Periodic classification of elements gr10, 2020-21

Periodic classification of elements

Classification of elements
● The earlier attempt to classify elements involve dividing them into metals and
nonmetals.
● The classification was done by Antoine Laurent Lavoisier (Father of
chemistry).
● This method failed because most of the elements were metals and a very few
were non metals.
● Some elements resembled metals and nonmetals and hence they could not
be classified.

Dobereiner's triads
● In the year 1817, Johann Wolfgang Dobereiner,
a German chemist classified elements having
similar properties into groups called triads.
● When the elements were arranged in groups of 3, in the
increasing order of atomic masses, the middle element
has the atomic mass and properties roughly the average
of the other two elements.

Alkali metal group Alkaline earth metal group
Element At: mass Element At: mass
Li 7 Ca 40
Na 23 Sr 88
K 39 Ba 137
Halogen group
Element At: mass
Cl 35.5
Br 80
I 127

Newlands law of Octaves - John Newland
When elements are arranged in the increasing order of their atomic masses (from
H to Th), the properties of the 8th element resembled with the properties of the
first element. This was compared to the octaves found in music. Hence called
Newlands law of octaves.

Demerits of Newland’s Law of Octaves
● It was found that the Law of Octaves was applicable only upto calcium, as after
calcium every eighth element did not possess properties similar to that of the
first.
● It was assumed by Newlands that only 56 elements existed in nature and no
more elements would be discovered in the future. But, later on, several new
elements were discovered, whose properties did not fit into the Law of Octaves.
● In order to fit elements into his Table, Newlands adjusted two elements in the
same slot, but also put some unlike elements under the same note. Cobalt and
nickel are in the same slot and these are placed in the same column as fluorine,
chlorine and bromine which have very different properties than these elements.
Iron, which resembles cobalt and nickel in properties, has been placed far away
from these elements. Thus, Newlands’ Law of Octaves worked well with lighter
elements only.

Mendeleev’s Periodic Law - Dmitri Ivanovich
Mendeleev
When Mendeleev started his work , 63 elements were known.
He examined the relationship between the atomic masses of
the elements and their physical and chemical properties.
“ The properties of elements are a periodic function of their
atomic masses”

Features of Mendeleev’s Periodic table.
● The vertical columns were called groups and horizontal
rows were called periods. There were 6 periods and 8
groups in the table.
● The elements in each group resembles with each other in
many properties.
● All the elements in the group show the same valency.
● The properties like atomic size, MP and BP changed
gradually in any period while going from left to right.

Advantages of Mendeleev’s Periodic table.
● Mendeleev could classify all the 63 elements discovered at that time
on the basis of similarity in properties.
● He left gaps for elements yet to be discovered. He named them by
prefixing the sanskrit numeral, Eka (One) to the name of the
preceding element in the same group. Eg: Scandium, gallium and
germanium have properties similar to Eka-Boron, Eka-Aluminium and
Eka-Silicon respectively.
● He predicted the properties of the undiscovered elements and helped
in the discovery of these elements.
● His periodic table could accommodate noble gases when they were
discovered.
● He could correct the masses of certain elements with respect to their
position in the table.

Limitations of Mendeleev’s Periodic table
● A correct position for hydrogen could not be given in the
periodic table. Hydrogen resembles alkali metals and halogens.
● The position of isotopes could not be explained.
● Wrong order of atomic mass of some elements could not be
explained. Eg: Cobalt (Atomic mass 58.9) appeared before
nickel (Atomic mass 58.7).
● Atomic mass do not increase in a regular manner. So it was not
possible to predict how many elements could be discovered
between two elements.
● He does not make any attempt to separate metals from
nonmetals.

Position of Hydrogen
● Hydrogen resembles alkali metals -
They combine with halogens, oxygen
and sulphur to form compounds
having similar formula.
● Like halogens, hydrogen is diatomic and it combines with
metals and nonmetals to form covalent compounds.

Modern Periodic Law
Henry Moseley showed that atomic number is a more
fundamental property than atomic mass.
“ Properties of elements are periodic function
of their atomic number”

● Atomic number (Z) is the number of protons present in an
atom.
● Z increases by one in going from one element to the next.
● Prediction of properties of elements was more precise
with the arrangement of elements in the increasing order
of atomic number.

