2.6.

1 Subshells
The line spectrum shows that the lines represent transitions between
the principal shells are in fact split into finer lines.
This indicates the presence of subshells with different energy values
in each principle shell.
The number of subshells in a principal shell is the same as the
principal quantum number of the principal shell.
In any subshell, the shell represented:
f- fundamental
d- diffuse ENERGY VALUE
p- principal
s- sharp (lowest energy)

Principal Shell n No. of subshell Symbol n= 3

First 1 1 1s
n= 2
Second 2 2 2s, 2p

Third 3 3 3s, 3p, 3d

n= 1

Fourth 4 4 4s, 4p, 4d, 4f

principal shell
Fifth 5 5 5s, 5p, 5d, 5f

On the line spectra (in the presence of a magnetic field) shows that each subshell is
further made up of several orbitals where the electrons are placed.
 The number of orbitals depends on the type of subshell.

Subshel No. of Symbol

l orbitals

s 1 S
p 3 px , p y pz
,

d 5 d xy ,d yz , d xz , d x − y ,d z
2 2 2

f 7

The arrangement of the orbitals in order of increasing energy (in an empty

atom) is as follows:

1s

2s 3s

3s 3p 3d

4s 4p 4d

5s 5d

6s

[1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s and so on]
 2.7 Atomic Orbitals
An orbital
to the region (or volume) in space around the nucleus
where the probability of finding a particular electron is high.

An orbit

the circular path of an electron around the nucleus

1. The s orbitals is spherical (like a round ball) with its nucleus in the centre.
The radii of the s orbitals increases with the principal quantum numbers.
2. The p orbital has a ‘dumb bell’ shape. The 3 p orbitals are arranged along
the x, y and z axes in space.

3. The shape of the five d orbitals are shown below.

 PRACTICE TO DRAW THE ORBITALS

2.8 Filling of Electrons

Pauli Exclusion
Principle
The Aufbau
Principle Hund's Rule

3 Rules
Arrangement
of Electrons
The A ufbau Principle

States that electrons m ust occupy

available orbitals of low est
energy first before they fill orbitals of higher
energy
The arrangem ent of the orbitals in order of
increasing energy. Each orbital is
represented by a square box.
The three boxes representing the three p
orbitals are joined, indicating that they have
the sam e energy, i.e. they are degenerate
orbitals.

Pauli Exclusion Principle

a) States that each orbital (i.e. each box) can be occupied by 2

electrons of opposite spins only.
b) The spins of the electron are represented by | and |
c) Hence the total number of electrons in an orbital or set of
orbitals is as follows.

Type of orbital Maximum no. of electrons

s 2x1=2
p 2x3=6
 d 2 x 5 = 10
f 2 x 7= 14

d) The total number of electrons that can occupy a principal shell is 2n2.

Principal Shell n Maximum no. of electrons

First 1 2
Second 2 8
Third 3 18
Fourth 4 32
Fifth 5 50

Hund's Rule

 In a given set of orbitals of equivalent energy

degenerate orbitals
(e.g the 3 p orbitals or the 5 d orbitals), elecrons tend
to occupy the orbitals singularly (with parallel spins)
first pairing up.

For example: the 2 electrons in a p orbitals is

and not
2.9 Electronic Configuration of Atoms

1. Hydrogen (Z= 1) has only one electron. Therefore , it will occupy the
lowest energy level/orbital available, i.e. the 1s orbital.

This is written as 1s1 (or 1).

2. The electronic configuration of helium (Z= 2) is

This is written as 1s2 (or 2).

3. The lithium atom (Z= 3) has three electrons. The first two go into the
1s orbital, while the third e- fills the next available empty orbital of 2s

This is written as 1s2 2s1 (or 2.1)

4. Beryllium (Z= 4) has the electronic configuration of

This is written as 1s2 2s2 (or 2.2)

5. Boron (Z= 5) has five e- . The first four go into the 1s and 2s orbitals
while the fifth electron occupies one of the empty 2p orbitals.
 This is written as 1s2 2s2 2p1 (or 2.3)

6. Carbon (Z= 6) has six e-. The 6th e- does not occupy the same orbital
as the fifth electron, but insteads occupies one of the two remaining
empty 2p orbitals (Hund’s Rule).
and not

this is written as 1s2 2s2 2p2 (or 2.4).

