over, so it wants 4 more.

Carbon forms a lot of bonds
that we call "covalent".

These are bonds where the
atoms actually share electrons,

so the simplest carbon
compound ever, methane,

is carbon sharing 4 electrons

with 4 hydrogen atoms.

Hydrogen only has 1 electron,

so it wants its first S-orbital full.

Carbon shares its 4 electrons
with those 4 hydrogens,

and those 4 hydrogens each
share 1 electron with carbon,

so everybody's happy.

This can all be represented
with what we call

Lewis dot structures.

Gilbert Lewis, also the guy
behind Lewis acids and bases,

was nominated for the Nobel Prize 35 times

and won none.

This is more nominations
than anyone else in history,

and roughly the same number
of wins as everyone else.

Lewis disliked this a great deal.

He may have been the most
influential chemist of his time.

He coined the term photon.

He revolutionized how we
think about acids and bases.

He produced the first the
first molecule of heavy water,

and he was the first
person to conceptualize

the covalent bond that we're
talking about right now.

But, he was extremely
difficult to work with.

He was forced to resign
from many important posts,

and was also passed up
for the Manhattan Project,

so while all of his colleagues
worked to save his country,

Lewis wrote a horrible novel.

Lewis died alone in his laboratory

while working on cyanide
compounds after having had lunch

with a younger, more charismatic colleague

who had won the Nobel prize

and worked on the Manhattan Project.

Many suspect that he killed
himself with the cyanide compounds

that he was working on,

but the medical examiner said heart attack

without really looking into it.

I told you all that because,

well, the little Lewis structure
that I'm about to show you

was created by a deeply troubled genius.

It's not some abstract scientific
thing that has always existed.

Someone, somewhere, thought it up,

and it was such a marvelously useful tool,

that we've been using it ever since.

In biology, most compounds can be shown

in Lewis structure form.

One of the rules of thumb
when making these diagrams

is that some elements tend
to react with each other

in such a way that each atom
ends up with 8 electrons

in its outermost shell.

That's called the octet rule,

because these atoms want to complete

their octets of electrons
to be happy and satisfied.

Oxygen has 6 electrons in its outer shell,

and needs 2, which is why we get H2O.

It can also bond with
carbon, which needs 4,

so 2 double bonds to 2
different oxygen atoms,

you end up with CO2,

that pesky global warming gas,

and also the stuff that plants
and, thus, all life are made of.

Nitrogen has 5 electrons
in its outer shell.

Here's how we count them.

There are four placeholders.

Each wants two atoms,

and like people getting on a bus,

they prefer to start out not
sitting next to each other.

I'm not kidding about this.

They really don't double
up until they have to.

We count it out.

1, 2, 3, 4, 5.

So, for maximum happiness,

nitrogen bonds with 3 hydrogens,

forming ammonia,

or with 2 hydrogens, sticking
off another group of atoms

which we call an amino group.

And if that amino group
is bonded to a carbon

that is bonded to a carboxylic acid group,

you have an amino acid.

Sometimes electrons are shared
equally within a covalent bond

like with O2.

That's called a non-polar covalent bond,

but often one of the
participants is more greedy.

In water, for example, the oxygen molecule

sucks the electorns in,
and they spend more time

around the oxygen than
around the hydrogens.

This creates a slight positive
charge around the hydrogens

and a slight negative
charge around the oxygen.

When something has a charge,
we say that it's polar.

It has a positive and negative pole.

This is a polar covalent bond.

Ionic bonds occur when
instead of sharing electrons,

atoms just donate or accept an electron

from another atom completely

and then live happily as
a charged atom or ion.

Atoms would, in general,
prefer to be neutral

but compared with having
the full electron shells

is not that big of a deal.

The most common ionic
compound in our daily lives?

that would be good old table
salt, NaCl, sodium chloride,

but don't be fooled by its deliciousness.

Sodium chloride, as I
previously mentioned,

is made of 2 very nasty elements.

Chlorine is a halogen,

or an element that only needs one proton

to fill its octet,

while sodium is an alkali metal,

an element that only has
one electron in its octet.

They will happily tear
apart any chemical compound

they come in contact with,

searching to satisfy the octet rule.

No better outcome could occur
than sodium meeting chlorine.

They immediately transfer electrons

so sodium doesn't have its extra

and the chlorine fills its octet.

They become Na+ and Cl-,

and are so charged that
they stick together,

and that stickiness is
what we call an ionic bond.

These chemical changes
are a big deal, remember?

Sodium and chlorine just
went from being deadly

to being delicious.

They're also hydrogen bonds,

which aren't really bonds, so much.

So, you remember water?

I hope you didn't forget about water.

Water is important.

Since water is stuck together
with a polar covalent bond,

the hydrogen bit of it is a
little bit positively-charged

and the oxygen is a
little negatively-charged.

When water molecules move around,

they actually stick together a little bit,

hydrogen side to oxygen side.

This kind of bonding happens
in all sorts of molecules,

particularly in proteins.

It plays an extremely important role

in how proteins fold up to do their jobs.

It's important to note here,

bonds, even when they're written
with dashes or solid lines,

or no lines at all, are
not the same strength.

Sometimes ionic bonds are
stronger than covalent bonds,

though that's the exception
rather than the rule,

and covalent bond strength varies hugely.

The way that those bonds
get made and broken

is intensely important to how
life and our lives operate.

Making and breaking bonds
is the key to life itself.

It's also like if you were
to swallow some sodium metal,

the key to death.

Keep all of this in mind as
you move forward in biology.

Even the hottest person you have ever met

is just a bunch of
chemicals rambling around

in a bag of water.

That, among many other things,

is what we're gonna talk about next time.