Periodic Classification of Elements class 10 chemistry
Need
for the Periodic Classification of Elements:-
When
there were only 31 elements it was relatively easy to study the properties of
these chemical elements individually. Due to discovery of new elements it is
difficult to study the properties of every element individual and making it
difficult to remember the behavior and properties of these elements. This
created the need for the periodic classification of elements.
Advantages
of classification:
- Classification
of elements provides as a fixed pattern in which the element change their
properties.
- It
is also make easy to understand how similar elements form the compounds and so
predict the compound formation.
- Helps in discovering new elements
1.
Lavoisier's Classification (1789)
- 33 elements
Lavoisier's
classification was based on the basis of the physical properties of the
elements, such as hardness, malleability and lustre. He classified the elements
into two groups, i.e. as metals and non metals. The first elements,
he classified, was sodium and lead.
Limitation:-
Hardness,
malleability and Luster were found to be the only common properties of sodium
and lead otherwise the two elements are entirely different.
So this method of classification was found to
be inadequate for classification of all the elements.
1.
Johann Wolfgang
Döbereiner(1829)
He grouped the elements
with similar chemical properties into clusters of three, called 'Triads'. When
elements were arranged in order of their increasing atomic mass, the atomic
mass of the middle element was approximately the arithmetic mean of the other two
elements of the triad.
Traid 1:
Element |
Atomic mass |
Lithium (Li) |
7 |
Sodium (Na) |
23 |
Potassium (K) |
39 |
Traid 2:
Element |
Atomic mass |
Chlorine (Cl) |
35.5 |
Bromine (Br) |
80 |
Iodine (I) |
127 |
Traid 3:
Element |
Atomic mass |
Calcium (Ca) |
40 |
Strontium (Sr) |
88 |
Barium (Ba) |
137 |
Drawback of Triad
Classification
Dobereiner could classify only 9 elements ( 3 Traids) in such manner,
out of all that were known at the time.
- All the known
elements could not be arranged in the form of triads.
- This law did not hold good for
elements with very low or very high atomic mass.
Example: The arithmetic mean of the atomic masses of fluorine 19 and bromine 80, which comes to 49.5, varies significantly from the atomic mass of chlorine, which is 35.5.
Newland's Classification (53 element)
John Newlands arranged the known
elements in the order of increasing atomic masses. He started with the element
having the lowest atomic mass (hydrogen) and ended at thorium which was the
56th element.
He found that every eighth element had properties
similar to that of the first. He compared this to the octaves found in music.
Therefore, he called it the 'Law of Octaves'. It is known as 'Newlands' Law of
Octaves'.
In Newlands' Octaves, the properties of lithium and sodium were found to be the same. Sodium is the eighth element after lithium.
Drawbacks of Newland's Octave Law
1.
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.
2. 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.
Ex. 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.
3. Newlands' Law of Octaves worked well with lighter elements only.
Advantages of New/land's Octave Law
- This
law provided a basis for the classification of element into groups of elements
having similar properties.
- This
law provided a wider scope to arrange all known elements into a tabular form.
1. Mendeleev’s Periodic Table
At the time of Mendeleef, only 63 elements had
been discovered.
Mendeleev’s Periodic Law: The physical and chemical properties of
the elements are a periodic
function of their atomic masses.
Mendeleev
arranged the elements known at that time in order of increasing atomic masses and this arrangement was
called periodic table. He found the oxides and hydrides of elements, and on
basis of their oxides and hydrides he arranges elements in a group.
Elements
with similar characteristics were present in vertical rows called groups. The
horizontal
rows were known as periods.
Description of Mendeleev’s Periodic Table
(i) In the periodic table, the elements are
arranged in vertical rows called groups and horizontal rows known as periods.
(ii) There are eight groups indicated by Roman
Numerals as I, II, III, IV, V, VI, VII, VIII. Group VIII consists of nine
elements which are arranged in three triads.
(iii) There are seven periods (numbered from 1
to 7) or, horizontal rows in the Mendeleev’s periodic table.
Merits of Mendeleev's Periodic Table:
1. Mendeleev was first to properly
arrange and classify all elements and was able to explain the periodicity in
their chemical properties.
2. Mendeleev left some
gaps for undiscovered elements. He predicted that some elements would be
discovered in future. He also predicted properties of those undiscovered
elements.
There were many vacant spaces in Mendeleev's
periodic table for the elements to be discovered. Ex. Eka-Boron, Eka-Aluminium and Eka-silicone .These
elements were discovered as Sc ,Ga and Ge with same features as he
predicted.
3. Mendeleev corrected the atomic masses of
certain elements with the help of their expected positions and properties.
Demerits
in Mendeleev’s Periodic Table
(i) Hydrogen has been placed in group IA along
with alkali metals. But it also resembles halogens of group VII A in many
properties. Thus, its position is the Mendeleev’s periodic table is
controversial.
(ii) The isotopes of an element have different atomic masses but
same atomic number. Since, periodic table has been framed on the basis of
increasing atomic masses of the elements, different positions must have been
allotted to all the isotopes of a particular element.
Therefore – protium (H-1),
deuterium(H-2), and tritium (H-3) would occupy varying positions in Mendeleev’s table.
(iii) Although the elements in the
Mendeleev’s periodic table have been arranged in order of their atomic masses,
but in some cases the element with higher atomic mass precedes the element with
lower atomic mass. Ex. Place
cobalt before Nickel though Cobalt has more mass.
(iv) According to Mendeleev, the elements placed
in the same group must resemble in their properties. But there is no similarity
among the elements in the two sub-groups of a particular group.
