This is the present accepted form of the
structure of the atom,
in which, the electrons revolve around the
nucleus of the atom
in fixed orbits or shells.
So are these electrons distributed randomly?
That is, can there be any number of electrons
in any shell?
Well, no.
There’s a definite pattern, a definite
distribution of electrons
around the nucleus which is known as the
Electronic Configuration of the atom.
It's the definite arrangement of electrons
around the nucleus of the atom. 
To get the electronic configuration, it
follows
a certain set of rules. These rules were
given by two scientists,
Bohr and Bury, the two scientists. They gave
a set of rules
and these are known as Bohr and Bury Scheme.
So let’s see what the Scheme says.
The first rule of the scheme is- The
maximum number of electrons
in a particular shell is given by 2n squared.
And this ‘n’
stands for the number of shell, that is,
the first shell,
it has n is equal to 1.So, if you
substitute this in the formula we get
two
into one square, which is two.
The second shell
has n is equal to 2. So if you
substitute it in the formula,
we get 8, 18, 32 and so on.
So this means that the first shell can have maximum
2 electrons,
the second shell can have maximum 8
electrons,
the third shell can have maximum 18
electrons,
and so on.
This is a helium nucleus, it has 2
protons
And two neutrons. Now let’s add electrons
to this.
So as we add electrons, observe 
what happens.
So you see that the third electron that I added,
automatically moves to the second shell.
Even if I try to add it in the first shell,
you see that
it occupies or it takes position in the second
shell,
and not in the first shell. This is
because by ‘2n squared’ rule,
we know that the first shell can accommodate
a maximum of two electrons.
So the third electron cannot be 
accommodated in the first shell and so it
takes position in the second shell.
Similarly, if I add one more electron,
it takes its position in the second
shell and not in the first shell.
So if you remember the names that are given
to these shells.
So, the first shell is known as the K Shell.
It can have a maximum of two shells.
The second shell, known as the L Shell,
can have a maximum of eight electrons.
M Shell can have 18,
N, 32 and so on.
Let’s move to the second rule.
The second rule says that
‘Electrons are not accommodated in a given shell,
unless the inner shells are filled.
That is, the electrons are filled in a stepwise manner’.
Let’s see.
Here we have a nucleus. So as we add
electrons, the electrons first
occupy the first shell. That is,
the K Shell.
According to Bohr, the energy of the shells
is related to their size.
So the first shell is smallest in size.
So it has the lowest energy.
Now when this shell gets filled,
then the electrons occupy in the second shell.
So this shell, that is the L Shell,
has energy more than the first shell.
So when this shell gets filled, then the
electrons go to the next shell, that is the M Shell,
which has the highest energy
out of these three shells.
As, the size of the
shells increase, the energy of the shell also increase.
So we see that the electrons
are occupied in the shells
in a stepwise manner, that is,
first the K Shell gets filled,
then L, M, N and so on.
The third rule says that
‘The maximum number of electrons
that can be accommodated in the outermost orbit
is eight’.
So the outermost orbit cannot have more
than eight electrons.
So let’s revise the three rules.
So according to the first rule,
the electrons are occupied in the shells
following the rule 2n squared,
that is, the first shell can have a maximum of two
electrons,
second shell can have maximum eight
electrons and so on. 
The second rule says that the shells
are filled in a stepwise manner.
That is, the electrons first occupy 
the K Shell,
then L Shell and so on.
The third rule says that the maximum
number of electrons that can be
accommodated in the outermost shell
is eight. So the outermost shell cannot
have more than eight electrons.
Let's see. Let’s try to write the
electronic configurations
of a few atoms. So here we have Oxygen.
It has 8 electrons.
So let’s revise the rules.
So according to the first rule,
the electrons are occupied following the 
rule, 2n squared.
So the first shell of Oxygen, it can
accommodate 2 electrons.
Now Oxygen has 8 electrons.
So the first shell occupies 2 electrons.
Remaining 6 electrons,
go to the next shell. 
So this is for the K Shell,
this is for the L Shell
Now since it has only 8 electrons, so,
we leave the other factors.
The second rule says that the shells are filled in a
stepwise manner. So the way we write 
electronic configuration is, we first
write the number of electrons 
that are occupied in the first shell,
then we write a comma, then we write
the number of electrons
present in the next shell.
Since it has only two shells, that is, the 
eight electrons can be accommodated
in the two shells itself. So we enclose it
within brackets.
This is how we write the electronic configuration.