7. Nitrogen (Z= 7) has the electronic configuration of

This is written as 1s2 2s2 2p3 (or 2.5)

8. Oxygen (Z= 8) has eight e-. The eight e- will have to pair up one of
the unpaired electrons in one of the half-filled 2p orbitals.
This is written as 1s2 2s2 2p4 (or 2.6)

9. Repeating the process, the electronic configuration of argon (Z= 18)

This is written as 1s2 2s2 2p6 3s2 3p6 (or 2.8.8).

10. Potassium (Z= 19) has nineteen e-. The 19 e- will occupy the 4s
orbital and not 3d orbital bc the 4s is of lower energy than that of the
3d (Aufbau Principle). The electronic configuration potassium:
 This is written as 1s2 2s2 2p6 3s2 3p6 4s1 (or 2.8.8.1).
Element Electrons Electronic Configuration
Hydrogen (H) 1 1s1
Helium (He) 2 1s2
Lithium (Li) 3 1s2 2s1
Beryllium (Be) 4 1s2 2s2
Boron (B) 5 1s2 2s2 2p1
Carbon (C) 6 1s2 2s2 2p2
Nitrogen (N) 7 1s2 2s2 2p3
Oxygen (O) 8 1s2 2s2 2p4
Fluorine (F) 9 1s2 2s2 2p5
Neon (Ne) 10 1s2 2s2 2p6
Sodium (Na) 11 1s2 2s2 2p6 3s1
Magnesium (Mg) 12 1s2 2s2 2p6 3s2
Aluminum (Al) 13 1s2 2s2 2p6 3s2 3p1
Silicon (Si) 14 1s2 2s2 2p6 3s2 3p2
Phosphorous (P) 15 1s2 2s2 2p6 3s2 3p3
Sulfur (S) 16 1s2 2s2 2p6 3s2 3p4
Chlorine (Cl) 17 1s2 2s2 2p6 3s2 3p5
Argon (Ar) 18 1s2 2s2 2p6 3s2 3p6
Potassium (K) 19 1s2 2s2 2p6 3s2 3p6 4s1
Calcium (Ca) 20 1s2 2s2 2p6 3s2 3p6 4s2
Scandium (Sc) 21 1s2 2s2 2p6 3s2 3p6 4s2 3d1
Titanium (Ti) 22 1s2 2s2 2p6 3s2 3p6 4s2 3d2
Vanadium (V) 23 1s2 2s2 2p6 3s2 3p6 4s2 3d3
Chromium (Cr) 24 1s2 2s2 2p6 3s2 3p6 4s1 3d5
Manganese (Mn) 25 1s2 2s2 2p6 3s2 3p6 4s2 3d5
Iron (Fe) 26 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Cobalt (Co) 27 1s2 2s2 2p6 3s2 3p6 4s2 3d7
Nickel (Ni) 28 1s2 2s2 2p6 3s2 3p6 4s2 3d8
 Copper (Cu) 29 1s2 2s2 2p6 3s2 3p6 4s1 3d10
Zinc (Zn) 30 1s2 2s2 2p6 3s2 3p6 4s2 3d10
Gallium (Ga) 31 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p1
Germanium (Ge) 32 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2
Arsenic (As) 33 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p3
Selenium (Se) 34 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4
Bromine (Br) 35 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
Krypton (Kr) 36 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
Rubidium (Rb) 37 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1
Strontium (Sr) 38 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
Yttrium (Y) 39 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d1
Zirconium (Zr) 40 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d2
Niobium (Nb) 41 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d4
Molybdenum (Mb) 42 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d5
Technetium (Tc) 43 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d5
Ruthenium (Ru) 44 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d7
Rhodium (Rh) 45 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d8
Palladium (Pd) 46 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 4d10
Silver (Ag) 47 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d10
Cadmium (Cd) 48 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10
Indium (In) 49 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p1
Tin (Sn) 50 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p2
Antimony (Sb) 51 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p3
Tellurium (Te) 52 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p4
11. For the electronic configuration of the transition elements (Z= 21
to Z=30) the energy level of the 4s orbital is lower than that of the 3d
orbitals. However, once the e-/ (s) is/ are filled into the 3d orbital, the
order is reversed. The 3d orbitals now have lower energy than the 4s
orbital.

