Chapter 5. Periodic Classification of Elements
Modern Periodic Table:
Atomic number of an element is more fundamental properties than its atomic masses.
In 1913, Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass as described below. Accordingly, Mendeleev’s Periodic Law was modified and atomic number was adopted as the basis of Modern Periodic Table.
The atomic number gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next. Elements, when arranged in order of increasing atomic number Z, lead us to the classification known as the Modern Periodic Table.
Modern Periodic Law:
‘Properties of elements are a periodic function of their atomic number in modern periodic table.’ This law is known as modern periodic law.
Position of Elements in the Modern Periodic Table :
The Modern Periodic Table has 18 vertical columns known as ‘groups’
and 7 horizontal rows known as ‘periods’.
Distinguish between Modern and Mendeleev’s Periodic table.
Modern Periodic table
Mendeleev’s Periodic table
(1) Elements are arranged in increasing order of their atomic numbers.
(2) There are 18 column and 7 raw.
(3) Isotopes are kept at the same place of their corresponding elements as they have same atomic numbers.
(4) There are no anomalies in the modern periodic table.
(1) Elements are arranged in increasing order of their atomic masses.
(2) There are 8 column and 6 raw.
(3) There is no position explained for isotopes.
(4) Mendeleev’s periodic table had a number of anomalies or defects.
The Position of Noble Gases:
Noble gases are placed in a separates group.
There are following reasons to place the noble gases in a separate group:
(i) Isotopes were discovered long after Mendeleev's periodic table, but they could be placed in without changing the previous table's setting.
(ii) These all are similar properties and they are very unreactive becausetheir valency is zero.
(iii) Their octaves are completed and they do not make any partnership to any element. They are also monoatomic.
Groups in the Periodic Table signify an identical outershell electronic configuration. On the other hand, the number of shells increases as we go down the group.
Elements have same valence electrons in a same group.
When we move forward in a period then we see that:
(i) These elements do not have the same number of valence electrons, but they contain the same number of shells.
(ii) The number of valence shell electrons increases by one unit, as the atomic number increases by one unit on moving from left to right in a period.
(iii) Each period marks a new electronic shell getting filled.
(iv) The maximum number of electrons that can be accommodated in a shell depends on the formula 2n2 where ‘n’ is the number of the given shell from the nucleus.
There are only 8 elements in 2nd and 3rd periods as L and M shell are filled which can not be filled more than 8 electrons.
Position of elements in Modern Periodic Table:
The position of an element in the Periodic Table tells us about its chemical reactivity. As you have learnt, the valence electrons determine the kind and number of bonds formed by an element.
Valency : As you know, the valency of an element is determined by the
number of valence electrons present in the outermost shell of its atom.