### Learning check

Once you have watched the video, check your learning with this quiz.

## Electron clouds

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. What an electron isn't and what an 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

Contains

1. One s orbital, 2s
• Spherical
• Maximally 2e
2. Three p orbitals, 2p
• Dumbbell-shaped
• Maximally 3 × 2e = 6e

In total: Maximally 8e in the L shell.

### The M and N shells

The M shell has three energy levels:

1. One s orbital, 3s
2. Three p orbitals, 3p
3. Five d orbitals, 3d

The N shell has four energy levels:

1. One s orbital, 4s
2. Three p orbitals, 4p
3. Five d orbitals, 4d
4. 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.

Argon, 18Ar:

 K L M 18p+ 2e– 8e– 8e–

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:

 K L M N 19p+ 2e– 8e– 8e– 1e–

Let's also look at the electron configuration for calcium, 20Ca:

 K L M N 20p+ 2e– 8e– 8e– 2e–

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:

 K L M N 21p+ 2e– 8e– 8e– 2e–

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:

 K L M N 31p+ 2e– 8e– 18e– 3e–

In krypton, 36Kr, the 4p orbitals have been filled up with 6e, leading to 8e in the N shell:

 K L M N 36p+ 2e– 8e– 18e– 8e–

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: The electron orbitals are filled according to the Madelung rule (the Aufbau principle).

### 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
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