So when the structure of electrons or the
atom was established, we knew that electrons
were spinning around the outside. At least
that was the theory. There was some sort of
Newtonian planetary model where effectively
the protons – the positive charge on the
protons – held the electrons effectively
via a simple harmonic force: centripetal acceleration,
constantly holding them in the same way a
satellite would orbit round a planet.
However, this rather simplistic model, which
implied that electrons existed only as particles,
was fundamentally flawed by the question:
if positive charges attract negative charges
– and we know they do, opposites attract
– why don’t electrons crash into the nucleus?
So why is it, like for example we see satellites
orbit the Earth eventually slowing down and
having lower and lower orbits until, eventually,
losing their orbital stability and crashing
into the ocean? Why do you not see electrons
crashing into the proton-rich nucleus?
This was a problem. So let’s have a look.
We’ve got here in the case of 35Cl and 37Cl
the same number of electrons and the same
number of protons. We’ve got here though,
in the case of each of our isotopes, a different
number of neutrons: 18, 19 and 20.
So, as I talked about in the previous slide,
if we have a positively-charged nucleus, why
isn’t the negative charge of the electron
result in the electron itself moving towards
the nucleus itself? What stops the electrons
from crashing into the nucleus? And this is
the basis of quantum mechanics: the understanding
that energy exists in small discrete packets.
And it was the Danish physicist Niels Bohr
who came up with a solution to this issue
of electrons not actually ending up crashing
into the nucleus in the early 20th century.
And he put forward a couple of ideas.
The amount of energy which an electron has
determines how far away it is from the nucleus.
In other words, the more energy it has the
further away it is from the grasp of the positive
charge. He also suggested that electrons could
only be at certain specific distances from
the nucleus. And whereas my analogy earlier
of a satellite orbiting around a planet means
that it could technically exist at a number
of different altitudes away from the Earth,
in this particular case we’re saying that
quantised energy means that electrons can
only exist at certain discrete distances away
from the nucleus of an atom.
So to use an analogy here of electrons and
quantisation of energy, we’ll take the example
of a staircase which is shown on the board.
A person shown on the board moves from one
step to the other but they can only be on
one step at a time and the higher up the staircase
they are, the greater the amount of potential
energy they have with respect to the ground.
This, if you recall, is the very basic level
of understanding for potential energy = mass
× gravity × height.
Also you can only move one step at a time.
It is not possible to move more than one step
at a time. And that, as shown here, means
that your movement from 4 to 2 is forbidden
but your movement from 3 to 2 is allowed.
So the movement of a person up a single step
or down a single step is permitted but more
than that is not.