Electron configuration

• What is an electron configuration?


  The arrangement of electrons in the orbitals of an atom, following specific rules like the **Aufbau principle**, **Pauli exclusion principle**, and **Hund's rule**.
• What is the Aufbau principle?


  Electrons fill the lowest energy orbitals first before moving to higher energy levels. Example: $1s$ fills before $2s$.
• What is the Pauli exclusion principle?


  No two electrons in an atom can have the same set of quantum numbers. This means an orbital can hold a maximum of **2 electrons** with opposite spins.
• What is Hund's rule?


  Electrons will fill degenerate orbitals (orbitals of the same energy) singly first, with parallel spins, before pairing up. Example: $p^3$ has one electron in each $p$ orbital.
• What is the electron configuration of sodium (Na)?


  $1s^2 2s^2 2p^6 3s^1$. Sodium has 11 electrons, with the last electron in the $3s$ orbital.
• What is a valence electron?


  Electrons in the outermost shell of an atom, which are involved in chemical bonding. Example: Sodium has **1 valence electron** in the $3s$ orbital.