The
Periodic
Classification of
Elements
EARLIER ATTEMPTS OF
CLASSIFICATION OF
ELEMENTS
The first classification of
elements was as metals
and non-metals. This
served only limited purpose
mainly because of two
reasons:
 1. All the elements were
grouped in to these two
classes only. Moreover the group containing metals
was very big.
 2. Some elements showed properties of both-metals
and non-metals and they could not be placed in any
of the two classes.

In 1864 John Alexander
Newland, an English
chemist, arranged in the
increasing order of their
atomic masses* every eighth
element had properties
similar to the first element.
1825
1835
1845
In 1829,
Dobereiner, a
German
scientist made
some groups of
three elements
each and called
them triads
1855
1865
1875
Henry Moseley, an
In 1870, Meyer
English physicist unaware of
discovered in the year
Mendeleev’s
1913 that atomic study has also
number, is the mostbeen working
fundamental property
on his periodic
of an element and table
not consisting
its atomic mass. of 56 elements.
1885 1895
1905
1915
1925
His work
1869 Dmitry Mendeleev**
involved the
a Russian chemist while
trying of
discovery
to classify elements transuraniu
discovered that
m elements
on arranging in the increasing
94 to 102.
order of atomic mass*,
elements with similar chemical
properties occurred
periodically.
Johan Dobereiner

1871. Johan Dobereiner
grouped elements with
similar properties during
1817 to 1829. He,
through his work saw
some pattern in threes.
The halogens chlorine, bromine, and iodine
possessed similar properties as well as the
alkali metals sodium, lithium, and potassium.
 His idea of 'triads' led others to look at the
same idea.

A. E. Beguyer de Chancourtois
When Dobereiner chose to
look at the properties of
elements, 1862 A. E.
Beguyer de Chancourtois
chose the atomic weights.
 His method used the idea
of writing the elements
down in a continuous and
uniformly spaced interval
list.

John Newland
In 1863 John Newland’s
viewed the arrangement as
best being presented in what
he called the Law of Octaves.
 It was much like the triads but
utilized the atomic weights.
 He noted that the elements
would repeat their chemical
properties every eighth
element.
 This was later found to be a very useful idea
when working with the arrangement of the
elements.

Dmitri Ivanovich Mendeleev
In 1869, Mendeleev discovered
that the atomic weights of
elements were related to the
periodic variation in their
properties.
 He arranged the elements
according to increasing atomic weights providing
intervals or periods w/c were not always of the
same length.
 This intervals showed a change in properties of
the elements from the first to the last element
of the table.

Lothar Meyer

In 1870, Meyer unaware
of Mendeleev’s study
has also been working
on his periodic table
consisting of 56
elements.

He also maintained that the properties of
the elements are periodic functions of
their atomic mass.
Dmitri Ivanovich Mendeleev
and Lothar Meyer
Dmitri Ivanovich
Mendeleev and
Lothar Meyer
worked on the idea
of atomic weights.
 Two people, working independently, came up with
what is today considered our working model for
the periodic table of the elements.
 Mendeleev published his work in 1869 while Meyer
did the same in 1870. Because Mendeleev's work
was published first he is usually recognized as the
father of the periodic table
Henry G. J. Moseley

1894. The work of H. G.
J. Moseley brought the
elements in to a new
arrangement (although
minimal).

He selected to arrange the elements by their
atomic number (number of protons) rather than
their weight. His work in 1914 led to what we
call the Law of Chemical Periodicity.
Henry G. J. Moseley
Periodic
Law
states that: the
physical and
chemical properties
of elements are a
periodic function of
atomic number.
Glenn Seaborg
Finishing out this illustrious
group of people is Glenn
Seaborg.
 His work involved the
discovery of transuranium
elements 94 to 102.
 This work led to the addition of the lanthanides
and actinides (lathanoids and actinoids) [some
say actinoins and lathanoins] in the periodic
table. He and his colleagues are credited with
discovering over 100 isotopes.

From those works the
Periodic Table emerged.
Period
The horizontal rows in the PT are known as
PERIOD or SERIES.
 The first period is the shortest period of all
and contains only 2 elements, H and He.
 The second and third periods are called short
periods and contain 8 elements each.

Fourth and fifth periods are long periods
and contain 18 elements each.
 Sixth and seventh periods are very long
periods containing 32 elements* * each.

Period
The sixth series is called Lanthanide
series
 And the last is called the Actinide
series

Group

A group, also known as a family, is a vertical
column in the periodic table of the chemical
elements. There are 18 groups in the standard
periodic table.

