The possible locations around an atom where electrons having specific energy values (quantum number) may be found. The term shell has been replaced with the term energy levels because the term shell insinuated that electrons circled the atom in fixed orbits like planets circle the sun. This model however is incorrect and outdated. Although the term shell is outdated, it is still often used grade school and high school science classes, as its concepts are less intimidating than are the use of terms like quantum number.