**WHY SHOULD THE ELECTRONS PRESENT IN AN ORBITAL HAVE OPPOSITE SPINS?

This is in accordance with quantum mechanics which leads to the derivation of spin quantum number. As per quantum mechanics all electrons should differ from each other in atleast one way, i.e. No two electrons should be exactly similar. Like no two humans are exactly alike similarly no electrons are exactly alike. In an orbital all the 3 quantum numbers are same for 2 electrons. So, inorder to make them different the fourth quantum number which is spin quantum number is assigned diffrently for them.i.e. +1/2 and -1/2, which indicates the direction of rotation.