The letters A and B were designated to
main group elements (A) and transition
elements (B).
Group
The stairway that starts from Boron
separates metals from the non metals.
 About 20 elements are nonmetals and
these are found above and to the right of
the stairway.
 Element found along the stairway are the
so called Metalloids

Group

The modern explanation of the pattern of
the periodic table is that the elements in a
group have similar configurations of the
outermost electron shells of their atoms:
as most chemical properties are
dominated by the orbital location of the
outermost electron
Group 1


Group 17
Element
Electronic configuration
Element
Electronic configuration
Li
Na
K
Rb
2,1
2,8,1
2,8,8,1
2,8,8,8,1
F
Cl
Br
I
2,7
2,8,7
2,8,8,7
2,8,18,8,7
All elements of group 1 have only one valence electron. Li
has electrons in two shells, Na in three, K in four while Rb
has electrons in five shells.
Similarly all the elements of group 17 have seven valence
electrons however the number of shells is increasing from
two in F to five in I.
The gradual filing of the third shell
can be seen below.
Element
Na
Electronic 2,8,1
configur
ation
Mg
Al
Si
P
S
Cl
Ar
2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
PERIODIC PROPERTIES




We have also learned that in a period the number of
valence electrons and the nuclear charge increases
from left to right. It increases the force of attraction
between them.
In a group the number of filled shells increases and
valence electrons are present in higher shells. This
decreases the force of attraction between them and
the nucleus of the atom.
These changes affect various properties of elements
and they show gradual variation in a group and in a
period and they repeat themselves after a certain
interval of atomic number.
Such properties are called periodic properties.
Periodic Properties and their
Variation in the Periodic Table
Valency in a period : the number of
valence electrons increases in a period.
 In normal elements it increases from 1 to
8 in a period from left to right.
 It reaches 8 in group 18 elements (noble
gases) which show practically no chemical
activity under ordinary conditions and
their valency is taken as zero.

Atomic radii
A number of physical properties like
density and melting and boiling points are
related to the sizes of atoms. Atomic size
is difficult to define.
 Atomic radius determines the size of an
atom. For an isolated atom it may be
taken as the distance between the centre
of atom and the outermost shell.

Atomic radii
Practically, measurement of size of an
isolated atom is difficult; therefore, it is
measured when an atom is in company of
another atom of same element.
 Atomic radii is defined as one-half the

distance between the nuclei of two atoms
when they are linked to each other by a
single covalent bond.
Variation of atomic radii in a
period


Atomic radii of 2nd and 3rd period elements are given in the table
below. What do you observe?
In a period, atomic radius generally decreases right.
2nd Period
3rd Period
190
Li
155
Na
160
Be
112
Mg
143
B
98
Al
132
C
91
Si
128
N
92
P
127
O
73
S
99
F
72
Cl
Can you explain this trend?
 You have learnt in the beginning of this section that in a period
there is a gradual increase in the nuclear charge.
 Since valence electrons are added in the same shell, they are
more and more strongly attracted towards nucleus. This
gradually decreases atomic radii.

Variation of atomic radii in a
group

What happens to atomic radii in a group?

Atomic radii increase in a group from top
to bottom.
Ionic radii
Ionic radius is the radius of an ion. On
converting into an ion the size of a neutral
atom changes.
 Anion is bigger than the neutral atom.
 This is because addition of one or more
electrons increases repulsions among
electrons and they move away from each
other.

Ionic radii
On the other hand a cation is smaller than
the neutral atom.
 When one or more electrons are removed,
the repulsive force between the remaining
electrons decreases and they come a little
closer.

Variation of ionic radii in
periods and groups
Ionic radii show variations similar to those
of atomic radii.
 Thus, ionic radii increase in a group.
 Ionic radii decrease in a period .

Ionization energy
Negatively charged electrons in an atom are
attracted by the positively charged nucleus.
 For removing an electron this attractive force
must be overcome by spending some energy.


The minimum amount of energy required to
remove an electron from a gaseous atom in its
round state to form a gaseous ion is called
ionization energy.
Variation of ionization energy
in a group and period
We have already seen earlier, that the force of
attraction between valence electrons and
nucleus decreases in a group from top to
bottom.
 What should happen to their ionization energy
values?



Ionization energy decreases in a group from top
to bottom.
The ionization energy increases in a period from
left to right.
Electron affinity



Another important property that determines the
chemical properties of an element is the
tendency to gain an additional electron. This
ability is measured by electron affinity.
It is the energy change when an electron is
accepted by an atom in the gaseous state.
By convention, electron affinity is assigned a
positive value when energy is released during the
process.
 The greater value of electron affinity, means
more energy is released during the process and
greater is the tendency of the atom to gain
electron.
Variation of electron affinity in
a group & period

In a group, the electron affinity decreases
on moving from top to bottom, that is,
less and less amount of energy is
released.

In a period, the electron affinity increases
from left to right, that is, more and more
amount of energy is released.
Electronegativity

Electronegativity is relative tendency of
a bonded atom to attract the bondelectrons towards itself.
Electronegativity is a dimensionless
quantity and does not have any units.
 It just compares the tendency of various
elements to attract the bond-electrons
towards themselves.

Variation of electronegativity
in a group & period
Electronegativity decreases in a group
from top to bottom.
 Electronegativity increases in a period
from left to right.

