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 (1s^{1}) and helium (1s^{2}) [2 elements]

Second period (n = 2) → Li (1s^{2} 2s^{1}), Be (1s^{2} 2s^{2}), B (1s^{2} 2s^{2} 2p^{1}) to Ne (2s^{2} 2p^{6}) [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)

3dtransition series → Sc (3d^{1} 4s^{2}) to Zn (3d^{10} 4s^{2})

Fifth period (n = 5) → 18 elements (Rb to Xe)

4dtransition 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 4finner 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 5finner 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.

Atomic number
Symbol
Electronic configuration
3
Li
1s^{2}^{ }2s^{1}(or) [He]2s^{1}
11
Na
1s^{2}^{ }2s^{2}^{ }2p^{6}^{ }3s^{1}(or) [Ne]3s^{1}
19
K
1s^{2}^{ }2s^{2}^{ }2p^{6}^{ }3s^{2}^{ }3p^{6}^{ }4s^{1}(or) [Ar]4s^{1}
37
Rb
1s^{2}^{ }2s^{2}^{ }2p^{6}^{ }3s^{2}^{ }3p^{6}^{ }3d^{10}^{ }4s^{2}^{ }4p^{6}^{ }5s^{1}(or) [Kr]5s^{1}
55
Cs
1s^{2}^{ }2s^{2}^{ }2p^{6}^{ }3s^{2}^{ }3p^{6}^{ }3d^{10}^{ }4s^{2}^{ }4p^{6}^{ }4d^{10}^{ }5s^{2}^{ }5p^{6}^{ }6s^{1}(or)[…
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