Here's why:

* Arsenic's atomic number is 33, meaning it has 33 protons and 33 electrons.

* Argon (Ar) is the noble gas that comes before arsenic on the periodic table. Argon has an atomic number of 18.

* [Ar] represents the electron configuration of argon, which is 1s² 2s² 2p⁶ 3s² 3p⁶.

* The remaining 15 electrons in arsenic are then distributed in the 3d, 4s, and 4p orbitals.

So, the full electron configuration of arsenic is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³. The noble gas shorthand simply uses [Ar] as a shortcut for the first 18 electrons.