CHAPTER-5
PERIODIC CLASSIFICATION OF ELEMENTS
Elements: All the elements have been divided into a few groups in such a way that
elements in the same group have similar properties
DOBEREINER'S TRIADS:-
When elements
are arranged in the order of increasing atomic masses, groups of three elements having similar chemical properties are obtained. The atomic
mass of the middle element of the triad being equal to the arithmetic mean of
the atomic masses of the other two elements.
The Alkali Metal Group
(i) All these elements are metals
(ii)All of them react with water to
form alkalis and hydrogen gas,
(iii)All of them have a valency of 1
example-Lithium,Sodium,Potassium
The Alkaline Earth Metal Group
(i)All these elements are metals
(ii)The oxides of all of them are
alkaline in nature
(iii)All these elements have a
valency of 2
example-calcium,Strontium,Barium
The Halogen Group
(i)All these elements are non-metals
(ii)All these elements react with water
to form acids
(iii)All these elements have a valency of
1
The limitation of Dobereiner’s classification
The limitation of Dobereiner’s classification was that it failed to
arrange all the then known elements in the form of triads of elements
having similar chemical properties. Dobereiner could identify only three
triads from the elements known at that time. So, his classification of
elements was not much successful.
NEWLANDS' LAW OF OCTAVES
When elements are
arranged in the order of increasing
atomic masses, the properties of the
eighth element (starting from a given
element) are a repetition of the
properties of the first element
Figure . Classification of elements based on Newlands’ law of octaves.
- Newlands' classification of elements based on his law of octaves, however, gave a very important conclusion that there is some systematic relationship between the order of atomic masses and repetition of properties of elements.
Limitations
- Applicable only upto calcium
- properties of new elements couldn't fit in it.
- in some cases properties of the elements were not same as defined by octave. eg Fe was placed far away from Co and Ni.
- noble gases could be placed without disturbing the existing order.
MENDELEEV’S PERIODIC TABLE
when elements are arranged in the order of increasing atomic masses, the elements with similar properties occur at regular intervals.
There were seven periods (horizontal rows) and eight groups (vertical columns) in the original periodic
table of Mendeleev.
We will now
describe two main features of Mendeleev’s periodic classification : gaps in the periodic table, and wrong
order of atomic masses of some of the elements.
Modern Periodic Law :- Given by Henry moseley in 1913
The properties of elements are a periodic function of their atomic numbers.
The modern periodic table was prepared by Bohr. It is also known as long form of periodic table.the arrangement of elements in the modern (long form) periodic table is based on their electronic configurations.
LONG FORM OF PERIODIC TABLE
4. Metallic Character
On moving from left to right in a period, the metallic character of elements decreases (but the nonmetallic character increases).
5. Chemical Reactivity
On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases.
6. 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.
- elements with similar properties could be grouped together
- some gaps were left for the undiscovered element
- noble gases could be placed without disturbing the existing order
Limitations
- no fixed position for hydrogen
- no place for isotopes
- no regular trend in atomic mass
- Co was placed before and Ni
Modern Periodic Law :- Given by Henry moseley in 1913
The properties of elements are a periodic function of their atomic numbers.
- When the elements are arranged according to increasing atomic numbers, then the elements having same number of valence electrons occur at regular intervals (or periods).
- Atomic number denoted by Z and equal to the number of protons in the nucleus of an atom
- Modern Periodic Table contains 18 vertical columns known as groups and 7 horizontal rows known as periods
- Number of the shells increase as we go down the group
- Position of the element in the periodic table tells about its reactivity
MODERN PERIODIC TABLE
(OR LONG FORM OF PERIODIC TABLE)
The modern periodic table was prepared by Bohr. It is also known as long form of periodic table.the arrangement of elements in the modern (long form) periodic table is based on their electronic configurations.
LONG FORM OF PERIODIC TABLE
CHARACTERISTICS OF PERIODS
1. Valence Electrons
On moving from left to right in a period, the number of valence electrons in elements increases from
1 to 8
2. Valency
On moving from left to right in each short period, the valency of elements increases from 1 to 4
and then decreases to 0 (zero).
3. Size of Atoms
On moving from left to right in a period of the periodic table, the size of atoms
decreases (or atomic size decreases).
On moving from left to right in a period, the metallic character of elements decreases (but the nonmetallic character increases).
5. Chemical Reactivity
On moving from left to right in a period, the chemical reactivity of elements first decreases and then increases.
6. 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.
CHARACTERISTICS OF GROUPS
1. Valence Electrons
All the elements of a group of the periodic table have the same number of valence electrons.
2. Valency
all the
elements in a group have the same valency.
3. Size of Atoms
On going down in a group of the periodic
table, the size of atoms increases (or atomic size
increases).
4. Metallic Character
On going down in a group of the periodic table, the
metallic character of elements increases.
5. Chemical Reactivity
(i) The chemical reactivity of metals increases on going down
in a group of the periodic table.
(ii) The chemical reactivity of nonmetals decreases on going down in a
group of the periodic table.
6. Nature of Oxides
On going down in a group of the periodic table, there is no change in the nature of oxides of elements.
Advantages of the Periodic Table
1. The periodic table has made the study of chemistry systematic and easy.
It acts as an aid to memory.
2. It is easier to remember the properties of an element if its position in
the periodic table is known.
3. The type of compounds formed by an element can be predicted by
knowing its position in the periodic table.
4. A periodic table chart is used as a teaching-aid in chemistry in schools and colleges. \
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