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4

Periodic Classification of Elements
You must have visited a library
...
In spite of
this if you ask for a particular book, the library staff can locate it easily
...
They are
arranged on shelves accordingly
...

In the last two lessons you have studied about the structure of atoms and their electronic
configurations
...
Electrons are filled in various shells and subshells in a
fairly regular fashion
...
Such
trends in their physical and chemical properties were noticed by chemists in the nineteenth
century and attempts were made to classify elements on their basis long before structure of
atom was known
...
Finally we shall learn about some properties of
elements and their variations in the periodic table
...

4
...
This served only limited
purpose mainly because of two reasons:
1
...
Moreover the group
containing metals was very big
...
Some elements showed properties of both-metals and non-metals and they could not
be placed in any of the two classes
...
Now we shall learn about some
of them
...
1
...
All three elements of a triad were
Sodium, Na
23
similar in their properties
...
Also, same was the case with their other properties
...
They form
a Dobereiner’s triad
...
Two more
examples of Dobereneir’s triads are given below
...
5
80
127

40 + 137
= 88
...
5 + 127
Mean of the first atomic masses of the and third elements =
= 81
...

4
...
2 Newland’s law of Octaves
In 1864 John Alexander Newland, an English chemist noticed that “when elements are
arranged in the increasing order of their atomic masses* every eighth element had properties
similar to the first element
...
It was due to its
similarity with musical notes where, in every octave, after seven different notes the eighth
note is repetition of the first one as shown below
...
9)

(9
...
8)

(12
...
0)

(16
...
0)

Na

Mg

Al

Si

P

S

Cl

(23
...
3)

(27
...
1)

(31
...
1)

(35
...
1)

Ca
(40
...
And starting from sodium? It is potassium
...
Similarly, aluminnium is the eighth element from boron it shows
properties similar to it
...
Because of this shortcoming his work was
not received well by the scientific community
...

4
...
3 MENDELEEV’S PERIODIC LAW AND PERIODIC TABLE
4
...
1a Mendeleev’s periodic law
Dmitry Mendeleev** a Russian chemist while trying to classify elements discovered that
on arranging in the increasing order of atomic mass*, elements with similar chemical
properties occurred periodically
...

A periodic function is the one which repeats itself after a certain interval
...
Now we shall learn about the arrangement of elements on the basis of the
periodic law
...

A tabular arrangement of the elements based on the periodic law is called periodic
table
...
He placed this element below the first
element and thus started the second row of elements
...

∗ Then known as atomic weight
** Also spelled as Mendeleef or Mendeleyev

: 66 : Periodic Classification of Elements
PERIODIC TABLE (Modified form of Mendleeff’s Table)
P
E
R
I
O
D
I
C
S

Group :

I

II

Oxide:
Hydride:

R2O
RH

RO
RH2

A
1
2
3
4

5

First
series
second
series
First
series
second
series
First
series

6

B

B

H 1 (At
...
)
1
...
Wt
...
939
Na 11
22
...
102
Cu 29
63
...
012
Mg 12
24
...
08
Zn 30
65
...
47

IV

V

VI

VII

R2O5
RH4

R2O5
RH3

RO3
RH2

R2O7
RH

A

B

Sr 38
87
...
87

Cs 55
132
...
811
Al 13
26
...
011
Si 14
28
...
96

Ga 31
69
...
40

Ge 32
72
...
82

Hg 80
200
...
37
Actinide
Elements
89-103

B

N7
14
...
974

Zr 40
91
...
999
S 16
32
...
99
Se 34
78
...
906
Sb 51
121
...
49
Pb 82
207
...
94
As 33
74
...
69
*Rare
Earths
57-71

A

Ti 22
47
...
905

Ba 56
137
...
97

Fr 87
(223)

7

A

III
R2O3
RH3

Po 84
(210)

Zero
Noble
gases

B

Tc 43
(99)

He 2
4
...
183
Ar 18
39
...
85 58
...
71
Kr 36
83
...
07 102
...
4

I 53
124
...
85

Ta 73
180
...
998
Cl 17
35
...
939
Br 35
79
...
94
Te 52
127
...
98

A

VII

Xe 54
131
...
2

O s 76 Ir 77 Pt 78
190
...
2 195
...
91 140
...
91 144
...
35 151
...
25 158
...
50 164
...
26 168
...
04 174
...
04 (231) 238
...
4
...
1
...
4
...
What do you observe?
Here, elements are arranged in tabular form in rows and columns
...

1
...
You can see that
there are seven periods in the periodic table
...

2
...
e
...

3
...
You must have noticed that these
are nine in number and are numbered from I to VIII and Zero (Roman numerals)
...
Groups I to VII are subdivided into A and B subgroups
...

