Atom 1. The Clash of Titans (20 of 30)

Atom

Gold is solid, mercury is liquid.
Gold is inert, mercury is highly toxic.
And yet they differ by just one electron.
Gold has 79 and mercury has 80.
So how does one tiny electron make all that difference?
What Pauli did was pluck another quantum rule out of thin air.
Remember Bohr's multi-storey atom?
The nucleus is the ground floor with the electrons progressively filling the floors above.
Pauli said there's another quantum rule which states crudely that each floor can only accommodate a fixed number of electrons.

So if we want to add another electron to the atom, it has to check for a vacancy in the top floor.
And if that floor is full, another floor or shell is created above it for the electron.
In this way, a single electron can radically change the shape of the atom and this, in turn, affects how the atom behaves and how it fits together with other atoms.
So Pauli's principle really is the basis upon which the whole of chemistry, and ultimately biology, rests.
Pauli's Exclusion Principle was a major breakthrough for Bohr's quantum mechanics.
For the first time, it seemed to offer us a real understanding of the incredible variety in the world around us and possibly life itself.
Its success blew a large hole in Einstein's defence of the old physics.
And like quantum jumping, it was straight out of the weird rule book of atomic physics.
Pauli didn't explain why his principle worked.
He said it just did.
Einstein and the traditionalists hated it.
For them, this sounded like arrogant, unscientific nonsense.
But they needed to hit back, and hit back hard.
So far, the debates about the new atomic physics had been polite and gentlemanly.
Now the two sides wheeled out their biggest guns.