12.2.2 - State how orbitals are labeled. Limit this to n < 5

Having identified the evidence for energy shells and sub-shells as well as the existence of orbitals they must be named to avoid confusion.

The main energy levels are represented by the letter "n".

The Energy Levels are numbered n=1 (also called the ground state) n=2, n=3, n=4, etc all the way to n=infinity (the levels converge to the maximum possible energy that an electron can have before it escapes from the atom. After this point the nucleus no longer can hold the electron and it is said to be at infinite distance)

Atomic orbitals are the regions of space in which there is a 99% probability of finding a particular electron.

In the first energy level there is only one orbital - designated the letter "s"

In the second energy level there are s and p sub-shells. The "s" sub-shell contains only one s orbital and the p sub-shell comprises three p orbitals

n = ( level 1: s) (level 2: s and p) (level 3: s, p and d) (4: s, p, d and f)

Note:
Historically the letters used to designate the sub-shells come from words used to describe the lines seen on the spectra studied at the time. s stands for the term "sharp" , "p" for "principal, "d" for "diffuse" and "f" for "fundamental" - IB students do not need to know this.

12.2.3 - State the relative energies of s, p, d and f orbitals

The relative energies of the subshells are: s < p < d < f (within a given energy level)

Note:
It should be remembered that in spite of this order the 4s electrons fill up before the 3d electrons when applying the Aufbau principle, and that there are anomalies in the configurations of chromium, [Ar] 4s1 3d5 and copper, [Ar] 4s1 3d10 due to the extra stability of half full and full sets of "d" orbitals respectively.

12.2.4 - State the number of orbitals at each energy level

This is usually shown in graphical form. Click on the link to see the Aufbau principle in practice.

The number of orbitals at each level : s=1, p=3, d=5, f=7

Level 1: has only one s orbital

Level 2: has one s and three p orbitals

Level 3: has one s, three p and five d orbitals

Level 4: has one s, three p, five d and seven f orbitals

12.2.5 - Draw the shape of an s orbital and the shapes of the px, py and pz orbitals

Shapes of orbitals:

  • s orbital is a sphere around the nucleus.
  • p orbitals are shaped like a figure 8 (and there are 3 of them at 90 degrees around the nucleus)

s- orbitals / p- orbitals
1s orbital /
2s orbital /
3s orbital /
/ Px orbital /
Py orbital /
Pz orbital /

12.2.6 - State the Aufbau principle. Reference should be made to Hund's rule

The Aufbau principle just means the way the electrons fit into the atomic orbitals in order of ascending energy. The first electron goes into the lowest enrgy orbital available (the 1s orbital) the next electron pairs up with it in the same orbital and the third electron (that of lithium) fits into the next orbital up, the 2s orbital.

The rules for filling up the orbitals are as follows:

1. Electrons always enter the orbital of lowest energy

2. If there are two or more degenerate orbitals (meaning that they have the same energy) then the electrons will singly occupy the degenerate orbitals until the orbitals are all singly occupied, after which they will pair up one at a time. This is known as Hunds rule

3. There cannot be more than two electrons in any one orbital.