$\rightarrow$ The atom consists of a small positively charged nucleus at its center.

$\rightarrow$ The whole mass of the atom is concentrated at the nucleus and the volume of the nucleus is much smaller than the volume of the atom.

$\rightarrow$ All the protons and neutrons of the atom are contained in the nucleus.

$\rightarrow$ Only certain orbits known as discrete orbits of electrons are allowed inside the atom.

$\rightarrow$ While revolving in these discrete orbits electrons do not radiate energy. These orbits or cells are represented by the letters $K , \,L , \,M , \,N$ etc. or the numbers, $n=1,\,2,\,3,\,4$, …….. as shown in below figure.