5
...
They show
regular gradation in their physical properties and chemical reactivities
...

4
...
3c Merits of Mendeleev’s periodic classification
1
...

2
...
These vacant spaces were for
elements that were yet to be discovered
...
The element gallium was discovered four
years later and its properties matched very closely with the predicted properties of ekaaluminium
...
Does it mean that this periodic table was perfect? No
...
Now we shall discuss the
defects in this classification
...
3
...

The following were the main defects in it:
1
...
Therefore, it could neither be placed with alkali metals
(group I ) nor with halogens (group VII )
...
Position of isotopes
Different isotopes of same elements have different atomic masses, therefore, each one
of them should be given a different position in the periodic table
...

3
...
For example, Argon (39
...
1)
CHECK YOUR PROGRESS 4
...
Elements A, B and C constitute a Dobereiner’s triad
...
How many elements were included in the arrangement given by Newland?
3
...
How many groups were originally proposed by Mendeleev in his periodic table?
5
...
Mendeleev’s periodic table had some blank spaces in it
...
What name was given to the element whose properties were similar to the element
eka-aluminium predicted by Mendeleev?
4
...
Atomic number,
(Z), of an element is the number of protons in the nucleus of its atom
...
This discovery changed
the whole perspective about elements and their properties to such an extent that a need
was felt to change the periodic law also
...

4
...
1 Modern periodic law
After discovery of atomic number the periodic law was modified and the new law was
based upon atomic numbers in place of atomic masses of elements
...
Now we shall learn about the modern periodic table which finally emerged
...
2
...

Many versions of this periodic table are in use but the one which is most commonly used
is the Long Form of Modern Periodic Table
...
3
...
4
...
4
...
Now let us learn the main features of
this periodic table
...
2
...
Each column is called a group
...
Group 1
on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr) and group 18 on
extreme right side position contains noble gases (He, Ne, Ar, Kr, Xe and Rn)
...
You can see in case of group 1 (alkali metals) and group 17
elements (halogens) that as one moves down a group, more and more shells are added
...
Li has electrons in two shells,
Na in three, K in four while Rb has electrons in five shells
...

●

Elements present in groups 1 and 2 on left side and groups 13 to 17 on the right side of
the periodic table are called normal elements or representative elements
...
They are also called typical or main group elements

Periodic Classification of Elements : 69 :
●

Elements present in groups 3 to 12 in the middle of the periodic table are called
transition elements
...
Their two outermost shells are incomplete
...
In transitions elements, the electrons are added
to incomplete inner shells
...


Group 18 on extreme right side of the periodic table contains noble gases
...

●
Inner transition elements:14 elements with atomic numbers 58 to 71 (Ce to Lu) are
called lanthanides# and they are placed along with the element lanthanum (La), atomic
number 57 in the same position (group 3 in period 6) because of very close resemblance
between them
...
They are shown
also separately below the main periodic table along with lanthanides
...
2
...
Each row is called a period
...

●

In each period a new shell starts filling up
...
For example, in elements of 3rd period, the third shell
(M shell) starts filling up as we move from left to right@
...

The gradual filing of the third shell can be seen below
...

The second and third periods are called short periods and contain 8 elements each
...

Sixth and seventh periods are very long periods containing 32 elements* * each
...
In transitions elements, the electrons are added to incomplete inner shells
...

* These elements have been named after the 1st elements actinium present in their position in the periodic
table
...
Elements 114, 116 and 118 have been reported only
recently
...
2
...
The long form of modern periodic table is therefore free of
main defects of Mendeleev’s periodic table
...
Position of isotopes
All isotopes of the same elements have different atomic masses but same atomic number
...

2
...
For
example, atomic numbers of argon and potassium are 18 and 19 respectively
...

CHECK YOUR PROGRESS 4
...
According to the modern periodic law the properties of elements are periodic function
of which property of theirs?
2
...
How many group and periods are present in the long form of periodic table?
4
...
The elements that are present in the right hand portion of the periodic table are metals
or non-metals?
6
...
3 PERIODIC PROPERTIES
In the previous section we have learnt about the main features of the Modern Periodic
Table
...
It increases the force of attraction between them
...
This decreases the force of attraction between them and the nucleus of the atom
...

Such properties are called periodic properties
...

4
...
1 VALENCY
(a) Valency in a period : You have already learnt in the previous section that the number
of valence electrons increases in a period
...
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
...
What do you observe? Valency
of normal elements with respect oxygen increases from 1 to 7 as shown below for
elements of third period
...

Group
Element
No
...

