Hunds Rule. Every orbital in a sublevel is singly occupied before any orbital is doubly occupied. The term with maximum multiplicity lies lowest in energy.
Electrons will prefer to go to different orbital with parallel spin instead of choosing single orbital for anti parallel coupling of electron. According to hund's first rule, a set of degenerate orbitals are singly occupied first, before the second slot in any of the orbitals are populated. The last of the three rules for constructing electron arrangements requires electrons to be placed one at a time in a set of orbitals within the same sublevel. Every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.
This chemistry video explains what is the aufbau's principle, hund's rule, and pauli's exclusion principle and how it relates to orbital diagrams, electron.
Example 1 consider the different ways in which a pair of electrons might be arranged in p orbitals. All of the electrons in singly occupied orbitals have the same spin (to maximize total spin). Applying the rst rule to carbon selects the three states 3p0, 3p1 and 3p2. Hund's first rule states that the maximum multiplicity of electrons is lowest in energy.
