PERIODIC
CLASSIFICATION OF
ELEMENTS
In Class IX we have learnt that
matter around us is present in
the form of elements,
compounds and mixtures and
the elements contain atoms of
only one type.
Do you know how many elements
are known till date?
At present, there are 118 elements
known to us.
All these elements have different
properties.
Out of these 118, only 94 are
naturally occurring.
As different elements were being
discovered, scientists gathered more
and more information about the
properties of these elements.
They found it difficult to organise all
that was known about the elements.
They started looking for some pattern in
their properties, on the basis of which
they could study such a large number of
elements with ease.
Need For Classification
1. Classification makes study of elements
easy.
2. Helps us in understanding the properties
of the elements and their compounds.
3. Helps to compare and differentiate the
properties of elements and compounds
from different groups.
4. Helps to keep the elements with similar
properties together.
EARLIER ATTEMPTS AT
THE CLASSIFICATION
OF ELEMENTS
The first attempt to classify elements resulted in
grouping the then known elements as
metals and non-metals.
DÖBEREINER’S TRIADS
In 1817, Johann
Wolfgang Döbereiner, (a
German Chemist) made
an attempt to arrange
the elements with
similar properties into
groups having 3
elements each.
He called these groups
as triads.
Döbereiner’s Showed:
When 3 elements in a triads
are arranged in the order of
increasing atomic masses.
The atomic mass of the
middle element was roughly
the average of the atomic
masses of the other two
elements.
Atomic Average atomic mass of
Elements
weight first and third elements
Cl 35.5
35.5 + 127 = 81.25
Br 80
2
I 127
EXAMPLES
Atomic Average atomic mass of
Elements
weight first and third elements
Ca 40
40 + 137 =
Sr
2
Ba 137
Atomic Average atomic mass of
Elements
weight first and third elements
Li 7
7 + 39 =
Na 23
2
K 39
Limitation
Importance
Döbereiner could
This method is identify only three
significant in triads from the
predicting the atomic elements known at
mass and properties that time.
of the middle
element. The elements which
were discovered later
didn’t follow this
trend.
NEWLAND’S LAW OF OCTAVES
In 1866, John Newlands,
an English scientist,
arranged the then
known elements in the
order of increasing
atomic masses.
Found that every eighth
element has properties
similar to that of the
first.
Newland’s Law of Octaves
He compared this to the
octaves found in music,
i.e.,
Therefore, called
sa, re, ga, ma, pa, da, ni the Law of Octaves.
do, re, mi, fa, so, la, ti.
Advantages
1. The law established a framework for
classifying elements with comparable
features into groups.
2. Newlands law was the first to be based
on the atomic weight, i.e. linking
element properties to atomic masses.
3. This law grouped lighter elements
together. For example lithium, sodium,
and potassium were combined.
Limitation
1. The law was applicable only upto calcium (after
calcium every eighth element did not possess
properties similar to that of the first).
2. Properties of new elements discovered did not
fit into the law of octaves.
3. In a few cases, Newlands placed two elements in
the same slot to fit elements in the table.
4. Grouped unlike elements under the same slot.
Thus, Newlands’ Law of Octaves worked well with
lighter elements only.
MENDELÉEV’S PERIODIC
TABLE
Dmitri Ivanovich Mendeléev, a Russian chemist, was the
most important contributor to the early development of
a Periodic Table of elements wherein the elements were
arranged on the basis of their:
- fundamental property,
- the atomic mass, and
- the similarity of chemical properties.
When Mendeléev started his work,
63 elements were known.
What did Mendeleev do?
● Examined the relationship between the atomic
masses of the elements and their physical and
chemical properties.
● Among chemical properties- He concentrated on
the compounds formed by elements with oxygen
and hydrogen.
Why?
1. O and H are very reactive
2. They are readily found
3. They form compounds with most elements.
What did Mendeleev do?
● The formulae of the hydrides and
oxides formed by an element were
treated as one of the basic
properties of an element for its
classification.
Mendeleev’s Observation
● He observed that elements with similar
properties fall in the same vertical
column.
● These vertical column are called
groups, and horizontal rows of
elements are called periods.
● On this basis, Mendeleev gave a
Periodic Law-
‘The properties of elements are the
periodic function of their atomic masses’.
