Classification of Elements and Periodicity in Properties
Modern Periodic Table
Electronic Configuration in Periods
Period indicates the value of ‘n’ (principal quantum number) for the outermost or valence shell.
Successive periods in the periodic table are associated with the filling of the next higher principal energy level (n = 2, n = 3, etc).
First period (n = 1) → hydrogen (1s1) and helium (1s2) [2 elements]
Second period (n = 2) → Li (1s2 2s1), Be (1s2 2s2), B (1s2 2s2 2p1) to Ne (2s2 2p6) [8 elements]
Third period (n = 3) → filling to 3s and 3p orbitals gives rise to 8 elements (Na to Ar)
Fourth period (n = 4) → 18 elements (K to Kr) − filling of the 4s and 4p orbitals
3d orbital is filled up before 4p orbitals (3d orbitals → energetically favourable)
3d-transition series → Sc (3d1 4s2) to Zn (3d10 4s2)
Fifth period (n = 5) → 18 elements (Rb to Xe)
4d-transition series starts at Ytterbium and ends at Cadmium.
Sixth period (n = 6) → 32 elements; electrons enter 6s, 4f, 5d, and 6p orbitals successively. Elements from Z = 58 to Z = 71 are called 4f-inner transition series or lanthanoid series (filling up of the 4f orbitals).
Seventh period (n = 7) → electrons enter at 7s, 5f, 6d, and 7p orbitals successively. Filling up of 5f orbitals after Ac (Z = 89) gives 5f-inner transition series or the actinoid series.
Electronic Configuration in Groups
Same number of electrons is present in the outer orbitals (that is, similar valence shell electronic configuration).
Electronic configuration of group 1 elements is given in the following table.
1s2 2s1(or) [He]2s1
1s2 2s2 2p6 3s1(or) [Ne]3s1
1s2 2s2 2p6 3s2 3p6 4s1(or) [Ar]4s1
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1(or) [Kr]5s1
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1(or)[…
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