Atomic orbitals represent three-dimensional spaces around a nucleus where electrons in different energy levels are most likely to be found. The first energy level is nearest the nucleus and has the lowest energy. It contains a single spherical s orbital. The second energy level has four orbitals— one s orbital and three dumbbell-shaped p orbitals, which point along the x, y, and z axes. The third energy level adds five d orbitals to its s and p orbitals. Four of the five d orbitals are clover-shaped, while the fifth one has a unique shape. In an orbital diagram, a box represents an orbital and arrows represent electrons. Each orbital can hold up to 2 electrons, one with a counterclockwise spin and another with a clockwise spin. Electrons in their ground state fill the atomic orbitals from lowest to highest energy to achieve the lowest-energy configuration. For example, the eleven electrons of sodium are distributed as two electrons in the 1s orbital, two in 2s, six in 2p, and one in the 3s orbital.