The greatest problem in understanding the atomic structure is: the shell structure (ring diagram) and the orbital structure (box diagram), which remain disconnected. The concepts of subshells and orbitals (and suborbitals) is not effectively presented either in the ring diagram or in the box diagram. Quantum numbers are treated as though they are extraneous to the ring diagram or even the box diagram. What has been presented here is a Primer on atomic shell structure.

1. Empowering Students’
Understanding of the
Atomic Shell Structure – Part 1
Shells, Subshells, and OrbitalsB
D
C D
D
Dr. Renuka Rajasekaran
Chemist and Chemistry Educator
PhD (Chemistry); PhD (Chemistry Education)
rekharajaseran@gmail.com
The greatest problem in understanding the atomic structure is: the shell structure (ring diagram) and
the orbital structure (box diagram), which remain disconnected. The concepts of subshells and
orbitals are not effectively integrated either in the ring diagram or in the box diagram. Quantum
numbers are treated as though they are extraneous to both diagrams. There is not enough emphasis
at the fundamental level, on the fact that the shell structure forms the basis for the periodic table.
Here is a simpler treatment of the atomic structure for the high school students and the early
chemistry courses in college. This Part 1 in the Series.
Progressive Chemistry Learning Series
Volume 1, 2006, pp. 1-7.

2. B
D
C D
D
Shell Names
7th-Q Shell
6th-P Shell
5th-O Shell
4th-N Shell
3rd-M Shell
2nd- L Shell
1st- K Shell
In 1913, Neils Bohr proposed the Discrete Orbits Model or
Planetary Model. According to this model, electrons revolve
around the nucleus in circular orbits, which lie at fixed distances
from the central nucleus. Bohr’s theory was improvised by other
scientists and the following is a summary of those
improvisations:
Shells are made up of subshells and subshells are made up of
orbitals. It is in the orbitals that electrons reside, There are four
kinds of subshells: S, P, D, and F. Subshells are imaginary zones
in the shells. It is in these zones that orbitals are present.
Orbitals are like apartments for the electrons. The S subshell is a
single room apartment and contains only one orbital called the s
orbital. The P subshell is a three-room apartment and contains a
set of three orbitals called the p orbital. Each p orbital is
therefore a set of three p orbitals; px, py, and pz. The D subshell is
a five-room apartment and contains a set of five d orbitals:
dz
2, dxy, dxz, dyz , and dx
2
-y
2. The F subshell is a seven-room
apartment and contains seven f orbitals.
Note that the S subshell is present in all shells, starting from the
first shell. The P subshell begins only in the second shell and it is
present in all the shells excepting the first shell. The D subshell
begins only in the third shell and ends in the sixth shell; that is,
the D Subshell is present in third, fourth, fifth, and sixth shells
and is not present in the 7th shell. The F subshell begins only in
the fourth shell. It ends in the fifth shell. That is, the F subshell is
present only in the fourth and the fifth shells.
See that the D subshell begins in the third
shell and ends in the sixth shell.
See that the F subshell
begins in the fourth shell
and ends in the fifth
shell.
A “P”
subshell is
like a
property
that has a
three-room
apartment
A “S” subshell
is like a property
that has a one-
room apartment
A “D” subshell is like
a property that has a
five-room apartment
A “F” subshell
is like a
property that
has a seven-
room apartment
Each room is called a suborbital; Each suborbital can
hold a maximum of TWO electrons. Each room has a
specific name. the s orbital has no suborbital.
p orbital: px, py, pz
d orbital: dxy, dyz, dxz, dx^2-Y^2, dz^2
Names of f orbitals are little more complex and
are not required for high school chemistry courses.
Note: Upper Case –
Lower Case Distinction: S
shell has s orbitals. P
shell has p orbitals. D
shell has d orbitals. F
shell has f orbitals. The
number before the lower
case letter represents the
number of the shell.
Nucleus
1s
ATOMIC STRUCTURE FOUNDATIONS: Ring Diagram of Shell Structure