Group
Element
No
...
Therefore, they all have the same valency
...
Similarly valency of all group 17 elements, halogens, is
1 with respect to hydrogen and 7 with respect to oxygen
...
3
...
Atomic size is difficult to define
...
For an isolated atom it may be taken as the distance between the centre of atom
and the outermost shell
...
It
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
...
3
...
What do you observe? In a period, atomic radius generally decreases from left to
right
...
Since valence electrons are added
in the same shell, they are more and more strongly attracted towards nucleus
...

4
...
2b 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
...

Element
Li

Atomic radius
155

Element
F

Atomic radius
72

Na
K

190
235

Cl
Br

99
114

Rb

248

I

133

As we go down a group the number of shells increases and valence electrons are
present in higher shell and the distance of valence electrons from nucleus increases
...
Also, the number of filled shells between valence electrons and nucleus increases
...
Both the factors decrease the force of attraction
between nucleus and valence electron
...

4
...
3 Ionic radii
Ionic radius is the radius of an ion
...
Anion is bigger than the neutral atom
...
On
the other hand a cation is smaller than the neutral atom
...

4
...
3a Variation of ionic radii in periods and groups
Ionic radii show variations similar to those of atomic radii
...
You can see such increases in groups 1 and 16 elements from the data given below
...
It can be seen from the data of ionic radii in picometer
for 2nd period elements given below
...
Also, the trend in radii of cations is seen in Li+ and Be2+and in radii
of anions is seen in N3–, O2– and F–
...
3
...

For removing an electron this attractive force must be overcome by spending some energy
...
It is measured in unit of kJ
mol-1
...
Stronger the force of attraction, greater is the value of ionization energy
...
If second electron is removed the ionization energy is called the second ionization
energy
...

4
...
3a Variation of ionization energy in a group
We have already seen earlier, that the force of attraction between valence electrons and
nucleus decreases in a group from top to bottom
...
This can be
seen from ionization energy values (in kJ mol-1) of groups 1 and 17 elements given below
...
3
...
As a consequence of this, the ionization energy increases in a
period from left to right
...

2nd Period Elements
Element
Li

Be

B

C

N

O

F

Ne

Ionization
Energy

899

801

1086

1400

1314

1680

2080

3rd Period Elements
Element
Na

Mg

Al

Si

P

S

Cl

Ar

Ionization
Energy

738

578

786

1021

1000

1251

1521

520

496

4
...
5 Electron affinity
Another important property that determines the chemical properties of an element is the
tendency to gain an additional electron
...
It is

: 74 : Periodic Classification of Elements

the energy change when an electron is accepted by an atom in the gaseous state
...
The energy change is measured in the unit kJ mol-1
...
Greater the value of electron affinity, more energy is released during the process
and greater is the tendency of the atom to gain electron
...

4
...
5a Variation of electron affinity in a group
In a group, the electron affinity decreases on moving from top to bottom, that is, less and
less amount of energy is released
...

Group 1
Element
Electron affinity
Li
58

Group 17
Element
Electron affinity
F
333

Na
K

53
48

Cl
Br

348
324

Rb

45

I-

295

4
...
5b Variation of electron affinity in a period
In a period, the electron affinity increases from left to right, that is, more and more amount
of energy is released
...

2nd Period elements
Element
Electron affinity

Li
58

Be
-

B
23

C
123

N
0

O
142

F
333

3rd Period elements
Element
Electron affinity

Na
53

Mg
-

Al
44

Si
120

P
74

S
200

Cl
348

4
...
6 Electronegativity
You have learnt in the previous section that electron affinity of an element is a measure of
an isolated atom to attract electrons towards it self
...
Mostly we come across atoms which are bonded to other atoms
...
This property is
known as electronegativity
...
Electronegativity is a dimensionless quantity
and does not have any units
...
The most widely used scale of electronegativity
was devised by Linus Pauling
...
You will learn in the
next chapter how it helps to understand the nature of chemical bond formed between two
atoms
...


Periodic Classification of Elements : 75 :
Group 1
Element
Electronegativity
Li
1
...
0

Na
K

0
...
8

Cl
Br

3
...
8

Rb

0
...
5

What do you observe? Electronegativity decreases in a group from top to bottom
...

2nd Period Elements
Element
Electronegativity

Li
1
...
5

B
2
...
5

N
3
...
5

F
4
...
9

Mg
1
...
5

Si
1
...
1

S
2
...
0

Now what do you observe? Electronegativity increases in a period from left to right
...
3
...
Metallic character of an element largely depends upon its ionization energy
...

4
...
7a Variation of metallic character in a group
You know the variation of ionization energy in a group
...

This can best be seen in elements of group 14
...

Group 14
Element

Nature

C

Non-metal

Si

Metalloid

Ge

Metalloid

Sn

Metal

Pb

Metal

4
...
7b Variation of metallic character in a period
How does metallic character change in a period? Metallic character of elements decreases
in a period from left to right as shown below for 3rd period elements
Element

Na

Mg

Al

Si

P

S

Cl

Character

Metal

Metal

Metal

Metalloid

Non-metal

Non-metal

Non-metal

: 76 : Periodic Classification of Elements

CHECK YOUR PROGRESS 4
...

1
...

2
...

3
...
Electronegativity _______________ in a period from left to right and
_______________ in a group from top to bottom
...
Metallic character of elements _______________ from top to bottom in a group
...
Ionization energy of the 1st element in a period is _______________ in the entire
period
...
It served only limited
purpose
...

Dobereiner grouped elements into triads
...
He could group only a few elements into triads
...

Newland tried to see the periodicity of properties and stated his law of octaves that,
“When elements are arranged in the increasing order of their atomic weights every
eighth element has properties similar to the first”
...

Mendeleev observed correlation between atomic masses and other properties and stated
his periodic law as, “The chemical and physical properties of elements are a periodic
function of their atomic weights”
...
It consists of seven horizontal rows called periods and numbered
from 1 to 7
...

Main achievements of Mendeleev’s periodic table were (i) inclusion of all the known
elements and (ii) prediction of new elements
...

Moseley discovered that atomic number and not atomic mass is the most fundamental
property of elements
...

This is the modern periodic table
...
Its long form has been accepted
by IUPAC
...
It is free of main

Periodic Classification of Elements : 77 :

•
•

A
...


3
...


5
...

1
...

3
...
Elements belonging to same group have same
number of valence electrons and thus show same valency and similar chemical
properties
...
Atomic and ionic
radii and metallic character increase while ionization energy , electron affinity and
electronegativity decrease in a group from top to bottom
...

TERMINAL EXERCISES
Multiple choice type questions
...

Ionization energy of an element increases with an increase in atomic number
...

Out of P3+, S2- and Cl- ions Cl- ion is the smallest one
...
The first member of lanthanide series of elements is lanthanum
...

1
...

3
...

5
...

7
...

9
...

11
...

Name the group and period of element having the atomic number 21
...

State Newland’s law of Octaves
...

How many groups were present in Mendeleev’s Periodic Table and give their numbers
...

List two main achievements of Mendeleev’s periodic table
...
Define atomic radius
...
What is ionization energy? How does it vary in a group? Give two reasons for it
...
Which element of the following has the highest ionization energy?
Na, Ba and Cl
15
...
What do you understand by ‘periodicity’ of properties? Explain taking metallic
character of elements as an example
...
Potassium is more reactive than sodium
...

18
...
To which group of
periodic table does it belong? Explain
...
The following is a portion of periodic table
...

1 2
3-15 16
17
18
H
He
C
D
A
E
B
(i) Out of A and B which one has lower ionization energy ?
(ii) Which is bigger atom C or D?
(iii)

Which is the most electropositive element of all?

(iv)

Which is more metallic in nature D or E?

Periodic Classification of Elements : 79 :

(v) Which is more non-metallic in nature C or D?
(vi)
Which is the least electronegative element of all?
ANSWERS TO CHECK YOUR PROGRESS
4
...
Atomic mass of the middle element B must be nearly equal to the average of the other
two elements A and C
...

3
...

5
...

7
...

Gallium

4
...
Atomic number
2
...

Position of isotopes
ii
...

Grouping of dissimilar elements
iv
...

3
...
Alkaline earths
5
...
32
4
...

2
...

4
...

6
...

Atomic number: It is the number of protons in the nucleus of the atom of an element
...

Dobereiner’s triad: A group of three chemically similar elements in which the atomic
mass and properties of the middle element are mean of the other two
...
By convention, it is assigned a positive value when energy is
released during the process
...

Groups: The vertical columns present in periodic table
...
e
...

Ionization energy: It is the minimum amount of energy required to remove an electron
from an isolated gaseous atom in its ground state to form a gaseous ion
...

Modern periodic law : The chemical and physical properties of elements are a periodic
function of their atomic numbers
...

Noble gases: The elements present in group 18 on extreme right side of the periodic
table
...

Normal elements: These are the elements present in groups 1 and 2 on left side and
groups 13 to 17 on the right side of the periodic table whose only outermost shells are
incomplete
...

Periodic table: A tabular arrangement of the elements based on the periodic law
...

Transition elements: These are the elements present in groups 3 to 12 in the middle
of the periodic table whose two outermost shells are incomplete

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