Question
Sat March 14, 2015 By: Varsneya Srinivas

How have you distributed the electrons in different shells? It doesn't follow the formula 2n squared & the rule that last shell will have 8 electrons.

Expert Reply
Hanisha Vyas
Mon March 16, 2015
Distribution of Electrons in the Orbits
  • According to Bohr’s model, electrons occupy certain stable orbits or shells. Each shell has a definite energy.
  • These orbits or shells are represented by the letters K, L, M, N… or the numbers 1, 2, 3, 4…
  • The maximum number of electrons present in the shell is given by the formula (2n2), where n is the orbit number or shell number.
  • The maximum number of electrons in different shells is as follows:
    • The first orbit or K shell will have 2 × 12 = 2 electrons.
    • The second shell will have 2 × 22 = 8 electrons.
    • The third shell will have 2 × 32 = 18 electrons.
    • The fourth shell will have 2 × 42 = 32 electrons and so on.
  • The maximum number of electrons which can be accommodated in the outermost orbit is 8.
  • The orbits or shells are filled in a step-wise manner.
  • Electrons are not accommodated in a given shell unless the inner shells are filled.

Octet Rule

The Octet rule states that- ‘The maximum number of electrons that the outermost shell of an electrically neutral and chemically stable atom can have is 8.’

Exception: If the atom has only one shell, it can hold only 2 electrons. For example, hydrogen and helium can have only 2 electrons (duplet).

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