3. ATOMIC STRUCTURE FOUNDATIONS: Ring Diagram of Shell Structure
A B
D
D
D
Q Shell – one S subshell and one
P subshell – maximum of 8
electrons only
P Shell one S subshell, one P
subshell, and one D subshell –
Maximum of 18 electrons
O Shell - one S subshell, one P
subshell, one D subshell, and
one F subshell – Maximum of 32
electrons
N Shell – one S subshell, one P
subshell, one D subshell, and
one F subshell – Maximum of 32
electrons
M Shell – one S subshell, one P
subshell, and one D subshell –
Maximum of 18 electrons
L Shell – one S subshell and one
P Subshell – Maximum of 8
electrons
K Shell – one S subshell –
Maximum of TWO electrons
When fully filled, a s orbital will hold two (1 x
2 = 2) electrons; a p orbital will hold six
electrons (3 x 2 = 6); a d orbital will have ten
electrons (5 x 2 = 10); and a f orbital will have
fourteen electrons.
See the Octet Configuration of the Valence
Shell, typical of a fully filled shell as in
Noble Gases. However, remember that
Helium will have only two electrons in its
valence shell because its valence shell is the
first shell with a maximum capacity of only
two electrons.
See that the d orbitals begin
in the third shell and ends in
the penultimate shell.
See that the f orbitals begins in the fourth shell
and ends in the fifth shell. The f orbitals in the 4th
shell are called 4f orbitals; the f orbitals in the 5th
shell are called 5f orbitals.
S subshells filled
with electrons in
their one single
orbital per shell
(I x 2 = 2
electrons).
P subshells filled with
electrons in their three
sub orbitals, namely px.
Py, and pz (3 x 2 = 6
electrons in all, two
per each in the set of
three suborbitals).
F subshells filled
with electrons in
their seven orbitals
(7 x2 =14 electrons
in all, two per each
in the set of seven
sub orbitals).
D subshells filled with
electrons in their five
orbitals (5 x 2 = 10
electrons in all, two
per each in the set of
five suborbitals).
Shell Names
7th-Q Shell
6th-P Shell
5th-O Shell
4th-N Shell
3rd-M Shell
2nd- L Shell
1st- K Shell
Nucleus
1s
2s
3s
4s
5s
6s
7s
2p3p4p5p6p
7p
3d
4d
5d
6d
4f
5f

4. ATOMIC STRUCTURE FOUNDATIONS: Ring Diagram of Shell Structure
A B
D
D
D
We have already seen that in each orbital can hold two
electrons. How two spinning electrons remain stable and
continue to spin and revolve was a great curiosity.
In order to explain the stability of spinning pairs of electrons in
the orbitals of the shells, Wolfgang Pauli proposed (in 1925)
what is called the Pauli Exclusion Principle, which is given below:
“No two electrons in the same orbital will have all the four
quantum numbers the same. They will differ in the spin quantum
number.”
The planetary model of the atom emphasizes that electrons are
spinning bodies; that is they spin on their own axis and also revolve
around the nucleus.
Shell Names
7th-Q Shell
6th-P Shell
5th-O Shell
4th-N Shell
3rd-M Shell
2nd- L Shell
1st- K Shell
Nucleus
Quantum Numbers are the particulars that describe an electron; in
other words Quantum Numbers are like the address of an
electron. There are four quantum numbers: The Principal
Quantum Number; The Azimuthal Quantum Number; The
Magnetic Quantum Number; and the Spin Quantum Number.
Table 1 provides basic details about quantum numbers.
A detailed discussion on Quantum Numbers is beyond the scope of high school chemistry curriculum.
Some details about the Quantum Numbers are provided in Table 2 as well. However, if you are a high
school student, it is enough if you know that the spin quantum number of electron can be either +1/2
or ─1/2. +1/2 represents the electron spin up (clockwise direction); ─1/2 represents the electron spin
down (counterclockwise direction). We thus understand from the Pauli Exclusion Principle that the two
electrons within the same orbital cannot spin in the same direction but only in the opposite direction)

5. ATOMIC STRUCTURE FOUNDATIONS: Ring Diagram of Shell Structure
B
D
D
D
How Quantum Numbers Work is illustrated
here in Table 2. As already stated, for high
school chemistry, quantum numbers may be
ignored.
Nucleus
1s
spin quantum number of
electron can be either
+1/2 or ─1/2. +1/2
represents the electron
spin up (clockwise
direction); ─1/2
represents the electron
spin down
(counterclockwise
direction).
Magnetic Quantum Number
indicates how many suborbitals
(rooms) are present in the
biggest orbital in that shell.
Azimuthal quantum
Number (ℓ) tells you how
many subshells are present
for a given n
Principal Quantum Number
(n) represents the Shell to
which an electron belongs
Principal Quantum Number
7-Q Shell
6-P Shell
5-O Shell
4-N Shell
3-M Shell
2- L Shell
1- K Shell
Table 2: Understanding Quantum Numbers

6. ATOMIC STRUCTURE FOUNDATIONS: Ring Diagram of Shell Structure
A B
D
D
D
Did you know that the Periodic Table is based
on the Shell Structure of the Atom?
Nucleus
1s

7. The shell structure constitutes the most
fundamental foundation for
understanding the atomic structure.
Also, atomic structure is a growing
concept and runs through curricula
from upper elementary through middle
school to high school, and extends
beyond into college studies and higher
education.
However, students develop very many
misconceptions and struggle through
learning difficulties as they pursue the
atomic structure content presented to
them by text books and other media,
including classroom teaching.
In fact, teachers themselves develop a
lot of misconceptions about atomic
structure and therefore struggle to
present the content effectively to their
students.
The greatest problem in understanding the atomic
structure is: the shell structure (ring diagram) and
the orbital structure (box diagram), which remain
disconnected. The concepts of subshells and
orbitals (and suborbitals) is not effectively
presented either in the ring diagram or in the box
diagram. Quantum numbers are treated as though
they are extraneous to the ring diagram or even
the box diagram.
What has been presented here is a
Primer on atomic shell structure.
Follow the Progressive Chemistry
Learning Series to access the other
parts.
Progressive Chemistry Learning Series, Volume 1, 2006, pp. 1─7.