Modern Periodic Table

The arrangement of elements on the basis of their atomic masses left many anomalies in the position of different elements in the periodic table. Moreover the existence of isotopes showed that the atomic mass of an element is not the fundamental property of an element.

The modern periodic table is the result of discovery of atomic number by Moseley in 1914.

Based on the concept of atomic number Bohr, Werner and Bury proposed the Modern Periodic Law which states that,

"The physical and chemical properties of all elements are periodic Junctions of their atomic numbers.

In modern periodic table, also known as Bohr's Long Form Of Periodic Table, elements are arranged in order of their increasing atomic number. The elements having similar properties are repeated at regular intervals. As atomic number is related to the number of protons in an atom, so the real basis of periodicity of properties is due to recurrence of similar valency shell configuration of the next element in the same group. The modern periodic table is shown in table 4.4

The modern periodic table contains seven horizontal rows called periods and sixteen vertical columns called groups.

4.1.6 Periods:

The elements within a period have dissimilar properties from left to right across any period, the physical and chemical properties of elements change from metallic to non metallic along a period.

All periods except the first, start with an alkali metal with one electron in their valence shell and end up with zero group element with valence shell having 8 electrons, except He which has only 2 electrons.

The First Period:

It contains only two elements i.e. H and He. This period signifies the competition of K-shell or first orbit. It is the shortest period with two elements.

The Second and Third Period (Short Periods):

Each of these periods contains 8 elements. They signify the filling up of L-shell and M-shell respectively.

The second period starts with Li and ends up with Ne; whereas the third period starts with Na and ends at Ar.

The Fourth and Fifth Period (Long Periods):

Each of these periods contains 18 elements. In these periods the electrons fill M and N shells. Fourth period starts from K and ends at Kr. Fifth period starts from Rb and ends at Xe.

The Sixth Period (Longest Period):

It contains 32 elements. It starts from Cs and ends with Rn.

Besides, fourteen elements called Lanthanides, are placed at the bottom

of periodic table.     

The Seventh Period (Incomplete Period):

It starts with Francium (Fr). This period is incomplete as to date about 109 elements have been discovered.

This period also includes a group of fourteen elements starting from Actinium (Ac). These elements are called Actinides. They are also placed at the bottom of the table.


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