Table 5.4 Mendeléev’s Periodic Table (Published in a German journal in 1872)
Note:
Group
In the formula for
oxides and hydrides at
the top of the columns, Periods
the letter ‘R’ is used to
represent any of the
elements in the group.
Features of Mendeleev’s Periodic Table
1. The table contains 8 vertical columns, and 6
horizontal rows.
2. Mendeleev left gaps in his table for the
elements not discovered at his time.
He named these elements by prefixing a
Sanskrit numeral, Eka (One), divi (two), tri
(three), etc., to the name of the preceding
similar element in the same group.
Example: Eka-boron, Eka-aluminium, Eka-silicon,
etc., after their discovery were named as scandium,
gallium, and germanium respectively.
Features of Mendeleev’s Periodic Table
3. He predicted the atomic masses and
properties of several elements which
were not known at that time.
One BIG strength of this model
4. In order to group elements having
similar properties together, at some
places, Mendeleev had to place an
element with a slightly greater atomic
mass before an element with a slightly
lower atomic mass.
5. Noble gases were discovered much
later after Mendeleev’s periodic table.
They were placed easily in a new group
called zero group of Mendeleev’s table,
without disturbing the existing order.
Limitations of Mendeleev’s Periodic Table
1. Position of Hydrogen
He could not assign a correct position of hydrogen, as
hydrogen resembles alkali metals as well as halogens.
Like alkali metals- hydrogen combines with halogens,
oxygen and sulphur to form compounds having similar
formulae.
E.g., HCl, NaCl; H2O, Na2O.
Like halogen metals- hydrogen also exists as diatomic
molecules and it combines with metals and non-metals
to form covalent compounds.
Limitations of Mendeleev’s Periodic Table
2. Position of Isotopes
Isotopes are placed into same positions thought they
have different atomic masses.
Example- Element chlorine has two isotopes of
chlorine, Cl-35 and Cl-37. Placing of these two isotopes
in the same group of the periodic table could not be
explained by Mendeleev’s periodic law.
Limitations of Mendeleev’s Periodic Table
3. Wrong order of Atomic masses of some
elements could not be explained
When placed few elements, those posses higher
atomic mass were placed before elements having
a lower atomic mass,
e.g., Agron (39.9) placed before Potassium (39.1),
Cobalt (58.9) placed before Nickel (58.7).
Q. List any three observations which posed a
challenge to Mendeleev’s Periodic Law.
ANSWER
i) No fixed position of H in the periodic table.
ii) Position of isotopes not clear.
iii) Atomic mass does not increase in a regular
manner.
MODERN PERIODIC TABLE
In 1913, Henry Moseley showed that the atomic
number of an element is a more fundamental
property.
He gave :
1. Modern Periodic Law : Properties of elements
are a periodic function of their atomic number.
2. Modern Periodic Table : Arrangement of the
elements in increasing order of their atomic
number
It did away with the limitations of earlier
models :
● As isotopes have same atomic number, but different
atomic masses (due to different number of
neutrons in them), they could now be placed at
same position.
● Prediction of properties of elements could be made
with more precision when elements were arranged
on the basis of increasing atomic number.
It did away with the limitations of earlier
models :
● Position of H was sorted!
This table has 18 vertical Modern Periodic Table
columns, known as groups
& 7 horizontal rows known
as periods.
Features of Groups
Groups are not divided into sub-groups.
Elements present in a group have same no. of
valence electrons.
Elements have same valency.
No. of shells increases as we go down the group.
Elements in groups have identical chemical
properties.
The physical properties of elements in a group
vary gradually.
Features of Periods
Elements of a period do not have the same no
of valence electron.
They contain same no of shells.
The no of valence shell increases by one unit as
the atomic no. as the atomic no increases by
one unit on moving from left to right.
Chemical properties of elements changes as
the no. of valence shell electrons change.
The no. of elements in periods is based on how
electrons are filled into various shells.
The maximum number of electrons that can be
accommodated in a shell depends on the formula
2n2 where ‘n’ is the number of the given shell from
the nucleus.
For example,
K Shell – 2 × (1)2 = 2, hence the first period has 2
elements.
L Shell – 2 × (2)2 = 8, hence the second period has 8
elements. The third, fourth, fifth, sixth and seventh
periods have 8, 18, 18, 32 and 32 elements
respectively.
