The electron configuration, or the way the electrons are distributed in an atom

The atom and its structure

The individual particles that make up matter are atoms. All that surrounds us are atoms. The elements are the sum of their atoms – iron is made of iron atoms, copper is made of copper atoms, etc.

So what does an atom consist of?

Most of us know the answer: positively charged protons, negatively charged electrons, and neutrons without any charge. Is this the correct answer? Absolutely, but if we look at an atom the way a chemist does, we answer that the atom consists of two basic elements: the nucleus and the surrounding electron cloud.

Atomic nucleus

The nucleus of the atom is its centre and constitutes its most important part. It consists of protons (positively charged) and neutrons (electrically neutral). These are not non-divisible components. Both protons and neutrons have an internal structure – they are made of smaller particles called quarks. A proton is made of two upper quarks and one lower quark. A neutron, however, has one upper quark and two lower quarks in its structure.

Electron cloud

An atom does not have a clearly defined edge – this is due to the presence of an electron cloud. An electron cloud is the area of the greatest probability of the presence of an electron (important: the path along which the electron moves cannot be clearly determined. It is only possible to determine the probability of finding it in different areas in space). An electron cloud consists of electrons orbiting around an atomic nucleus. Right next to the nucleus, the density of the electron cloud is the highest, while the further away from the nucleus, the more diffused the cloud is.

Quantum description of the atom

The state of each electron in an atom is described by the wave functions. The wave functions are a mathematical solution to the Schrödinger equation. In turn, this equation can be solved if several basic conditions are introduced. For this reason, quantum numbers have been used. Quantum numbers that uniquely describe the quantum state of each electron in a given atom are briefly characterised below:

• principal quantum number n:

is responsible for the energy of the electron. Has the values of successive natural numbers. It can range from 1 to infinity. In practice, this is not the case and most often n ranges from 1 to 7. Levels with the same principal quantum number are called the electron shell.

• azimuthal quantum number l:

defines energies more precisely. The value of the azimuthal quantum number determines the subshell of a given atomic shell. The shape of the atomic orbitals also depends on the value of this number. The azimuthal quantum number has values from zero to (n-1).

• magnetic quantum number m:

the value of a magnetic quantum number depends on the azimuthal quantum number. The magnetic quantum number m has values from – l to l (including 0). Thanks to the knowledge of the magnetic quantum number, the mutual positions of the orbitals in space are determined, which gives information on the number of orbitals at a given sub-level.

• spin quantum number:

while moving around the atomic nucleus, electrons also move around their own axis. This movement is called the spin and the spin quantum number is associated with it. It has only two values: + ½ and – ½. Each atomic orbital may contain two electrons with different value of the spin quantum number.

When describing quantum numbers, it is impossible not to mention one of the fundamental laws in chemistry, namely the Pauli exclusion principle. According to this principle, an atom cannot contain two electrons with the same quantum numbers. The electrons in the atom must differ in value of at least one quantum number.

Electron shells and subshells

The atomic nucleus is surrounded by an electron cloud in which, with a certain probability, an electron can be found. These electrons are arranged on appropriate electron shells. Simply put, the electron shells are levels with the same principal quantum number n. The shell furthest from the atomic nucleus is called the valence shell – the electrons orbiting on this shell are called the valence electrons (they create chemical bonds between atoms of different elements or atoms of the same element). Each electron shell is identified by a letter. So, for n = 1, the letter is K, for n = 2 the letter is L, etc (for n 1 to 7, the letters are: K to Q).

Each of the electron shells in an atom consists of subshells. The subshells are defined by the azimuthal quantum numbers l. On the subshells, there are electrons that have precisely defined, equal energy values. The subshells also have a specific ‘capacity’ – they can contain 2*(2*l+1), where l is the azimuthal quantum number. The subshells also have their letter designations: s, p, d, f, g, h, etc.

Electron configuration

For the correct determination of the electron configuration in an atom, it is necessary to know the order of energy levels (the order of individual subshells and shells according to increasing energy value). Configuration is just the assignment of individual electrons to energy levels.

There are two energy states of an atom: the ground state and the excited state. We observe the ground state when the electrons are distributed on individual orbitals according to the expansion rules. It then has the lowest energy. If the atom receives a certain amount of energy, then the electron can be transferred from the lower-energy orbital to the free higher-energy orbital – then we are talking about the excited state of the atom.

Thus, in order to find the correct electron configuration of an atom in the ground state, individual orbitals must be filled according to the increasing energy, observing the Pauli exclusion principle. According to these principles, the so-called full configuration notation featuring the numbers of successive shells, letter designations of successive subshells and a notation of the number of electrons in specific orbitals, is created. The abbreviated electron configuration notation initially contains the core in the form of an electronic configuration of a noble gas, which is supplemented by the remaining electrons.