(v) In some
cases, elements with similar properties have been placed in different groups.ex-
Cu and Hg.
Modern Periodic Table- by Henry Moseley (1913)
Modern Periodic Law- The modern periodic law
states that the physical and chemical properties of the elements are the
periodic functions of their atomic numbers.
Why atomic number and not atomic mass?
Atomic
mass is the total mass of the protons and neutrons present in a nucleus of an
atom. Whereas, the atomic number is the number of protons in a nucleus. Also,
the number of protons in the nucleus is equal to the electrons present outside
the nucleus.
We
know that the nucleus is deep-seated inside an atom. But the electrons outside
it, especially the ones in the outermost shell, are free to move around. Hence
they take part in chemical reactions. For this reason, the properties of an
element depend on the atomic number rather than the atomic mass.
Classifications of Elements in the Periodic Table
The modern periodic table of elements can be
broken down into 4 blocks –
1.
the
s-block, the p-block,( representative elements)
2.
the
d-block, (Transition Elements)
3.
f-block.(
Inner Transition Elements)
This classification is based on the types of
orbitals that contain the valence electrons of the element. For example,
s-block elements have valence electrons in s-orbitals and d-block elements have
their valence electrons in d-orbitals.
The modern periodic table consists of 18
vertical columns and 7 horizontal rows.
Groups in the Modern
Periodic Table
·
Groups are the vertical columns in the
modern or long form of the periodic table.
·
There are 18 groups in the periodic table.
·
These groups are numbered from 1 to 18.
Each group consists of elements having the same outer shell electronic configuration
|
Group number |
Group name |
Property |
|
Group 1 or IA |
Alkali metals |
They form strong alkalis with water |
|
Group 2 or IIA |
Alkaline earth metals |
They also form alkalis but weaker than group
1 elements |
|
Group 13 or IIIA |
Boron family |
Boron is the first member of this family |
|
Group 14 or IVA |
Carbon family |
Carbon is the first member of this property |
|
Group 15 or VA |
Nitrogen family |
This group has non-metals and metalloids |
|
Group 16 or VIA |
Oxygen family |
They are also known as chalcogens |
|
Group 17 or VIIA |
Halogen family |
The elements of this group form salts. |
|
Group 18 |
Zero group |
They are noble gases and under normal
conditions they are inert. |
Periods in the Modern
Periodic Table
·
Periods are the horizontal rows in the
modern or long form of the periodic table.
·
There are 7 periods in the periodic table.
·
These are
numbered as 1, 2, 3, 4, 5, 6 and 7 from top to bottom.
·
The 1st period consists of only two
elements i.e hydrogen & helium. So called very short perod.
·
While the 2nd and 3rd period consists of 8
elements each and called as short period
·
The 4th and 5th period consists of 18
elements each called long period.
·
On the other hand, the 6th period consists
of 32 elements and called longest period.
·
The long form of the periodic table
consists of a separate panel at the bottom. It consists of 14 elements of the
6th period called the lanthanoids . And 14 elements in the 7th period called the actonides.
·
Each period represents the number of
shells or energy levels present in an atom of an element.
Trends of properties in Modern Periodic
table:
1) Atomic Radius
Atomic radius is one-half the distance between the nuclei of two
adjacent atoms
Across
the table, the atomic number increases moving left to right and the effective
nuclear charge also increase i.e. when moving left to right across a period the
nucleus has a greater pull on the outer electrons and the atomic radii
decreases.
when move down a group in the table,
the number of filled electron shells increases. However, the valence electrons
keep the same effective nuclear charge, As a result, the
nucleus has less of a pull on the outer electrons and the atomic radii are
larger.
2) Ionization Energy (Ionization Potential)
The ionization energy (I.E. or I)
is the energy that is required to completely expel an electron from a
gaseous atom or an ion. The Ionization Energy is always positive.
The energy required to remove one
valence electron is the first ionization energy, the second ionization energy
is the energy required to remove a second valence electron, and so on.
As you move left to right across a
period, ionization energies increase. Also, when you go down a
group, ionization energy decreases. This is because size decrease and
effective nuclear force increase.
3) Electron Gain Enthalpy
The energy released when a neutral
isolated gaseous atom accepts an extra electron to form the gaseous negative
Ion. Therefore, the electron gain
enthalpy is negative.
As you move left to right across a
period, electron gain enthalpy increase. Also, when you go down a
group, electron gain
enthalpy decreases.
This is because size decrease and effective nuclear force increase.
4) Electronegativity
Electronegativity of an element is the tendency of
its atoms to attract the shared pair of electrons towards itself in a covalent bond. The higher the
value of the electronegativity, the more strongly that element attracts the
shared electrons.
Electronegativity decrease from top
to bottom in groups, and increases from left to right across
periods. Thus, fluorine is the most
electronegative element, while francium is one of the least
electronegative.
5) Electropositivity:
Electropositivity can be defined as the
tendency of an atom to donate electrons and form positively charged
cations.
Electropositivity increase from top to bottom
in groups, and decrease from left to right across periods.
6) Metallic Character of a Metal
Metallic character refers to the level of
reactivity of a metal. Metals tend to lose electrons in chemical reaction, as indicated by their
low ionisation energies. Within a compound, metal
atoms have a relatively low attraction for electrons. This is indicated by
their low electronegativities.
The most reactive metal is
caesium, which is not found in nature as a free element.
Metallic properties decrease left to
right in a period while increase from top to bottom.
7) Non-metallic Character of a
Metal
element fluorine is the most
reactive nonmetal.
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