Question

why half filled and fully filled orbitals are mor stable

Answer 1

The stability of an atom depends on the strength with which its electrons are bound to it. As you move across a row of elements the nuclear charge increases by one for each element, but the shielding of the electrons within a subshell is incomplete as far as the other electrons in that subshell are concerned. Thus. the EFFECTIVE nuclear charge each electron feels (the sum of the attraction of the electron to the nucleus and repulsion for all the other electrons) increases as you move the right in a row. For example, as you move from (say) a lanthanum (+3) ion to a gadolinium (+3) ion, as the number of 4f electrons increases from 0 to 7 and the effective nuclear charge continues to increase each step along the way (from La³⁺ to Gd³⁺). However, there is an extra effect to consider: electrons of the same spin avoid each other better than electrons of opposite spin. This is a consequence of the Pauli principle - no two electrons with the same spin quantum number can ever be in the same place at the same time, but there is some (small) probability that electrons of opposite spin can be in the same place at the same time.

In half-filled shells electrons have the same spin, so they avoid each other better than the NEXT electron added the the subshell can avoid all the electrons of opposite spin. For this reason, as you fill a subshell past the half-filled point, the (destabilizing) electron-electron repulsion is greater for additional electrons - but the effective nuclear charge still steadily increases as you move to the right in the row of elements until the subshell if filled. After a subshell is filled, electrons are forced into higher energy orbitals in the next shell or subshell.

Answer 2

look up "S characteristics" that should help you take a shortcut if you don't understand me comment back, I'll explain