New names added to the periodic table.
● 113 - Nh -Nihonium - Japan
● 115 - Mc - Moscovium - Moscow region
● 117 - Ts - Tennessine - Tennessee
● 118 - Og - Oganesson - Yuri Oganessian

General features of long form periodic table
● The elements are arranged in the increasing order of
atomic number.
● There are 7 horizontal rows called periods.
● There are 18 vertical columns called groups.
● Elements having similar valence electrons are placed in
the same group.
● Each period starts with the filling of a new shell.

● The 14 elements after Lanthanum are called lanthanoids.
● The 14 elements after Actinium are called actinoids
● To limit the size of the periodic table and to keep the
elements with similar properties together, Lanthanoids
and Actinoids are placed at the bottom of the periodic
table.
● The elements of group1, 2 and 13-17 (s-block and p-
block) are called representative elements or main group
elements. They have completely filled inner shells but
incomplete valence shell.
● The elements of group 3-12(d-block) are called
transition elements. They have incomplete outer shells

● The elements placed at the bottom of the periodic table (f-
block) are called inner transition elements. They are also
called rare earth metals because they occur rarely in the
earth's crust.
● Group 18 elements are called noble gases or inert
gases. Their outer orbits are completely filled and are
unreactive.
● A zig-zag line separates metals from nonmetals by some
elements called metalloids or semimetals (B, Si, Ge, As,
Sb, Te and Po)
● Metals are found on the left side of the periodic table and
nonmetals on the right side.

Important points to note
● Elements in any group have same number of valence
electrons.
● Size of the atom decreases across a period and increases
down a group.
● Each period starts with a new shell.
● Maximum number of electrons in each shell is given by
the formula 2n2, where n is the number of the given shell.
(K=1, L=2, M=3, ….)

Characteristics / trends of Modern periodic table
1. Valency - It is the combining capacity of an element. It
denotes the number of electrons lost or gained during a
chemical reaction.
● On moving from left to right in a period, valency of the
elements first increases from 1 to 4 and then decreases to
zero.
● All the elements in a group has same valency, as they
have same number of valence shell electrons.

2. Atomic size:
● On moving from left to right in a period, the atomic size
(atomic radius) of the atom decreases.
● As we move from left to right in a period, the atomic
number increases, which means the number of protons
also increases. Due to the large positive charge on the
nucleus, the electrons are pulled in more closer to the
nucleus and size of the atom decreases.
● Down a group size of atom increases, as new shells are
added up.

3. Metallic character:
● Metals are found on the left hand side of the table and
nonmetals on the right hand side.
● Metals has a tendency to lose electrons - electropositive.
● Nonmetals has a tendency to gain electrons -
electronegative.
● On moving from left to right in a period, the electropositive
character decreases and electro negative character
increases.
● On moving from left to right in a period, the tendency of
atoms to lose electrons decreases and tendency of atoms to
gain electron increases.

● On moving down a group the metallic character of the
elements increases or the electropositive character of the
element increases.
● As we go down in group 17, the electronegative character
(nonmetallic character) of elements decreases.
● Fluorine is the most electronegative element.

4. Chemical reactivity.
● Chemical reactivity of metals increases on going down a
group.
● Chemical reactivity of nonmetals decreases on going
down a group
● On moving from left to right in a period, the chemical
reactivity of elements first decreases and then increases.

5. Nature of oxides.
● On moving from left to right in a period the basic nature of
oxides decreases and the acidic nature of oxides
increases.
● On moving down a group there is no change in the nature
of oxides of elements
● All elements of group 1 form basic oxides
● All the elements of group 17 form acidic oxides

6. Ionisation Energy
● The energy required to remove the most loosely bound
electron from a gaseous neutral atom in the ground state
is known as Ionisation energy.
● It is expressed in Kj/mol.
● Ionisation energy increases with the increase in atomic
number across a period, due to the increase in nuclear
charge and decrease in atomic size.
● Ionisation energy decreases down a group due to the
increase in atomic size and increase in the shielding
effect.

Reactivity of metals and nonmetals

Periodic classification of elements gr10, 2020-21

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