3d 4s

3d
4s

Therefore the electronic configuration of scandium (Z= 21) is

1s2 2s2 2p6 3s2 3p6 3d1 4s2 (or 2.8.9.2) and not

1s2 2s2 2p6 3s2 3p6 4s2 3d1

The first electron ionized off from the scandium atom is from the 4s
orbital and not the 3d orbital.
12. The electronic arrangement of chromium (Z= 24) is
[Ar] 3d5 4s1 and not [Ar] 3d4 4s2
Because the first arrangement, where the five 3d orbitals are exactly
half filled, are more stable energetically.

Energetically more stable

Energetically less stable

13. The electronic configuration of copper (Z= 29) is

[Ar] 3d10 4s1 and not [Ar] 3d9 4s2
because a completely filled d subshell is more stable.

Energetically more stable

 Energetically less stable

2.9.1 Electronic Configuration of Ions

1. In the formation of cations (+ve ions), electrons are removed in

reverse order from of electron filling. That is, the last electron id
removed first.
 The electronic configuration of oxygen (Z= 8) is

 The first electron to be removed is from the paired electron in

the 2p orbital. Thus, the electronic arrangement for the O+ ion is

This is written as 1s2 2s2 2p3 (or 2.5)

2. In the formation of anions (-ve ions), electrons are added in the same
manner as the filling of electrons in the neutral atoms.

 The electronic configuration of fluorine is

 The fluorine ion, F- will have the following configuration:

   

2.10 The Modern Periodic Table

2.10.1 Electronic Configuration and The Periodic Table

1. All elements in the Periodic Table are grouped under 7 Periods (the
horizontal rows) and 18 Groups (the vertical columns).

a) Elements in Group 1 and Group 2 are known as the s-block

elements because their valence electrons occupy the s orbitals.

Group 1 3Li Na
11 K
19 37 Rb Cs
55 Fr
87
Valence shell 2s1 3s1 4s1 5s1 6s1 7s1
configuration

Group 2 Be
4 Mg
12 20 Ca Sr
38 Ba
56 88Ra
Valence shell 2s2 3s2 4s2 5s2 6s2 7s2
configuration

b) Elements in Group 13 (or III) to Group 8 (or VIII) are known as

the p-block elements because the highest orbital filled with
electron is the p orbitals.

Group 13 14 15 16 17 18
Element 13Al 14Si 15P 16S 17Cl 18Ar

Valence shell 3s2 3p1 3s23p2 3s23p3 3s23p4 3s23p5 3s23p6

configuration
 c) Elements in Group 3 through Group 12 are known as the d-block
elements where filing of electrons involves the d orbitals.

Group 3 4 5 6 7 8 9 10 11 12
Element 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn

Valence 3d14s 3d24s 3d34s 3d54s 3d54s 3d64s 3d74s 3d84s 3d104s 3d104s
2 2 2 1 2 2 2 2 1 2
shell
configuratio
n

d) Elements with proton numbers 58 71 (Lanthanides series)

and from 90 103 (Actanides series) are known as the f-block
elements.

Proton 58 59 60 61 62 63 64 65 66 67 68 69 7 71
number 0
Element Ce Pr Nd Pm Sm Eu G Tb Dy Ho Er Tm Y Lu
d b

2. The group number of an element indicates the number of valence

electrons while the period number indicates the outermost principal
shell that is filled with electrons.
E.g. bromine (proton number = 35) is found in Period 4 and Group
17 of periodic table. Valence shell configuration is 4s2 4p5..
3. Eleven of the known elements are gases under room conditions:
H2, He, Ne, Ar, Kr, Xe, Rn, N2, O2, F2, and C12.
4. Only 2 elements are liquids at room conditions: Hg and Br2.