(N/A) $\rightarrow$ 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 shells are represented by the letters $K, L, M, N$ etc.,or the numbers $n=1, 2, 3, 4, \dots$ as shown in the figure.