And the third rule says that the maximum
number of electrons in the outermost shell
cannot be more than eight.
So since we have only six electrons in the
outermost shell,
so it also obeys the third rule.
So this is the electronic configuration of Oxygen atom. 
So for Oxygen, the electronic configuration is (2, 6).
That is, the first shell or the K Shell,
has 2 electrons,
and the second shell, or the L Shell has 
6 electrons.
Similarly, if we have an atom, Neon atom.
It has 10 electrons.
So the first shell can have 2 electrons,
and the remaining 8 electrons are present
in the second shell, that is, the L Shell.
Now let's take another example, Sodium.
It has 11 electrons.
So from the first rule, we know that
the first shell can have
2 electrons. Now Sodium has
11 electrons.
So the second shell can have eight electrons.
Remaining 1 electron, it goes to the third shell.
So from rule two, we know that the electrons,
they occupy shells in a stepwise manner.
So how do we write it?
We write the electrons in the first shell, 
followed by the second shell and the
third shell.
And then we enclose it within brackets.
And we see again in this case, the number of
electrons in the outermost shell 
is not more than eight.
So it also obeys the third rule.
So the electronic configuration of Sodium
is (2, 8, 1) and it’s not (2, 9).
So we have to follow the 2n squared rule in this.
Similarly, in other atom, Argon atom.
This has 18 electrons.
It also obeys the 2n squared rule.
So the first shell has 2 electrons.
This is the K Shell.
L Shell has 8 electrons.
M Shell has 8 electrons. So the electronic configuration
for Argon is (2, 8, 8).
Let's take another example- Calcium atom.
It has 20 electrons.
Let’s see how it follows the rules.
Okay, so according to the first rule, it
should occupy
the electrons based on the 2n squared rule.
So, we have
Calcium, it has 20 electrons.
So the first shell can accommodate
2 electrons,
the second shell can accommodate 8 electrons,
and the third shell can accommodate 18 electrons.
We are left with 10 electrons, this means 
that the third shell can accommodate 10 electrons.
Now, the second rule says that the
shells are filled in a stepwise manner.
 
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So if we have to fill them, we get
2, 8 and 10.
But now when we see the third rule,
it says that the maximum number of 
electrons
that can be accommodated in the outermost
shell is 8.
If you see here, there are 10 electrons in the outermost shell.
This is wrong.
There cannot be more than 8 electrons
in the outermost shell.
The outermost shell can have only maximum
number of 8 electrons.
So how do we write
the electronic configuration
of Calcium?. We write
2, 8. Since this shell can have maximum 8 electrons,
we write 8. Remaining 2 electrons go to the last shell.
This is the electronic configuration of
Calcium.
Remember we do not write
(2, 8, 2, 8). Why? Because this is wrong.
When we are distributing these 10 electrons,
we do not give the third shell 2 electrons
and the last shell 8 electrons.
The shells are filled in a stepwise manner.
So when we are filling electrons,
we first have to give
8 electrons to the third shell,
The remaining number of electrons, 
they go to the last shell.
So in this case it has 10 electrons,
So the third shell gets 8 electrons,
Remaining 2 electrons go to the last shell.
So this is the electronic configuration of Calcium.
It obeys the third rule, the second rule and the first rule.
That is why this arrangement,
that is, (2, 8, 10) is wrong,
because it violates the third rule. 
So the electronic configuration of Calcium is
2, 8, 8, 2.
That is, the K Shell has 2 electrons, L 8,
M Shell 8 and N Shell 2.
So we never write (2, 8, 10) because any time
you see
the valence, the outermost shell having more
than 8 electrons, this is wrong.
So always keep in mind,
that the outermost shell
cannot have more than 8 electrons.
The Magnesium atom consists of 12 electrons.
What is the
electronic configuration of Magnesium?
So again, let’s recall the rules,
the Magnesium has 12 electrons.
By the 2n squared rule, the first shell
can have 2 electrons,
the second shell can have 8 electrons.
Remaining 2 electrons
go to the last shell.
The second rule says that
the shells are filled in a stepwise
manner.
So we first fill the K Shell,
then the L Shell,
then the M Shell which has 2 electrons.
This is the electronic configuration and
this does not violate the third rule,
which says that the maximum number of
electrons that can be accommodated
in the outermost shell is 8.
We see that the outermost shell has 2 electrons.
So it obeys all the rules.
So this is the Bohr and Bury Scheme
for arranging the electrons in the shells.
