Electrons are actually not tiny marbles of negatively charged matter.
- The electron is “smeared” in time and space.
- One cannot determine the electron’s speed and its position at the same time.
- One can only determine where an electron is with some specific probability.
Because of this, we talk about elecron clouds; a "cloud" where there is some probability that the electron is.
Shells and orbitals
All the electrons in a shell have the same average energy. The electrons may reside in different energy levels (orbitals) within the same shell.
- All the electrons in a certain shell have the same average energy.
- Each orbital may contain max 2e–.
The K shell
Contains a single orbital, 1s.
- Since the K shell only contains one orbital (1s), and each orbital maximally contains 2e–, there can be only two electrons in the K shell.
The L shell
- One s orbital, 2s
- Maximally 2e–
- Three p orbitals, 2p
- Maximally 3 × 2e– = 6e–
In total: Maximally 8e– in the L shell.
The p orbitals
The M and N shells
The M shell has three energy levels:
- One s orbital, 3s
- Three p orbitals, 3p
- Five d orbitals, 3d
The N shell has four energy levels:
- One s orbital, 4s
- Three p orbitals, 4p
- Five d orbitals, 4d
- Seven f orbitals, 4f
Note: The 3d and 4f orbitals overlap.
- The energy of the electrons in the 4s orbitals are lower than the energy of the electrons in the 3d orbitals.
- The 4f orbitals are filled before the 3d orbitals.
The Madelung rule (the Aufbau principle)
Let's look at the electron configurations for a few atoms.
Note: The 3s and 3p orbitals in the M shell are completely filled.
In potassium, 19K, the next electron ends up in the 4s orbital of the N shell, because there its energy is lower than in the 3d orbital of the K shell.
- This is why there are never nine electrons in the outermost shell, only eight electrons at most.
The electron configuration for 19K:
Let's also look at the electron configuration for calcium, 20Ca:
In calcium, the N shell's 4s orbital is filled in calcium. The next electron (in scandium, 21Sc) must be placed in one of the M shell's 3d-orbitals:
When the M shell's 3d-orbitals are filled, we can start adding electrons the N shell's 4p orbitals. In gallium, 31Ga, there are 10e– in the M shell's 3d orbitals, and the last electron ends up in the 4p orbital of the N shell. This means there are 3e– in the N shell:
In krypton, 36Kr, the 4p orbitals have been filled up with 6e–, leading to 8e– in the N shell:
The rest of the orbitals are filled in order according to the Madelung rule (the Aufbau principle). The arrows indicate in which order the orbitals are filled:
Writing ground states
Using the Madelung rule, we can determine the electron configuration for e.g. argon 18Ar. Each superscripted number indicates the number of electrons in the orbitals:
- 1s2 2s2 2p6 3s2 3p6
Another example, zinc 30Zn:
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10
Since the first five orbitals are identical to 18Ar, we can also write the electron configuration for zinc 30Zn like this:
- [Ar] 3d10 4s2