PERIODIC CLASSIFICATION OF ELEMENTS

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PERIODIC CLASSIFICATION OF

ELEMENTS

Classification

of

Elements

The need to
simplify and
organize the
study
of
elements and
their large number
of
compounds
led
to the
development
of the
periodic

table.


Dobereiner’s

Triad

The

initial

attempt

towards

classification

of

elements

was

made

by

J
.
W
.

Dobereiner,

a

German

chemist
.

He

tried

to

arrange

the

elements

with

similar

properties

into

groups
.

He

identified

certain

groups

of

three

elements

with

similar

properties
.

He

called

these

groups

of

three

elements

as

Dobereiner’s

Triads
.

He

showed

that

when

the

three

elements

of

a

triad

were

arranged

in

order

of

their

increasing

atomic

masses,

the

atomic

mass

of

the

middle

element

was

roughly

the

average

of

the

atomic

masses

of

the

other

two

elements
.

Element

Atomic

Mass

Li

6.9

Na

23

K

39

6.9


39


22.95

2

For

example,

Li,

Na,

K

;

Ca,

Sr,

Ba

;

S,

Se,

Te

and

Cl,

Br,

I

constitute

Dobereiner’s

triads
.

The

idea

of

Dobereiner’s

triad

was

rejected

because

it

could

not

be

applied

to

all

the

elements

known

at

that

time
.

Newlands

Law

of

Octaves

Next

attempt

towards

classification

of

elements

was

made

by

John

Alexander

Newland

who

gave

Newlands’s

Law

of

Octaves
.

According

to

this

law,

when

elements

were

arranged

in

increasing

order

of

their

atomic

masses,

the

properties

of

every

eighth

element

was

similar

to

the

first

one
.

Limitations

of

Newlands

Law

of

Octaves

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1.

The
Newlands’ law of octaves was rejected because it could be applied only
to
elements
having
atomic masses upto 40
u
,
i.e.,

calcium

2.

When
noble
gases
were
discovered, the
properties
of the
ninth element were
now
similar
to
that
of
the

first.

Mendeleev’s Classification

Mendeleev classified
the elements on the
basis
of
similarity in physical properties

and

similarity in
the
formulae
of
their hydrides
and
oxides
.

Mendeleev’s Periodic

Law

Mendeleev’s Periodic Law states that
the
physical
and
chemical properties
of the
elements are a
periodic function
of
their atomic

masses.


Salient

Features

of

Mendeleev’s

Classification

(a)

In

Mendeleev’s

periodic

table,

elements

are

arranged

in

order

of

their

increasing

atomic

masses

in

such

a

way

that

elements

with

similar

properties

are

placed

in

the

same

vertical

column

called

the

group
.

To

be

sure

that

elements

with

similar

properties

fell

in

the

same

group,

Mendeleev

had

to

place

an

element

with

slightly

greater

atomic

mass

(i
.
e
.
,

Te)

before

an

element

of

slightly

lower

atomic

mass

(i
.
e
.
,

I)
.

Similarly,

cobalt

was

placed

ahead

of

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nickel.

(b)
Mendeleev
even
left some
gaps
in
the
periodic table
for those
elements which
were yet
to be
discovered. For example, gallium
and

germanium.


Mendeleev’s

Periodic

Table

consists

of

seven

horizontal

rows

called

periods

and

eight

vertical

columns

called

groups
.

The

periods

are

numbered

from

1

to

7

while

the

groups

are

designated

as

I,

II,

III,

IV,

V,

VI,

VII

and

VIII
.

Except

group

VIII,

each

group

is

divided

into

two

subgroups

designated

as

A

and

B
.

Limitations
of
Mendeleev’s Periodic

Table

Anomalous position
of
hydrogen
Position
of

isotopes

Wrong
order
of
atomic masses
of
some elements
Uncertainty in prediction
of new

elements.

1.

2.

3.

4.

