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How are orbitals related to energy levels?
Each period has its orbitals. The higher the period, the further electrons are from the core of an atom, the easier it is to separate said electrons from these orbitals.
Orbital is basically a an area where with 95% probability is an electron. This electron oscillates within his orbital, but does not move away.
However, by adding energy to this electron, it oscillates even more and will eventually separate itself from its orbital. Depending on how much energy you added, the freed electron will either completedly leave the atom, or will fall into another, unoccupied orbital present in the same period. This is only true for the valence sphere (the upmost layer of the atom´s capsule).
If the atom falls into unoccupied orbital, it means we "excitated" the atom - and now such atom would be capable of making up to two simple bonds at once, where before were none.
The higher period, the higher layer of electrons in capsule we are talking about. And the higher this layer is, the further these electrons are from the positive core. Since they are soo far away, it is easier to separate them, or excitate them.
Another thing worth mentioning: certain types of orbitals (s, p) require more or less energy to separate their electrones.