CBSE Class 10 Answered

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.

 First orbit or K shell will be = 2 X 1² = 2, Second orbit or L shell will be = 2 X 2² = 8,   Third orbit or M shell will be = 2 X 3² = 18, Fourth orbit or shell will be = 2 X 4² = 32 and so on.

The maximum number of electrons that can be accommodated in the outermost orbit is 8.

Further,

Starting with the energy level/shell closest to the nucleus electrons fill the different shells in order of increasing energy.

Energy levels or shells can be divided into sub-shells, which can hold different numbers of electrons.

Within each sub-shell there are one or more orbitals. Each orbital can hold a maximum of two electrons (of opposite spin).