Modern

Periodic

Table

Henry

Moseley

in

1913

showed

that

atomic

number

was

a

more

fundamental

property

of

an

element

than

its

atomic

mass
.

Therefore,

atomic

number

or

electron

number

was

adopted

as

the

basis

of

classification

of

elements
.


Modern

Periodic

Law

Mendeleev’s

periodic

law

was

thus

modified

to

Modern

periodic

law

which

states

that

the

properties

of

elements

are

a

periodic

function

of

their

atomic

number
.

Salient

Features

of

Modern

Periodic

Table

When

the

elements

are

arranged

in

increasing

order

of

their

atomic

numbers,

the

anomalies

of

Mendeleev’s

periodic

table

are

removed
.

However,

the

position

of

hydrogen

still

remains

anomalous
.

It

can

be

placed

either

along

with

alkali

metals

of

group

1

or

along

with

halogens

of

group

17

of

the

Modern

periodic

table
.

1.

2.

In the
Modern
or
Long form
of
the
periodic table,
elements
are arranged
in
increasing order
of
their
atomic

numbers.

3.

The
Modern periodic table
is based upon
electronic configuration
of

elements.

4.

The
periodicity in properties
of
elements is
due
to
periodicity in their
outer
electronic
configurations.

5.

The

numbers

2,

8,

8,

18,

18

and

32

after

which

the

properties

of

elements

get

repeated
are the
magic
numbers
on which
this
classification is

based.

6.

The Modern
periodic table consists
of
18
vertical
columns
called
groups
and 7
horizontal rows called

periods.

7.

Each

period

starts

with

the

filling

of

electrons

in

a

new

electronic

shell

and

the

elements
in
a
period have consecutive atomic

numbers.

Valence

Electron

All

the

elements

in

a

group

have

identical

outer
-
shell

electronic

configuration
.

However,

the

number

of

inner

filled

energy

shells

increases

as

we

go

down

the

group
.

Further,

the

elements

in

a

group

do

not

have

consecutive

atomic

numbers
.

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Group

No.

1

2

13

14

15

16

17

18

Atomic No.

11

12

13

14

15

16

17

18

Symbol

Na

Mg

Al

Si

P

S

Cl

Ar

Electronic Configuration

2, 8,

1

2, 8,

2

2, 8,

3

2, 8,

4

2, 8,

5

2, 8,

6

2, 8,

7

2, 8,

8

No. of valence

electron

1

2

3

4

5

6

7

8

Valency

1

2

3

4

3

2

1

0

Atomic Radii

186

136

125

117

116

104

99

190

Element

Atomic

No.

E.C.

Atomic

Radii

Period

Two

Li

3

2,

1

152

Period

Three

Na

11

2, 8,

1

186

Period

Four

K

19

2, 8, 8,

1

231

Valency

The valency
of
elements in
a group
is fixed but in
a
period first it
increases
from
1 to 4
and then
decreases
to

zero.

Valency

Increases and then decreases
in
a

period


Remains the
same
down

the
group



Atomic
size/Atomic

Radii

The
atomic size decreases across
a
period from left
to
right
but
increases down
a
group.


Atomic Size /

Radii

Decreases from left to right
in
a

period

Increases from
top
to
bottom

in
a

group



Metallic

and

Non
-
Metallic

Character

Across

a

period,

the

metallic

character

decreases

while

the

non
-
metallic

character

increases
.

Conversely,

on

moving

down

a

group,

the

metallic

character

increases

while

the

non
-
metallic

character

decreases
.


Metallic

Character

Decreases along a

period


Increases

down
the

group

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Nature
of

Oxides

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The oxides
of
metals are
basic
while
those of
non
-
metals are
acidic
in

nature.


The

periodicity

in

the

properties

of

elements

such

as

valency,

atomic

size

and

metallic/non
-
metallic

character

can

be

explained

on

the

basis

of

outer
-
shell

electronic

configuration

of

the

elements
.

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