The correct answer is 18. Explanation & Key Takeaways: • The nuclear shell model is a model of the atomic nucleus. • It uses the Pauli exclusion principle to explain the nucleus structure in terms of energy levels. • Shell Model of the Atom • It explains the arrangement of different electrons present around the nucleus of an atom as per energy levels. • The atomic shell model explains the structure of atoms. • The negatively charged fundamental particles which are known as electrons are considered to occupy diffuse shells in the space that surrounds the positively charged nucleus. • The shell which is closest to the nucleus is the first shell. • The shells are designated as: • K First shell • L Second shell • M Third shell and so on • The sequence in which the electrons occupy the shell is given as 2, 8, 18, 32, 50, 72, 98. • The maximum number of electrons that can occupy the energy level in an atom is found by using the following formula: Electron capacity = 2n^(2), Where n is the principal quantum number. • M shell i.e. is the third shell that can hold 18 electrons. Hence option 2 is correct.