1) My textbook tells me that chromium's electronic configuration is $\ce{[Ar]}\space 3d^5 4s^1$ instead of $\ce{[Ar]} \space 3d^4 4s^2$, as fully filled and half filled orbitals have more stability. But how does that happen? Why is the fully or half filled orbital more stable?

2) Following this method as we go down group 6 we would expect tungsten's last two orbitals to be filled as $5d^5 6s^1$ to attain more stability. But the actual configuration turns out to be $5d^4 6s^2$. Isn't this supposed to be "unstable"?