PERIODIC CLASSIFICATION OF ELEMENTS (Dobereiner’s Traids (1817) (Triads…
PERIODIC CLASSIFICATION OF
Dobereiner’s Traids (1817)
When the elements were written in order of increasing atomic masses the atomic mass of the middle was the average of the atom mass of the other two elements.
Only three triads were recognised from the elements known at that time.
Ca Sr Ba
Li Na K
Cl Br I
It made chemists look at elements in terms of groups of elements with similar chemical and physical properties.
This eventually led to rigorous classification of elements and the modern periodic table of elements, as we know, it was discovered.
Newlands’ Octaves (1864)
Every eighth element showed
similar physical and chemical properties when the elements were placed in the increasing
order of their atomic masses.
It was not valid for elements beyond calcium.
When new elements like noble gases were discovered, his table had no place for them.
The periodicity Newlands saw in every eighth element.
The properties of elements are the periodic
functions of their atomic weights.
There are seven periods in the periodic table.
Properties of elements in a particular period show regular gradation from left to right
There are nine columns and they are numbered from I to VIII and Zero
Groups I to VII are subdivided into A and B subgroups. Group Zero and VIII do not have any subgroups.
All the elements in a particular group are chemically similar in nature.
Mendeleev’s was the first classification which included all the elements known at that time and even the elements discovered later.
Mendeleev had left some gaps in the periodic table. These gaps were for the elements yet to be discovered.
Scientists still use Mendeleev’s method to predict the properties of undiscovered elements by looking vertically across groups and horizontally across periods of elements.
Hydrogen resembles alkali metals ( it forms H+ ion like Na+ ion ) as well as halogens( it forms H- ion like Cl-
ion ). Therefore, it could be neither placed with alkali metals ( Group I ) nor with halogens ( Group VII ).
There was no proper position to isotopes in the periodic table.
At certain places, an element of higher atomic mass has been placed before an element of lower atomic mass
The properties of elements are the periodic functions of their atomic numbers.
There are 18 groups in the long form of the periodic table.
Elements in a given group have similar electronic configuration and have same number of valence electrons.
Elements in groups 1 and 2 on the left side and groups 13 to 17 on the right side of the periodic table are called normal elements or representative elements. Their outermost shells are incomplete. They are also called main group elements.
Elements in groups 3 to 12 in the middle of the periodic table are called transition
elements. Their two outermost shells are incomplete. It should be noted that
electrons are added to the last incomplete shell only in case of normal elements. In
case of transition elements, electrons are added to incomplete inner shells.
Group 18 on the extreme right side of the periodic table contains noble gases.
Elements below the main body of the periodic table are called inner transition elements. They contain lanthanides and actinides.
In a group, atomic radius increases from top to bottom.
Ionic radius goes on increasing from top to bottom in a group.
They have the same valency.
Ionisation energy of an element goes on decreasing from top to bottom in a group in the periodic table.
Electron affinity of an element goes on decreasing from top to bottom in a group in the periodic table
Electronegativity of an element goes on decreasing from top to bottom in a group in the periodic table
Metallic character of an element goes on increasing from top to bottom in a group in the periodic table
There are seven periods in the periodic table.
In each period, a new shell starts filling up.
First period is the shortest period of all and it contains only two elements H and He.
Sixth and seventh periods are very long periods containing 32 elements each.
Second and third periods are called short periods and they contain 8 elements each.
Fourth and fifth periods are called long periods and they contain 18 elements each.
The number of valence electrons increases in a period.
In a period, atomic radius generally decreases from left to right.
Among metals the cationic radius and among non – metals the anionic radius goes on decreasing across the period from left to right.
The ionization energy goes on
increasing from left to right in a period.
Electron affinity increases from left to right across the period.
Electronegativity goes on increasing from left to right in a period.
Metallic character of the element goes on decreasing from left to right across a period in
the periodic table.
The modern periodic table is based on atomic number which is a more fundamental property of an atom than atomic mass.
The table shows why elements in the same group display similar properties. The table also shows how and why the properties of elements differ in the same period.
The periodicity is related to electronic configuration.