How to understand s p d …

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:

i. The first orbit or K shell will have 2 × 1² = 2 electrons.

ii. The second shell will have 2 × 2² = 8 electrons.

iii. The third shell will have 2 × 3² = 18 electrons.

iv. 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.