Final answer:
Energy levels or electron shells are areas within an atom where electrons reside, represented by fixed distances from the nucleus with discrete energy values. Each shell can only hold a specific number of electrons, and these shells fill sequentially with increasing energy levels corresponding to their distance from the nucleus.
Step-by-step explanation:
Energy levels, also called electron shells, are regions in an atom where electrons are likely to be located. These shells are characterized by fixed distances from the nucleus and have distinct energy levels. In fact, each shell can hold only a certain number of electrons, and electrons are added to the lowest energy level until it is full before moving on to the next. An electron shell may be thought of as an orbit around the nucleus; although electrons do not follow rigid orbits, they do have definite areas where they spend most of their time, corresponding to their energy levels.
Energy levels are often illustrated with the help of the Bohr model, in which the electrons are shown to inhabit concentric circles around the nucleus, resembling the steps of a staircase that cannot be occupied between the steps. The principal quantum number (n) signifies the shell number, such as n=1 for the first shell, closest to the nucleus, and this number increases with the distance from the nucleus. Additionally, the maximum number of electrons within a shell depends on its specific sublevel's orbitals, with a limit of two electrons per orbital.