Energy levels in an atom are characterized by which quantum number?

The principal quantum number is the correct characterization of the energy levels in an atom. It is denoted by the letter "n," where "n" can take on positive integer values (1, 2, 3, etc.). The principal quantum number defines the main energy level or shell in which an electron resides, and as "n" increases, the energy level of the electron increases, allowing it to be located further from the nucleus.

This quantum number is pivotal because it essentially dictates the size and energy of the orbitals that electrons occupy. For example, when comparing electrons in the first energy level (n=1) to those in the second energy level (n=2), electrons in n=2 possess more energy and are found at a greater average distance from the nucleus than those in n=1.

Understanding the principal quantum number is crucial for organizing the periodic table and predicting the behavior of electrons in different elements, particularly in terms of their reactivity and bonding capabilities.