Basic chemistry

What is Study of electrons?

What is the study of electronic layers of atoms and ions?

Study of electrons and electronic layers

  • Reminder: composition of an atom
  • State of the electrons
  • Study of the electronic layers of atoms and ions
  • Study of the electronic structure
  • Configuration of some atoms

Reminder: composition of an atom

The atom is composed of a nucleus, formed by nucleons. Nucleons are composed of protons and neutrons. The electrons, meanwhile, gravitate around the nucleus.

The atom consists of a certain number of electrons, which are thus around the nucleus, the nucleus representing practically the whole mass of the atom.

There are as many electrons as there are protons in an atom.


Electrons are particles several thousand times smaller than nucleons, carrying a negative elementary charge ( -e = -1.6 × 10 -19  coulomb ) and mass  m e = 9.1 x 10 -31  kg   (approximately a thousand times lighter than a nucleon).

Electrons are components of atoms and ions.

Number of electrons in an atom

An atom is always electrically neutral which means that the positive charges are offset by the negative charges. But an electron carries a charge that is exactly the opposite of that of a proton so there are as many electrons as protons.

The atomic number (Z) of a chemical element therefore indicates both the number of protons and the number of electrons .

Example: the atom of phosphorus has as atomic number Z = 15 so its nucleus contains 15 protons and since it is neutral it also has 15 electrons.

State of the electrons

The energetic state of an electron can not vary continuously but only in discontinuous leaps between defined stable states.

The electrons are thus distributed by “layers” called electronic layers which are often represented as concentric layers and which are called K, L, M.

Did you know ? Why do electronic layers begin to be named from the letter K and not, for example, from the letter A?

  • A little history…

The hydrogen atom has been an important study model because it is the simplest atom. The hydrogen atom can absorb or emit well-defined amounts of energy: this corresponds to the passage of the atom from one level of energy to another.

The jump of energy is then manifested by a line of emission in the spectrum of the atom. It was the scientist Balmer who discovered these transitions in the visible, first. Then Paschen discovered transitions in the infrared and Lyman in the ultraviolet.

However, these scientists suspected that there might be other transitions further into the infrared or the ultraviolet. Thus the terms K, L, M were given not to start at the beginning of the alphabet and to have a margin of names in both directions!

Study of the electronic layers of atoms and ions

The model of Niels Bohr

Niels Bohr is a Danish physicist, winner of the Nobel Prize for Physics in 1922.

In 1913 he published an atomic model that showed the central nucleus around which electrons gravitated.

These electrons had the ability to switch from one layer to another. The electrons occupy well-defined orbits, and the Bohr model has several characteristics: the electronic layers are located at well-defined distances from the nucleus and have electronic reception capabilities also finished.

Who proposed an atomic model to represent the electronic layers?Niels Bohr, Nobel Prize in Physics in 1922 for his work on quantum mechanics.

For further 

The representation in the K, L, M sub-layers is practical to exploit the Lewis model, nevertheless, the most interesting model and the most precise remains that of the atomic orbitals (or probability of presence of electrons in an electronic cloud).

This method, which exploits quantum mechanics, uses numbers called quantum numbers n, l, m, s that describe electronic clouds.

Electronic layers

We know that the electrons that revolve around the nucleus of an atom or an ion are not randomly distributed but occupy layers of different energies.

In the increasing order of energy and distance to the nuclei these layers are denoted by K, L M.  The layer K can contain 2 electrons while the layers L and M can receive 8 electrons.

The electrons begin by filling the first layers before occupying the following ones.

  • The layer K is therefore the first layer to be filled. She is closest to the nucleus. It is a layer where the electrons tend to be in a very stable state.
  • L layer can accommodate 8 electrons. It is a layer a little less stable than the K layer.
  • The M layer can also accommodate 8 electrons and is still a little less stable than the previous layer.

To go further: the layer M can actually receive 18 electrons but the chemistry program is limited to 8, which corresponds to the first three lines of the periodic table.

How is the electronic configuration of an atom represented?The electronic structure of an element can be schematised by concentric layers called K, L, M.

External layer concept

The outer electron layer is the last layer of the atom containing electrons.

The study of this layer is very important because it gives the number of covalent bonds that an atom can achieve and it also provides information on reactivity.

Study of the electronic structure

It allows to describe the distribution of electrons in the different layers of an atom or an ion. Layers with electrons are designated by their letter, denoted in parentheses and accompanied by exposing the number of electrons they contain.

Example:  the electronic structure of the magnesium atom is (K) 2 (L) 8 (M) 2
This means that the atom has 2 electrons in its first layer K, 8 electrons on the next layer L and 2 electrons on the layer M which is not complete.

How to write the electronic structure of an atom?

It is necessary first of all to know the total number of electrons that this atom possesses, which is indicated by its atomic number Z.

Then, just write the first layer and place the first 2 electrons. If there are electrons they are then placed on the L layer. If the L layer is complete then the remaining electrons are noted on the M layer.

Example of the aluminum atom

The atom of aluminum has as atomic number Z = 13 which means that it has a total of 13 electrons.
The first two electrons complete the first layer that we note (K) 2 .

It then remains 13 – 2 = 11 electrons: the following 8 electrons complete the layer L which gives (K) 2 (L) 8 .
There remains 13 – 2 – 8 = 3 electrons which therefore belong to the layer M.

The electronic configuration of the aluminum atom is therefore (K) 2 (L) 8 (M) 3 .

What is the electronic structure of aluminum?The aluminum has 13 electrons which are distributed on the electronic layers K, L and M.

Configuration of some atoms

Neon case (Z = 10)

The first two electrons complete the L: K 2 layer

Then, there remains: 10-2 = 8 electrons to distribute. These 8 electrons completely complete the next layer, L. So we have the following configuration: K 2 L 8

We can notice that Neon, which is a noble gas, has its two saturated electronic layers: it can not gain or lose electron, it is stable.

Carbon case (Z = 6)

The first two electrons will be placed on the L layer and the next 4 (since 6-2 = 4) will be placed on the layer M. We thus have as configuration: K 2 L 4

Carbon has 4 electrons on its outer layer.

Case of an ion, sodium ion Na + (sodium Z = 11)

For sodium, in its ionized Na + form, it has lost an electron, so its atomic number is 11-1 = 10

The first two electrons fill the first electronic layer L and the other 8 electrons fill the M layer as for the Neon: K 2 L 8

The sodium Na + has thus obtained the electronic configuration as the nearest rare gas.

Magnesium case (Z = 12)

The first two electrons are placed on the layer K. It remains (12-2 = 10) 10 electrons to place.

8 electrons saturate the L layer. There remain (10-8 = 2) 2 electrons which will be placed on the outermost layer, the layer M.

The configuration of the magnesium atom is thus the following: K 2 L 8 M 2 .

Note : magnesium can be ionized twice in Mg 2+ , this means that magnesium can lose 2 electrons of its outer layer M to obtain a more stable configuration, that of the nearest rare gas, ie neon configuration K 2 L 8 .

Sulfur case (Z = 16)

The first two electrons will fill the layer L. The next 8 electrons will then fill the layer M. There remains 16- (2 + 8) = 6 electrons.


The remaining 6 electrons will fill the M layer which is the outermost layer

The configuration of the sulfur atom is therefore K 2 L 8 M 6 .

The results are compiled in the following table:

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