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We use what is called the Hund's rule to fill up the atomic orbitals. Electrons fill up lower energy levels before they can fill higher ones. In this case we have 1st,2nd and 3rd energy levels . So electrons will fill up 1 then 2 then 3 in that order.
One thing to note too is that electrons will fill empty atomic orbitals before they can start pairing.
The atom in question has 8 electrons and thus 2 will go to the 1s orbital and the remaining 6 will go to the 2s and 2p respectively. When the 6 fill up the 2nd energy level orbitals , 2 will fill the 2s leaving 4 electrons . 3 of those will fill up the 2p orbitals (1 per orbital) and the remaining 1 will go to pair with one of the electrons in the 2p orbitals since the 3s is of a higher energy level.
The reason for all of this is due to two reasons I know of . The first one regarding the order of the filling of energy levels is due to atoms wanting to have the least potential energy possible which makes them very stable . If the atom was to have an electon which would be in a higher energy level this would make it unstable as the electron would be further away from the attraction of the nucleus making it veryyyyyyyy susceptible to being lost thus making the atom unstable . (Which is why Francium is highly reactive as it has an electron in a higher energy level)
The reason now for the filling of atomic orbitals is in order to minimise repulsions between electrons occupying the same energy level. Electrons will tend to be further away since they have the same charge. If electrons are closer to one another they tend to push each other away which makes the atom unstable . I hope I have answered you.
