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PRESENTER: Now let's consider say the lithium atom.
It has three electrons.
And so say we start with the lowest energy state, n equal 1.
So that will be a 1s.
I'll just draw a line.
And I'll draw arrows to represent the electrons.
So we got that from helium, the two electrons that's allowed.
So where does the third electron go?
Well if you try to put it in the 1s orbital then
all four quantum numbers of-- two of those electrons
are going to be the same.
So I can't put them in the 1s orbital.
I'll have to go to the next orbital.
I have to go to n equal 2.
So I have 2s, or I have a 2p.
No there are three 2p orbitals, three separate ones
for m sub l values, Negative 1, 0, and 1.
So I could put the third electron say in the 2s orbital.
It could be pointed up or down, either way.
Now look at beryllium atom.
There are four electrons.
I have the 1s orbital.
I have 2s orbital and the 2p orbitals.
And so starting with lowest energy first
I'll put two electrons in 1s level.
And then I have two more electrons.
So I could put one in the 2s.
I could put one in the 2p.
But the lowest energy found by nature
is when I put a second electron in the 2s level.
Electrons like to be paired many times.
And so the beryllium atom would have
a pair of electrons in the 1s level and a pair in the 2s.
We can't always predict it.
But we can do experiments and then see the trends.
And nature picks what is most stable.
OK now we go to the boron atom with five electrons.
So we have our 1s, our 2s, and our 2p.
And so the first two electrons go in the 1s.
The next two electrons go in the 2s.
Now we have our fifth electron.
The only place for it to go is in one of the 2p orbitals.
They can go in either one.
Now nature's going to pick exactly
which orbital it wants it to go, either the minus 1, 0, or 1.
So there are a variety of ways.
But those energies are only separated by a small amount--
[INAUDIBLE] --purposes.
It just doesn't matter which one we stick it in.
Now we can have a shorthand way of writing these diagrams.
So what we'll do is we'll use a superscript
for the number of electrons.
That's the number of electrons in an orbital.
So this case we would have two electrons in the 1s orbital.
So it would be a way of representing this diagram.
This shorthand notation would be a way
of representing this diagram.
If we have 1s 2, 2s 2 that would be a way of representing two
electrons in the 1s orbital and two in a 2s orbital.
For our boron atom we would have this shorthand notation
for that diagram.
The shorthand notation of writing what
are called electron configurations of the atoms.
Electron configurations are important because they dictate
the physical properties and chemical
properties of the elements.