Here's why:

* Atomic number represents the number of protons in an atom. This number is fundamental to an element's identity and determines the element's chemical properties.

* Electronic configuration describes the arrangement of electrons in an atom's energy levels and orbitals. This arrangement is directly related to the number of protons (and thus the number of electrons in a neutral atom).

Mass number is the total number of protons and neutrons in an atom's nucleus. While it plays a role in determining an atom's isotopes and its mass, it doesn't directly affect the electronic configuration.

Example:

* Carbon has an atomic number of 6, meaning it has 6 protons and 6 electrons. Its electronic configuration is 1s² 2s² 2p².

* Carbon-12 and Carbon-14 are isotopes of carbon. They have the same number of protons (6) and electrons (6), but different numbers of neutrons (6 and 8, respectively). Their electronic configurations are identical because their number of protons and electrons are the same.

In summary, the atomic number is the key factor in determining an atom's electronic configuration.