Answer
According to Hund's Rule, the pairing of electrons in a subshell does not take place until the time all the orbitals of that subshell are singly occupied.
Work Step by Step
According to the Hund's Rule, the electrons in an orbital are first singly occupied, with each of them having the same electron spin. After each orbital is singly occupied, the pairing of electrons takes place. Now the electrons have the opposite spin.
