The aufbau principle is introduced:
1) Pauli exclusion principle: No two electrons may have the same set
of quantum numbers (in an atom).
An atom with all the electrons in the lowest allowable energy level is in
the "ground state". To determine the lowest state, start with these
rules:
2) Given the other quantum numbers are identical, the lower n
is the lower the energy.
3) Given a choice of ms initially, +1/2 and -1/2
are equal.
4) Given the other quantum numbers are identical, the lower l
is the lower the energy.
5) Initially, before any opportunity to select ml
, all are of equal energy.
The quantum numbers to build up to boron are determined:
atom
|
electrons
|
n
|
l
|
ml
|
ms
|
H
|
1
|
1
|
0
|
0
|
+1/2*
|
He
|
2
|
1
|
0
|
0
|
-1/2
|
Li
|
3
|
2
|
0
|
0
|
+1/2*
|
Be
|
4
|
2
|
0
|
0
|
-1/2
|
B
|
5
|
2
|
1
|
1*
|
+1/2*
|
*arbitrary at this point.