Hi, I am new member of this forum, and I have one question for you. When the atom have 8 electrons in the last shell, why it have stable configuration? What makes the atom to be stable and what makes the atom to be reactive? I know few things about this. Generally speaking the CH2 molecule is very unstable. The carbon's configuration is: 1s^2 (means 2 electrons in the orbital), 2s^2, 2px^1, 2py^1, 2pz^0 So for methane to be produce one of the 2s electrons must go into the 2pz electron and the final configuration of carbon is: 1s^2, 2s^1, 2px^1, 2py^1, 2pz^1 so there are 4 bonds. But how that 2s electron went to the 2pz orbital. Where is the energy from? Best regards.