Electronic Configuration

The relative energies of various orbitals of single electrons depend on the value of the principal quantum number 'n' and is independent of the value of 'l'. This can be shown by an arrangement known as energy level diagram. The diagram given below illustrates the relative energy of various energy levels for hydrogen and hydrogen like atoms.








For a particular main energy level, the orbital having higher value of the azimuthal quantum number 'l' has higher energy. In general, energies of orbitals belonging to the same main energy level follow the order, 's' < 'p'< 'd'< 'f' .


Shape of s and p orbitals

The energies of atomic orbitals also describe their shapes. The shapes are uncertain, but predictions have been made by experimentation. Another difficult task is describing where an electron is. We can think of it as a wave, and describing its exact location is impossible for us to comprehend. Instead, we can think of it as the statistical probability of the electron being found at a particular place. An electron cloud is used for showing the probability of where an electron is using a dot-density diagram. The denser the dots are in the diagram, the more probability that an electron could be found there.
For example, these are dot-density diagrams for the s and p orbitals (cross sections):







Electron density relates to how much of an electron's charge is packed into a given volume. In dense places on the dot-diagram, there is a high concentration of electrical charge.
An s orbital's shape is spherical, but the p orbital's shape is quite different. They have two lobes extending out into three dimensional space. Since there are 3 p orbitals per energy level, the lobes extend out along the x-axis (px orbital), the y-axis (py orbital), and the z-axis (pz orbital).












Writing Electronic Configuration

For writing electron configurations, the key is to use the periodic table. To write the configuration of S2-, look at what element as has 2 more electrons than S and write the electronic configuration of that element (in this case [Ar]). For Ca2+ look at the element that has 2 fewer electrons than Ca (since Ca would have to lose 2 electrons to become a +2 ion).

The electronic configurations for the first 20 elements are:

H 1s¹

He 1s²

Li 1s²2s¹

Be 1s²2s²

B 1s²2s²2p¹

C 1s²2s²2p²

N 1s²2s²2p³

O 1s²2s²2p⁴

F 1s²2s²2p⁵

Ne 1s²2s²2p⁶

Na 1s²2s²2p⁶3s¹

Mg 1s²2s²2p⁶3s²

Al 1s²2s²2p⁶3s²3p¹

Si 1s²2s²2p⁶3s²3p²

P 1s²2s²2p⁶3s²3p³

S 1s²2s²2p⁶3s²3p⁴

Cl 1s²2s²2p⁶3s²3p⁵

Ar 1s²2s²2p⁶3s²3p⁶

K 1s²2s²2p⁶3s²3p⁶4s¹

Ca 1s²2s²2p⁶3s²3p⁶4s²