As you know, every atom consists of a nucleus and an electron shell. In the nucleus are both protons as well as neutrons. In the electron shell, on the other hand, the electrons of an atom. How exactly these electrons are distributed in the electron shell is determined by the so-called electron configuration specified.
Basics of electron configuration
the electron configuration of an atom indicates how exactly the electrons are distributed in the electron shell of the atom.
The electron distribution is using the Bohr-Sommerfeld atomic modelas well as the orbital model described. To clearly identify the electrons, four so-called quantum numbers used, which will be discussed in more detail later in this statement.
The Bohr-Sommerfeld atomic model
That Bohr-Sommerfeld atomic model is, as the name suggests, an extension of the Bohr’s atomic model by the German physicist Arnold Sommerfeld. Therefore, there is also talk of the Sommerfeld extension or the Sommerfeld atomic model.
Danish physicist Niels Bohr developed Bohr’s atomic model in 1913. According to Bohr’s atomic model, the positively charged protons and the neutrally charged neutrons are located in the nucleus of an atom. The electrons, on the other hand, move on so-called shells around the atomic nucleus. A shell can hold a maximum of 2n2 electrons.
The first electron shell can therefore accommodate a maximum of 2 × 12, i.e. two electrons. The second shell, on the other hand, can already hold eight electrons.
While Niels Bohr assumed circular orbits on which the electrons of an atom move, Arnold Sommerfeld assumed elliptical orbits. Since the circle is a special form of the ellipse, it is also possible, according to Sommerfeld, that electron shells can be circular.
The further away the electron orbit is from the nucleus, the more energy is needed to keep the electrons there. Because of this, the electron shells closest to the nucleus are occupied first. Then the more distant electron shells are also occupied.
If you want to refresh or deepen your knowledge of Bohr’s atomic model, you can find more on the subject in the detailed explanation of Bohr’s atomic model.
The orbital model
That orbital model is, in contrast to the Bohr-Sommerfeld atomic model, a three-dimensional representation of the electron distribution. In addition, it clearly designates the energy levels of the individual electrons. According to the orbital model, the electrons of an atom have a high probability (> 90%) in three-dimensional spaces around the atomic nucleus. These rooms are called orbitals designated.
You will find out exactly what these orbitals can look like later in this explanation. If you want to learn more about the orbital model, be sure to check out the accompanying explanation.
Electron Configuration – Quantum Numbers
Four parameters have been set up to clearly describe the orbitals and the electrons in them: the quantum numbers.
The four quantum numbers have specific values that can be assigned to the electrons. According to the Pauli principle only one combination of the values can be assigned to each electron, so that each electron within an atom can be described unambiguously.
You want to learn more about the Pauli principle? In the explanation of the Pauli principle you will find more exciting facts and detailed information on the subject.
There are four quantum numbers:
- the principal quantum number n,
- the secondary quantum number l,
- the magnetic quantum number m and
- the magnetic spin quantum number s.
The principal quantum number n
the principal quantum number n describes in which electron shell the respective electron is located according to the Bohr-Sommerfeld atomic model. she can only take natural numbers.
The electron shells are identified by letters for clear assignment. The shell with the lowest energy is called the K shell and all other designations follow alphabetically: L, M, N, O shell, and so on. These orbits also correspond to the periods of the periodic table.
Period (= n)shell1K2L3M4N……
The secondary quantum number l
the secondary quantum number l indicates the shape of the orbital in which the respective electron is located. It can assume values from zero to n – 1, where n stands for the principal quantum number.
So that the quantum number l is not confused with the main quantum number, the values of the secondary quantum number are also replaced by letters. The table below shows the orbital designations. Each orbital can hold exactly two electrons, but the orbitals themselves come in different abundances. There is only one s orbital per period, while there can be up to three p, five d, and seven f orbitals per period.
Size and energy levels of orbitals
You’ve already learned that an s orbital is basically the smallest orbital, followed by the p orbital, d orbital, and then the f orbital. However, this only applies to orbitals in close proximity to each other.
A 6th period s orbital is many times larger than one of the 1st period and also than a p orbital of the lower periods.
As you now know, the electrons that are further away from the nucleus have more energy. So they have a higher energy level. The energy levels of the electrons give you information about the order in which the orbitals are actually occupied.
