Is it necessary that filling up orbitals start from spin-up orientation only? And also, is it mandatory to fill the electrons with one kind of spin or they can be filled with spin up and spin down(as shown in the photofor clarifying my doubt)enter image description here

  • $\begingroup$ You either have them all spin up or all spin down. Not a mixture of up and down. Whether it's all spin up or all spin down is not important (refer to the link in Mith's comment). $\endgroup$ – orthocresol Oct 5 '15 at 18:39
  • $\begingroup$ Is there any proof? I am asking this just to make sure that I don't miss anything. $\endgroup$ – Kritika Waghmare Oct 5 '15 at 18:42
  • $\begingroup$ also chemistry.stackexchange.com/questions/15967/… $\endgroup$ – Mithoron Oct 5 '15 at 18:55