According to Bohr scheme of arrangement of electrons in an atom, the electrons that move around the nucleus have definite paths. Electrons revolve in a three dimensional region around the nucleus. The name of such a fixed path is called an orbital. The paths of electrons around the nucleus of atom are shells. Electrons present in each such shell or orbit have a definite energy. Therefore shells are also known as energy levels. . The shells are numbered 1,2,3,4…or K,L,M,N starting from the centre to the outside.
• Only after filling the inner shells with electrons outer shells are filled. As the
distance from nucleus increases the energy of the electrons in the shells increases and the attractive force between the nucleus and the electrons decreases.
Bohr model of atom has shells K,L,M,N……considered as principal energy levels and each principal energy level contains sub energy levels. These sub energy levels are subshells. In these subshells, there are regions where there is a high probability of finding electrons. These are known as orbitals. The maximum number of electrons that can be accommodated in an orbital is 2.
The names s,p,d,f are given to the subshells based on words indicating certain properties related to atomic structure of elements. S for sharp, p for principal, d for diffuse , f for fundamental. The s subshell has only one orbital with a spherical shape. The p subshell has 3 orbitals which are dumb-bell shaped, the d subshell has 5 orbitals and the f subshell has 7 orbitals.