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Atomic orbitals are three-dimensional regions around the nucleus where an electron is most likely to be found.

The first energy level is nearest to the nucleus and has the lowest energy. It contains a single spherical s orbital.

The second energy level contains four orbitals: one lower-energy 2s orbital and three higher-energy 2p orbitals. Each p orbital is dumbbell-shaped and points along the x-, y-, or z-axis.

The third energy level, along with s and p orbitals, also includes five d orbitals. Four of the 3d orbitals have a cloverleaf shape, while the fifth has two lobes and a ring around the center.

In an orbital diagram, a box represents an orbital, and arrows represent electrons. Each orbital can hold a maximum of two electrons with only opposite spins: one spin-up and one spin-down.

Electrons in the ground state fill atomic orbitals from lowest to highest energy to reach the most stable configuration. For example, sodium has 11 electrons. Two electrons with opposite spins fill the 1s orbital first. Similarly, two electrons fill the 2s orbital. Next, one electron enters each of the three 2p orbitals with the same spin. After that, electrons pair with opposite spins until all three 2p orbitals are filled. Finally, one electron enters the 3s orbital.