Position of the Elements in Modern
Periodic Table
In order to find the position of an element in the periodic
table, first write its electronic configuration and then find
period and group no. from its electronic configuration.
● The period no. of an electron = no. of electron
shells in its atom.
Example: if the atom of an element has 2 electron
shells (K and L), then it belongs to the 2nd period.
● If two or more elements have the same no. of
valence shells, then they belong to the same period
of the periodic table.
● The group no. of an element having upto two
valence electrons is = no. of valence electrons.
● If two or more elements have the same no. of
valence electrons then they belong to the same
group of the periodic table.
TRENDS IN THE MODERN
PERIODIC TABLE
The valency of an element is
determined by the number of valence
electrons present in the outermost
shell of its atom.
ATOMIC SIZE
● The term atomic size refers to the radius of
an atom.
● The atomic size may be visualised as the
distance between the centre of the nucleus
and the outermost shell of an isolated atom.
● It depends upon :
1. Effective nuclear charge
2. Number of shells
Variation Along a Group
❏ The atomic size increases down the group.
❏ This is because new shells are being added
as we go down the group.
❏ This increases the distance between the
outermost electrons and the nucleus so that
the atomic size increases in spite of the
increase in nuclear charge.
Variation Along a Period
❏ The atomic size decreases on moving from left to
right along a period.
❏ This is due to an increase in nuclear charge which
tends to pull the electrons closer to the nucleus
and reduces the size of the atom.
Metallic Properties
Elements having a tendency to lose one or more element
and form positive ions are called metals.
These are present on the left side as well as centre of
the periodic table.
The tendency of these elements to lose electrons is
called their metallic character, because if the
formation of positive ions, these are also called
electropositive elements.
Variation Along a Period and a Group
As the effective nuclear charge acting on the
valence shell electrons increases across a
period, the tendency to lose electrons will
decrease.
Down the group, the effective nuclear charge
experienced by valence electrons is
decreasing because the outermost electrons
are farther away from the nucleus.
Therefore, these can be lost easily. Hence
metallic character decreases across a period
and increases down a group.
Elements having a tendency to gain one or more element
and form negative ions are called non-metals.
These are present on the right side of the periodic
table.
These are also called electronegative elements. The
non-metallic character of the elements is due to their
electron accepting tendency.
Variation Along a Period and a Group
The electronegativity of the elements
increases along a period since the
non-metallic character increases.
Similarly, it decreases down the group
since the non-metallic character
decreases.
● Metalloids are elements which exhibits the
properties of both metals and nonmetals.
● In modern periodic table a zig-zag line separates
metals from nonmetals.
● The borderline elements- boron, silicone,
germanium, arsenic, antimony, tellurium, and
polonium are intermediate in properties so they
are called metalloids or semimetals.
Nature of Oxides
Oxides of the metals are of basic nature while
those of non-metals are acidic.
This means that along a period, the basic
character of the oxides of the elements
decreases while their acidic character increases.
On going down in a group of the periodic table,
the order is reversed, i.e., basic character of
oxides increases and acidic character of oxides
decreases.
Nature of Oxides
Oxide Name Nature
Na2O Sodium Oxide Strongly Basic
MgO Magnesium Oxide Basic
Al2O3 Aluminium Oxide Amphoteric
SiO2 Silicon Dioxide Weakly Acidic
P2O5 Phosphorous pentoxide Acidic
SO2 Sulphur Dioxide Strongly Acidic
Cl2O7 Chlorine Heptoxide Very Strongly
Acidic
* Amphoteric- react both as a base and as an acid.
Q. How does the atomic radius of the elements change
on going:
i) from left to right in a period, and
ii) down a group
In the Modern Periodic Table? Give reason in support
of your answer.
Ans. i) Atomic radius decreases.
Reason: Nuclear charge increases which tends to
pull the electrons closer to the nucleus.
ii) Atomic radius increases.
Reason: Number of shells increases on going
down the group.
Homework
Questions
1. According to Mendeleev’s periodic law
the elements were arranged in the
periodic table in the order of:
A. Increasing atomic number
B. Decreasing atomic number
C. Increasing atomic masses
D. Decreasing atomic masses
2. Element ‘X’ forms a chloride with the
formula XCl2, which is a solid with high
melting point. X would most likely be in the
same group of the periodic table as:
A. Si
B. Mg
C. Al
D. Na
