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It depends on the number of electrons. Say you go into an energy level with three orbitals, and you only have two electrons left. You would put one electron of spin up in one orbital and then another electron of spin up in the second orbital, and these would be parallel. If you had 6 electrons left to fill these three orbitals, then you would fill each orbital with a spin up electron and go back and fill each with a spin down electron, and then each of these electrons would be paired because there are two in each orbital of different spin.
According to the Hund's rule, it is said that we have to fill one into each sub shell of the orbital before doubling any of the sub shell with electrons. Therefore, in each of the sub shells, you cannot have an opposite spin of the electrons until each sub shell are filled with parallel spinning electrons.
electrons have a negative charge, so they like to be as far away from each other as possible. This is why Hund's Rule states that electrons have to be parallel, and in separate orbitals, before they are paired in the same orbital.
When you are drawing the spin diagrams, the electrons are paired if there are two arrows in the same orbital. Electrons are parallel if the spin arrows face the same direction but are in different orbitals in the same subshell.
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