The periodic table organizes the elements according to increasing atomic number. Early scientists like Dobereiner, Newlands, and Mendeleev made attempts to classify elements, with Mendeleev devising the first successful periodic table. Mendeleev arranged elements with similar properties into columns and left gaps for elements not yet discovered. The periodic table provides information on elemental properties like valency, atomic size, metallic character, and electronegativity based on an element's position.

1. Periodic Classification
of Elements
The organization of all
elements known to man.
Made by:
Karma Dolkar, Anmol Pant, Kavya
Mishra and Harshul Malik {XC}

2. Why Do We Need Classification?
• With the passage of time, new elements are
being discovered. To study them in an orderly
manner and to predict their properties easily, we
require classification. The following scientists
have made some significant contributions in the
achieving of the same:
• Johann Wolfgang Dobereiner
• John Newlands
• Mendeleev
• Henry Moseley

3. Early Attempts
Dobereiner’s Triads
• In 1817, Dobereiner, a German
scientist, tried to arrange the
elements with similar
properties into groups. He
called these ‘triads’.
• If placed in the order of their
increasing atomic masses,
Atomic mass of middle
element = Atomic mass of 1+
Atomic mass of 2/2
• A major drawback was that
only 3 triads were formed.
Nevertheless, his attempt was
lauded.
Newlands’ Law Of
Octaves
• In 1866, John Newlands, an
English scientist, placed
elements in the increasing
order of their atomic masses.
• He found that every 1st
element had properties similar
to every 8th one, thus
resembling octaves in music.
• Drawbacks: Applicable up to
Ca only, assumption that only
56 elements existed, placing of
unlike elements together.

4. Major Contribution
Mendeleev is regarded as the main
contributor. When he started his
work, only 63 elements were known.
He took the compounds that
elements made with oxygen and
hydrogen as one of the basic criteria.
He then pinned the elements
according to their similarity on a wall.
He noticed a periodic recurrence and
gave the Periodic Law: ‘The
properties of an element are the
periodic functions of their atomic
masses’.

6. Mendeleev’s Periodic Table
 Mendeleev realized that the
physical and chemical properties
of elements were related to
their atomic mass in a 'periodic'
way, and arranged them so that
groups of elements with similar
properties fell into vertical
columns in his table.
 Gaps and predictions Sometimes
this method of arranging
elements meant there were gaps
in his horizontal rows or
'periods'. But instead of seeing
this as a problem, Mendeleev
thought it simply meant that the
elements which belonged in the
gaps had not yet been
discovered. He was also able to
work out the atomic mass of the
missing elements, and so predict

7. Advantages of Mendeleev’s
Table
Systematic arrangements of 63 elements in groups and
periods.
corrected doubtful masses of some elements like Be, Pd, Pt
etc.
Predicted properties of elements which were not discovered -
like scandium , gallium , germanium , which were correct and
left gaps for them.

8. Limitations of
Mendeleev’s
Table
 The position of hydrogen was not correctly defined. It was placed in
group I although it resembles both the group I elements - the alkali
metals and the group VII elements-the halogens, in their properties.
 In some cases Mendeleev placed elements according to their
similarities in properties and not in increasing order of their atomic
masses. Thus, the position of these elements was not justified e.g.
cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).
 Isotopes were not given separate places in the periodic table although
Mendeleev's classification is based on the atomic masses.
 Some similar elements were grouped separately while some dissimilar
elements were grouped together. For example copper and mercury are
similar in their properties but were placed separately. Copper was
placed in group I although it did not resemble the elements of this
group.
 Mendeleev could not explain the cause of periodicity in the elements.
 The position for lanthanides and actinides were not included in this
table.

11. Periodic Table
The periodic table organizes the elements in a
particular way. A great deal of information about an
element can be gathered from its position in the
period table.
For example, you can predict with reasonably good
accuracy the physical and chemical properties of the
element. You can also predict what other elements a
particular element will react with chemically.
Understanding the organization and plan of the
periodic table will help you obtain basic information
about each of the 118 known elements.

12. Key to the
Periodic TableElements are
organized on the
table according to
their atomic number,
usually found near
the top of the
square.
The atomic
number refers to
how many protons
an atom of that
element has.
For instance,
hydrogen has 1
proton, so it’s
atomic number is
1.

13. Valency-
The valency of an element is determined by the
number of valence electrons present in the outermost
shell of its atom.
 LEFT TO RIGHT : When we move from left to right first the
valency increases from 1 to 4, then it decreases from 4 to 0.
 TOP TO BOTTOM : Valency remains the same.
TRENDS IN THE MODERN PERIODICTABLE

14. 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 the isolated atom. (picometer, 1pm =
1×10−12 m). Example -atomic radius of hydrogen atom is 37 pm.
 FROM LEFT TO RIGHT : Moving from left to right across a period,
electrons are added one at a time to the outer energy shell. Electrons
within a shell cannot shield each other from the attraction to
protons. Since the number of protons are also increasing, the effective
nuclear charge increases across a period. This causes the atomic radius to
decrease.
 FROM TOP TO BOTTOM : Moving down a group in the periodic table,
the number of electrons and filled electron shells increases, but the
number of valence electrons remains the same. The outermost electrons
in a group are exposed to the same effective nuclear charge, but electrons
are found farther from the nucleus as the number of filled energy shells
increases.

15. Metallic properties
 The metals like Na, Mg are towards the left-hand side of the
periodic table.
 As metals tend to lose electrons while forming bonds, that
is, they are electropositive in nature.
 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 they
can be lost easily.
 Hence metallic character decreases across a period and
increases down a group.

16. Electronegativity
Electronegativity measures an atom's tendency to attract and
form bonds with electrons. This property exists due to the
electronic configuration of atoms. Most atoms follow the octet
rule (having the valence, or outer, shell comprise of 8 electrons).
Because elements on the left side of the periodic table have less
than a half-full valence shell, the energy required to gain
electrons is significantly higher compared with the energy
required to lose electrons. As a result, the elements on the left
side of the periodic table generally lose electrons when forming
bonds. Conversely, elements on the right side of the periodic
table are more energy-efficient in gaining electrons to create a
complete valence shell of 8 electrons. The nature of
electronegativity is effectively described thus: the more inclined
an atom is to gain electrons, the more likely that atom will pull
electrons toward itself.

17.  From top to bottom down a group, electronegativity
decreases. This is because atomic number increases down a
group, and thus there is an increased distance between the
valence electrons and nucleus, or a greater atomic radius.
 From left to right across a period of elements,
electronegativity increases. If the valence shell of an atom is
less than half full, it requires less energy to lose an electron
than to gain one. Conversely, if the valence shell is more than
half full, it is easier to pull an electron into the valence shell
than to donate one.