Metallic and non-metallic
character
You know what are characteristic properties
of a metal? They are its electropositive
character (the tendency to lose electrons),
metallic luster, ductility, malleability and
electrical conductance.
 Metallic character of an element largely
depends upon its ionization energy.
 Smaller the value of ionization energy,
more electropositive and hence more
metallic the element would be.

Variation of metallic character
in a group & period
Metallic character of elements increases
in a group from top to bottom.
 Metallic character of elements decreases
in a period from left to right

Summary
Atomic radii
 Defined as one-half the
distance between the
nuclei of two atoms when
they are linked to each
other by a single covalent
bond.
Ionic size

The radius of an ion. On
converting into an ion the
size of a neutral atom
changes.
Metallic Property
They are its electropositive character (the
tendency to lose electrons), metallic luster,
ductility, malleability and electrical
conductance.
Summary
Ionization Energy

The minimum amount of
energy required to
remove an electron from
a gaseous atom in its
round state to form a
gaseous ion
Electron Affinity
 The tendency of an atom
to gain an additional
electron
Ionization Energy

The relative tendency of a bonded
atom to attract the bond-electrons
towards itself.
Summary
Variation
Group
Top to Bottom
Period
Left to Right
Atomic size
Ionization
Electron Affinity
Increasing
Decreasing
Decreasing
Decreasing
Increasing
Increasing
Electronegativity
Decreasing
Increasing
Metallic Property
Increasing
Decreasing
ACTIVITY 1

Arrange the elements Li, Al, B, and Br in
order of
– A. increasing atomic size
– B. decreasing ionization energy
– C. increasing electronegativity
ACTIVITY 2

Choose which is the larger member of each pair:
1.Mg and Mg
0
2.O and O
0
2
3.Fe and Fe
2
4.Cl and Cl
0
2
3
1
ACTIVITY 3


Given the hypothetical elements:
39
19
Fe
40
20
De
75
33
Li
28
14
Z
Determine the group and series where each
element will be found in the PT.
ACTIVITY 4

Select which is the
– smallest atom
– Biggest atom
– atom that would require the greatest
ionization energy
– Most electronegative
– Noble gas
– nonmetal
39
19
Fe
40
20
De
75
38
Li
28
14
Z
Group→
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
↓ Period
1
H
1.0
08
He
4.00
3
2
Li
6.9
41
Be
9.01
2
B
10.8
1
C
12.0
1
N
14.0
1
O
16.0
0
F
19.0
0
Ne
20.1
8
3
Na
22.
99
Mg
24.3
1
Al
26.9
8
Si
28.0
9
P
30.9
7
S
32.0
7
Cl
35.4
5
Ar
39.9
5
4
K
39.
10
Ca
40.0
8
Sc
44.9
6
Ti
47.8
7
V
50.9
4
Cr
52.0
0
Mn
54.9
4
Fe
55.8
4
Co
58.9
3
Ni
58.6
9
Cu
63.5
5
Zn
65.3
9
Ga
69.7
2
Ge
72.6
1
As
74.9
2
Se
78.9
6
Br
79.9
0
Kr
83.8
0
5
Rb
85.
47
Sr
87.6
2
Y
88.9
1
Zr
91.2
2
Nb
92.9
1
Mo
95.9
4
Tc
[98]
Ru
101.
07
Rh
102.
91
Pd
106.
42
Ag
107.
87
Cd
112.
41
In
114.
82
Sn
118.
71
Sb
121.
76
Te
127.
60
I
126.
90
Xe
131.
29
6
Cs
13
2.9
1
Ba
137.
33
Hf
178.
49
Ta
180.
95
W
183.
84
Re
186.
21
Os
190.
23
Ir
192.
22
Pt
195.
08
Au
196.
97
Hg
200.
59
Tl
204.
38
Pb
207.
2
Bi
208.
98
Po
[209
]
At
[210
]
Rn
[222
]
7
Fr
[22
3]
Ra
[226
]
Rf
[263
]
Db
[262
]
Sg
[266
]
Bh
[264
]
Hs
[269
]
Mt
[268
]
Ds
[272
]
Rg
[272
]
Uub
[277
]
Uut
[284
]
Uuq
[289
]
Uup
[288
]
Uuh
[292
]
Uus
[291
]‡
Uuo
[293
]‡
•Lanthanides
La
138.
91
Ce
140.
12
Pr
140.
91
Nd
144.
24
Pm
[145
]
Sm
150.
36
Eu
151.
96
Gd
157.
25
Tb
158.
93
Dy
162.
50
Ho
164.
93
Er
167.
26
Tm
168.
93
Yb
173.
04
Lu
174.
97
** Actinides
Ac
[227
]
Th
232.
04
Pa
231.
04
U
238.
03
Np
[237
]
Pu
[244
]
Am
[243
]
Cm
[247
]
Bk
[247
]
Cf
[251
]
Es
[252
]
Fm
[257
]
Md
[258
]
No
[259
]
Lr
[262
]
*
**
Thank you for listening!

End of Presentation!

Exam next meeting.
39
19
Fe
40
20
De
75
38
Li
28
14
Z