The modern periodic table of elements is based on Mendeleev’s observations. The only difference is instead of being organized by atomic masses, the modern table is arranged by atomic number (Z). Henry Moseley, in 1913, found that atomic number is more fundamental property of an element than its atomic mass. Atomic number helps in arranging the elements according to their electronic configuration as it is indirectly equal to the number of electrons of a neutral atom.
Atomic number of an element is equal to the number of protons in an atom of the element since number of protons equal to the number of electrons in a neutral atom.
Thus, atomic number of an element (Z) = number of protons present = number of electrons present in a neutral atom.
Why atomic number is better choice of classification than atomic mass?
This is because-
(1) Atomic number increases by one unit (not fractional)when we move from one element to the next. This pattern does not follow in atomic mass.
(2) Atomic number of an element is fixed and exclusively denote that particular element only. No two elements can have same atomic number.
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Modern Periodic Law
Henry Mosley modified Mendeleev’s periodic law by considering the fact that atomic number of an element is a more fundamental characteristic than its atomic mass. Modern periodic law states,
The properties of elements are a periodic function of their atomic number.”
It clearly means that when elements are arranged in their increasing order of atomic number, the properties of elements repeat after a certain intervals. This is known as periodicity in properties.
Why periodicity in properties occur?
The reason behind periodicity in the properties of the elements is their same number of valence electrons (electrons present in outermost shell) or similar (alike not identical) electronic configuration.
How are electrons distributed in different orbits(shells)? Or How can be found electronic configuration of an element?
According to Bohr’s model of an atom, only certain energy levels or orbits or shells around a nucleus is possible where electron can revolve without radiating energy . Such orbits or shells are represented by K, L, M, N, … or the numbers n = 1, 2, 3, 4…
The maximum number of electrons present in a shell can be found by a formula 2n², where ‘n’ is the orbit or shell or energy level number.
Shells or orbits or energy levels are always filled in a stepwise manner. It means, when inner orbit(s) is occupied with maximum number of electrons, then only next orbits can be filled. For e.g., when K shell is filled with 2 electrons(fully occupied)then only next electrons go in L shell. And when L shell is occupied with 8 electrons then next electrons go in M shell and process proceeds further.
Consider electronic configurations of the elements from Li(at.no.3) to Ar(at.no.18), which are arranged in following table in order of increasing atomic number in a period and similarity in their valence electrons kept in a group.
Li (at. no.= 3) has three electrons which are occupied in K (2 electrons) and L (1 electron) shells.
Thus, Li contains one valence electron. As the atomic number increases, the next coming electron go in L shell till it is occupied fully (Ne=2,8). Further , the next electron goes in M shell (Na=2,8,1) as K and L shells are completely occupied. It should be noticed that again Na has one valence electron which produce similarity with Li in properties. It is applicable with other elements also which have similar valence electrons and so similar properties.
Thus it is concluded that periodicity in properties occur only when elements have same valence electrons.