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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.

Atomic number

Symbol

Electronic configuration

3

Li

1s2 2s1(or) [He]2s1

11

Na

1s2 2s2 2p6 3s1(or) [Ne]3s1

19

K

1s2 2s2 2p6 3s2 3p6 4s1(or) [Ar]4s1

37

Rb

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1(or) [Kr]5s1

55

Cs

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 6s1(or)[…

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