So the next question becomes why would this
individual atoms interact? And I bet you can
see already. We've talked about the plus and
minus ions. Perhaps positive and negative are
attracted to each other. That's one of the types
of bonds we'll see. But another type of bond
involves purely the electrons. And the behavior
of the electrons on the atom. So it all has to do
with electron arrangement around the nucleus of
the atom. We have energy shells which we describe
as somewhat of a railroad track around the center
of the atom. Although that's not really the way
it works. And electrons can occupy in biology,
1 of 3 electron shells. Each of these electron
shells also has subshells or orbitals. The s
and p-orbitals, you may have heard of before.
We're going to look at those in a little bit more
detail. And there can only be in each of those orbitals
or subshells, there can only be a maximum of 2
electrons. So lets take a closer look at what we
mean by energy levels or energy shells. Here you
can see that in chemistry there are many energy
shells or energy levels. In biology luckily
we're only concerned with the first 3.
The first energy shell has only 1 orbital. It's
called the s-orbital. And the s-orbital has a spherical
shape. Only contains 2 electrons. And we denote
it with a spherical shape because the electrons
can essentially be anywhere in the cloud around
that nucleus. Now when we move out to the next shell
we see that there are p-orbitals. When we move
out to the second and third energy levels
we have to consider the octet rule. The octet rule
is just like it sounds. We have to have 8 electrons
in the second and third shells. So, 2 in the
first shell, 8 in the second and third.
We also have the s-orbital in the second shell.
But in addition to that we have 3p-orbitals.
Each of those can be filled with 2 electrons a
piece. So with 2 in the s and then 2 in each of
3p-orbitals, we could have a total of 8 electrons
in the second and third shell. Now thank goodness
in biology we only have to go to the third shell.
Otherwise things could become much more complicated.
But in general we just need to consider the first
shell that contains 2 electrons in 1s-orbital.
The second and third shells contain 1s-orbital
and 3p-orbitals. Each of which can only contain
2 electrons which bring us back to the octet rule.
So electrons can move between the energy levels.
And this is precisely what brings us to why atoms
are attracted to each other and how they might
want to steal electrons or share electrons. In
order to fill the octet rule, we might have to gain
or lose electrons. But even before gaining or
losing electrons, we have our different energy levels
as you can see depicted here in our figure.
And electrons can actually be pulled out away
from their original shell towards a higher level
shell. And it takes the energy input to put in that.
So you think about it as an excited state.
When kids get excited about going to a dance,
they jump around and there is lots of energy
around, right. So in order to have electrons
move up an energy level they need to have some
energy put in. The thought of going to the dance
perhaps could be this energy. So electrons can
move up their energy level and then once we release
the electron from that energy level back to
ground state there is an energy release.
That energy can then be utilized to fuel biological
processes. So again we can put energy in,
say from the sunlight to excite electrons and add
energy to a molecule. And we can also release energy
by moving electrons closer to the nucleus. And
utilize that energy for cellular processes.
as happens in cellular respiration which
we will explore later on in this course.
So electrons can become so excited that they
actually leave their original atom to join
another atom or one atom could have such a draw
for an electron that it steals it from the other.
So in this case we have A and B. When A loses
an electron and gives it to B, then B becomes
more negative. Because its becomes more negative
we call it reduced. So the process of gaining
an electron is called reduction. So, gaining
electron is reduction then the other molecule
is going to lose an electron. So in this case,
A is losing an electron and thus it becomes more
positive. When an atom becomes more positive, we
call it oxidised. So oxidation is the process
of losing an electron. Reduction is the case of
gaining an electron. And these two reactions
can be paired to be called redox reactions.
Generally when one thing loses an electron,
another thing gains an electron. So oxidation
and reduction or redox reactions are
something that we're going to run in to
frequently throughout this course.