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1. Chapter 5. Periodic
Classification of Elements
Nitesh Kumar
TGT Science, KV

2. Introduction
Matter all around is present in the form of
elements, compounds and mixtures. Element is
the most basic form of matter (Anything that
have mass and occupy space is called matter).
In an element all atoms are identical. Or we can
say that element is made up of only one type of
atoms. There are 118 known elements in our
universe. Nearly all the elements are found on
earth.

3. Why do we need to classify them?
Studying this much large number of elements
and their compounds is difficult. That is why we
need to classify them according to their
properties. We organise elements in order to find
a pattern of properties. This pattern help us to
classify elements in specific categories. The
classification table is known as Periodic Table.

4. 5.1 MAKING ORDER OUT OF CHAOS –
EARLY
ATTEMPTS AT THE CLASSIFICATION OF
ELEMENTS
Always remember similarity in properties is the basis of
classification

5. 5.1.1 Dobereiner’s Triads
Dobereiner identified some groups having three
elements each. He called these groups ‘triads’.
He found that when the three elements in a triad
are written in the order of increasing atomic
mass, the atomic mass of middle element was
roughly the average of the other atomic masses
of other two elements.

7. • For example, take the triad consisting of
lithium (Li), sodium (Na) and potassium (K)
with the respective atomic masses 7.0, 23.0
and 39.0. What is the average of the atomic
masses of Li and K?
• The average is (7+39)/2
Or 46/2
Or 23.

8. Table 5.1 table of elements
showing pattern in atomic masses.

9. 5.1.2 Newlands’ law of Octaves
John Newlands, an English scientist (1866) arranged
56 elements from hydrogen(H) to thorium(Th) in
the increasing order of their atomic masses. He
found that every eighth element had the same
properties as that of the first one. For example, Na
had the same properties as of Li and Be had the
same properties as that of Mg.
This is similar to the eight notes found in music.
Because of this similarity, Newlands called this law
as ‘law of octaves’. And we know this law as
‘Newlands’ law of octaves’.

10. He prepared the following table-

11. It was found that this law was applicable only upto
calcium. After calcium elements did not follow this
rule.
Newlands thought that only 56 elements existed in
nature. But later on, more elements were
discovered.
To fit these elements into his table, Newlands
adjusted two elements in the same slot. But also put
unlike elements in the same category. For example,
he put Co and Ni along with F, Cl, and Br which
have different properties. And put Fe far away from
Co and Ni.
With the discovery of noble gases, the law of
octaves became irrelevent.

12. 5.2 MAKING ORDER OUT OF CHAOS –
MENDELÉEV’S PERIODIC TABLE

13. Many scientists continued to search for a pattern
that correlated the properties of elements with
their atomic masses.
The main credit for classifying elements goes to
Dmitri Ivanovich Mendeléev, a Russian chemist,
wherein the elements were arranged on the basis
of their fundamental property, the atomic mass,
and also on the similarity of chemical properties.
When Mendeléev started his work, 63 elements
were known. He examined the relationship
between the atomic masses of the elements and
their physical and chemical properties.

14. He observed that most of the elements got a
place in a Periodic Table and were arranged in
the order of their increasing atomic masses. It
was also observed that there occurs a periodic
recurrence of elements with similar physical and
chemical properties. On this basis, Mendeléev
formulated a Periodic Law, which states that ‘the
properties of elements are the periodic function
of their atomic masses’. Mendeléev’s Periodic
Table contains vertical columns called ‘groups’
and horizontal rows called ‘periods’.

16. Question- Write down the achievements and
limitations of Mendeleev's periodic table.
(home work)

17. 5.3 MAKING ORDER OUT OF CHAOS –
THE MODERN PERIODIC TABLE

18. In 1913, Henry Moseley showed that the atomic
number (Z) of an element is a more fundamental
property than its atomic mass. Accordingly,
Mendeleev’s Periodic Law was modified and
atomic number was adopted as the basis of
Modern Periodic Table and the Modern Periodic
Law can be stated as follows-
‘Properties of elements are a periodic function
of their atomic number.’

21. 5.3.1 Position of elements in the modern
periodic table
Vertical columns are called ‘groups’. These are
18 in number. Horizontal rows are called
‘periods’. These are 7 in number.
All elements in a group have same number of
electrons in the outermost shell (valence
electrons).
All elements in a period have same number of
shells. But each next element is increased by one
electron.

22. Question- How many elements are there in the
first, second, third and fourth periods?
Number of electrons in a shell=
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 position of an element in the periodic table
tells about its chemical reactivity.
Note- 2 × (2)2 will be read as two.into.two.squared

23. 5.3.2 Trends in the Modern Periodic Table
The modern periodic table have regular patterns
or trends in the following parameters-
1. Valency
2. Atomic size
3. Metallic and non-metallic properties
4. Atomic mass
5. Ionic radii not in syllabus
6. Electronegativity

24. 1. Atomic size
The term atomic size refers to the radius of an atom.
The atomic size is the distance between the centre of
the nucleus and the outermost shell of an isolated
atom. The atomic radius of hydrogen atom is 37 pm.
(picometre, 1 pm = 10–12m).
Atomic radius decreases in 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.
Atomic size increases down the group. This is
because new shells are being added as we go down
the group.

25. 2. Metallic and non-metallic properties.
Metals are found on the left hand side of the
periodic table and non-metals are found on the right
hand side of the periodic table. A zig-zag line
separates metals from non-metals. The borderline
elements – boron, silicon, germanium, arsenic,
antimony, tellurium and polonium – are
intermediate in properties and are called metalloids
or semi-metals. Metals have a tendency to lose the
electrons, hence are electropositive in nature. Non-
metals have a tendency to accept the electrons,
hence are electronegative in nature.

26. Home work
1. Write an additional note on metallic and non-
metallic properties of elements.
2. Write a note on valency of the elements in the
periodic table
3. Intext questions and their answers.
4. Summary of the chapter.
5. Exercise questions and answers.