A half filled orbital is LESS LIKELY to be reactive than a fully filled orbital
Explanation: A fully filled orbital has the electronic configuration of a noble gas. That is, unreactive.
The quantum states of these orbitals(fully filled) are stable because the electrons in the sub shell are fully filled which makes it difficult for it to loose and gain electron unlike a half filled which is much more easier. It will take more energy for ionization to take place making it(fully filled) more stable.