Each possible electron orbit in Bohr's model has a fixed energy.The fixed energies an electron can have are called energy levels.The fixed energy levels of electrons are somewhat like the rungs of the ladder in Figure 5.3a.The lowest rung of the ladder corresponds to the lowest energy level.A person can climb up or down a ladder by going from rung to rung.Similarly,an electron can jump from one energy level to another.A person on a ladder cannot stand between the rungs.Similarly,the electrons in an atom cannot be between energy levels.To move from one rung to another,a person climbing a ladder must move just the right distance.To move from one energy level to another,an electron must gain or lose just the right amount of energy.In general,the higher an electron is on the energy ladder,the farther it is from the nucleus.

summary of principal energy levels,sublevels,and orbitals
principal energy level
n=1
n=2
n=3
n=4
number of sublevels
1
2
3
4
type of sublevel
1s(1 orbital)
2s(1 orbital),2p(3 orbitals)
3s(1orbital),3p(3 orbitals),3d(5 orbitals)
4s(1orbital),4p(3 orbitals),4d(5 orbitals),4f(7 orbitals)

atomic orbitals
Solving the schrodinger equation gives the energies an electron can have.These are its energy levels.For each energy level,the schrodinger equation aso leads to a mathematical expression,called an atomic orbital,describing the probability of finding an electron at various locations around the nucleus.An atomic orbital is ofen thought of as a region of space in which there is a high probability of finding an electron.
The energy levels of electrons in the quantum mechanical model are labeled by principal quantum numbers(n).These are assigned the values n=1,2,3,4,and so forth.For each principal energy level,there may be several orbitals with different shapes and at different energy levels.These energy levels within a principal energy level constitute energy sublevels.
Each energy sublevel corresponds to an orbital of a different shape,which describes where the electron is likely to be found.
Different atomic orbitals are denoted by letters.As shown in Figure 5.5,s orbitals are spherical,and p orbitals are dumbbell-shaped.Because of the spherical shape of an s orbital,the probability of finding an electron at a given distance from the nucleus in an s orbital does not depend on direction.The three kinds of p orbitals have different orientations in space.

Exceptional Electron Configurations
Copper,shown in figure 5.8,has an electron configuration that is an exception to the aufbau principle.You can obtain correct electron configurations for the elements up to vanadium(atomic number 23) by following the aufbau diagram for orbital filling.If you were to continue in that fashion,however,you would assign chromium and copper the following incorrect configurations.
These arrangements give chromium a half-filled d sublevel and copper a filled d sublevel.Filled energy sublevels are more stable than partially filled sublevels.Some actual electron configurations differ from those assigned using the aufbau principle because half-filled sublevels are not as stable as filled sublevels,but they are more stable than other configurations.