They follow the pattern of the periodic table, but the d and f orbitals are a bit more specific. But in order to be able to remember the cast, there is a simple scheme that you can always write down. This scheme will also Madelung scheme called.
The magnetic quantum number m
the magnetic quantum number m indicates the orientation of an orbital in space. It can assume the following values: -l, -(l – 1), … , -1, 0, 1, … , (l – 1), +l.
As you can see, the magnetic quantum number depends on the secondary quantum number l. Since p-orbitals have the secondary quantum number one, the valid value range for them is: -1, 0, 1.
The magnetic spin quantum number s
So far you have been able to describe in which shell the electron is, what kind of orbital it is in and how this orbital is spatially arranged. Now you only have to know which of the two electrons in an orbital it is in order to clearly identify the electron.
With the help of magnetic spin quantumzahl s Can you tell the two electrons in an orbital apart? s indicates the direction in which the spin, i.e. the rotation of the electron, is oriented.
The magnetic spin quantum number can take on two values depending on whether the electron rotates clockwise or counterclockwise with respect to the z-axis. In which order the orbitals occurring more than once in a period are occupied, according to the so-called Hund’s rule.
Hund’s rule
You already know that each bowl has to be full before you can fill another one. In addition, the orbitals are to be occupied in ascending order according to their energy level. But what about the orbitals that exist multiple times in the same period?
the Hund’s rule states that multiple orbitals are first all occupied by an electron. The electrons used first must all have the same spin. In principle, it does not matter whether the electrons used first have a spin of +1/2 or -1/2.
In the explanation of Hund’s rule you will find more detailed information about the four quantum mechanical laws.
Quantum Numbers – A Brief Overview
After you have received a lot of information about the individual quantum numbers, you will find a brief overview of all quantum numbers in the following table:
quantum number
values
expression
nomenclature
example (oxygen)
n
1, 2, 3, …
Size/period of the orbital
K,L,M,N,O
l=1
l
0, 1, … , n-1
shape of the orbital
s, p, d, f
p = 1
m
-l, … , +l
Orientation of the orbital in space
px , py , pz
px = -l = -1
s
-½, +½
spin of the electrons
–
-½
Creating an electron configuration
Electron configurations of the last inserted electron
As the first of two possibilities for an electron configuration, you describe the electron that was last added to the atom. This is the eighth electron for oxygen and the seventeenth for chlorine.
Why only the last electron? If you look at the periodic table, you will see that an element differs quantitatively from the element before and after only in one electron. In other words, since the number of electrons in the periodic table increases continuously in steps of one, the electron configurations in each step only differ by the newly added (last) electron.
You describe this electron using the previously learned quantum numbers, because you can use them to assign a specific recognition to each electron.
If one wants to describe the last electron of phosphorus, one obtains the following parameters with the help of the periodic table:
- It is in the 3rd period: n = 3
- It is in a p orbital: l = 1
- The orbital is oriented parallel to the z-axis: m = +1
- The orbital is simply occupied. Since it is commonly assumed that the first electron in an orbital has positive spin, s is +1/2 here. In theory, however, the spin could also be negative.
Electron configurations of elements
In the previous section you described an electron configuration for a single electron. Besides this electron configuration, you can also create an electron configuration for an entire atom. In this you describe how many electrons are in which shell and in which orbital of the atom.
If one describes the electron configuration of an element, one proceeds as follows:
- First you write down the principal quantum number of the orbital,
- then comes the orbital shape (s, p, d, f)
- and finally, write down the number of electrons that are in that orbital in superscript.
- This process is repeated for all occupied orbitals, starting with the lowest-energy orbital and ending with the highest-energy orbital. The individual electron configurations are lined up.
If you want to set up the electron configuration of carbon, you need the following information:
- n = 2 because carbon is found in the second period of the periodic table.
- The highest energy orbital according to the periodic table is a p orbital.
Thus, the electron configuration of carbon is: 1s2, 2s2, 2p2. To check that you haven’t forgotten any electrons, you can add up the superscript numbers. If this number corresponds to the total number of electrons in the atom, you haven’t forgotten an electron.
Truncated electron configuration
As you may know, carbon is at the beginning of the periodic table. The higher the atomic number of the elements, the higher their number of electrons. This allows the electron configuration of high atomic number elements such as lead…