CBSE Class 9 Answered

In 1913 Neils Bohr put forward the following postulates about the model of an atom:

• Electrons revolve around the nucleus only in certain permissible circular orbits. This orbit is called shell. The first shell from the nucleus is 'K' shell The subsequent shells are L, M, N, 0,..respectively.
• Electrons in each shell have been associated with definite amount of energy. Electrons in higher shell have more energy than those nearer to the nucleus.
• The energy of an electron remains constant as long as it revolves in its own shell. The shells are also called energy levels or stationary energy levels.
• According to Bohr's model, electrons occupy certain stable orbits or shells. Each shell has 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 numbers of electrons in different shells are 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 orbits or shells are filled in a step-wise manner.
  Electrons are not accommodated in a given shell unless the inner shells are filled.

Diagram to show the arrangement of energy levels: