We might say The Atomic Age started with the work of John Dalton.
Dalton proposed the Atomic Theory in 1803.
This picture of the world where everything is composed of atoms helped explain what had
been observed in chemical reactions.
For instance, different elements always combined to form chemical compounds in amounts that
were simple whole-number ratios.
Dalton proposed that each element had its own unique type of atom, with a certain characteristic
weight.
These atoms were very small, solid particles that were indivisible.
That was the model of the atom for almost a hundred years.
Since Dalton’s first conception, the model of the atom has evolved over time.
Each time new experimental observations were gathered that couldn’t be explained by the
atomic model of the day, the model had to be revised and refined.
For instance, the discovery of subatomic particles meant Dalton’s model, that said atoms were
indivisible...needed some work.
In 1897, J.J.
Thomson was the first to discover a subatomic particle, the electron, through his experiments
with cathode rays.
At this time, people weren’t sure if cathode rays were waves, or particles.
Thomson was using magnets and electrically charged plates to deflect cathode rays (and
thereby estimate their mass).
He showed that cathode rays must be made up of negatively charged particles that were
over a thousand times lighter than the smallest atom (that’s hydrogen).
Before this experimental result, it was thought that THE smallest particle WAS the hydrogen
atom.
To account for his observations, Thomson proposed the “plum pudding” model of the atom.
If you’ve never had plum pudding, that name may not make a lot of sense to you.
So imagine you have a dense chewy cake with little raisins all through it - maybe like
a raisin bagel, or a chocolate chip muffin, if you prefer.
The cake part is the bulk of the atom, and it’s positively charged, while the little
sweet bits of raisin, or chocolate would be the negatively charged electrons.
The Thomson model stood, for about a decade, until the experiments of Ernest Rutherford.
Rutherford and colleagues, in an experiment performed in 1909, found that a beam of alpha
particles (that’s a kind of positively-charged radiation), shot at a target of very thin
gold foil, mostly passed straight through to a detector behind the foil.
But occasionally the particle would be violently deflected back, as if it had hit something
massive.
As Rutherford said, “It was quite the most incredible event that has ever happened to
me in my life.
It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper
and it came back and hit you.
On consideration, I realized that this scattering backward must be the result of a single collision,
and when I made calculations I saw that it was impossible to get anything of that order
of magnitude unless you took a system in which the greater part of the mass of the atom was
concentrated in a minute nucleus.
It was then that I had the idea of an atom with a minute massive centre, carrying a charge.”
- Ernest Rutherford
The results implied that an atom must be made mostly of empty space, but with a small core
of positively charged material that contained most of the mass of the atom.
This was the nucleus.
Again, the atomic model had to be updated to take this new information into account.
There was a problem almost immediately seen with Rutherford’s model, however - if the
electrons are out in this empty space around the positively charged nucleus, what is keeping
them from just giving in to the electrostatic attraction towards the nucleus?
Why doesn’t the atom just collapse on itself, from all that attraction?
Niels Bohr, who did postdoctoral work in Rutherford’s lab, came up with an elegant solution to the
collapsing atom problem.
In a publication in 1913, Bohr suggested that the negatively charged electrons are found
in concentric circular orbits around the positively charged nucleus, much like how the planets
orbit the sun.
We sometimes refer to the Bohr model as the “planetary model.”
According to this model, the electrons are found at fixed energy levels, orbiting at
fixed distances from the nucleus.
The path closest to the nucleus has the lowest energy level, and the energy is generally
higher the farther the orbits are from the nucleus.
The farther a negatively charged electron is from the positively charged nucleus, the
less attraction the electron feels.
In the Bohr model, electrons are able to jump from one energy level to another, but they
are not found between levels.
Much like how you have to take one entire step up or down a ladder, but you’re never
found standing between rungs.
Thus an electron gains or loses a discrete package of energy each time it changes energy
levels.
We call this a “quantum” of energy, and that’s where we get the term “quantum
leap.”
The Bohr model explained a lot of observed chemical behavior, including why each element
has a certain number of electrons available for reactions, the electrons found in the
outermost orbits.
These “valence electrons” thus determine the chemical properties of an atom.
The Bohr model is indeed, elegant - it’s clear and pleasing, and yet - it, too, has
been supplanted by another model, the quantum mechanical model.
We will talk all about the quantum mechanical model of the atom in another video.