What are energy sub-levels?

The Sub-levels of energy In the atom are the form in which the electrons are organized in the electronic layers, their distribution in the molecule or atom. These energy sub-levels are called orbitals.

The organization of electrons in sub-levels is what allows chemical combinations of different atoms and also defines their position within the Periodic Table of Elements.

Sub-levels of energy

Electrons are arranged electronically in the atom in a certain way by a combination of quantum states. At the moment one of these states is occupied by one electron, the other electrons must be located in a different state.


Each chemical element in the Periodic Table consists of atoms, which in turn are composed of neutrons, protons and electrons. Electrons are negatively charged particles that are found around the nucleus of any atom, distributed in the orbitals of electrons.

Electron orbitals are the volume of space where an electron has a 95% chance of being found. There are different types of orbitals, with different shapes. In each orbital a maximum of two electrons can be located. The first orbital of an atom is where it is most likely to find electrons.

The orbitals are designated by the letters s, p, d and f, ie Sharp, Principle, Diffuse and Fundamental and are combined when atoms join to form a larger molecule. In each layer of the atom are these combinations of orbitals.

For example, in layer 1 of the atom are the S orbitals, in layer 2 there are S and P orbitals, inside layer 3 of the atom there are S, P and D orbitals and finally in layer 4 of the atom are all Orbitals S, P, D and F.

Also in the orbitals we find different sub-levels, which in turn can store more electrons. The orbitals at different energy levels are similar to each other, but occupy different areas of space.

The first orbital and the second orbital have the same characteristics as an S orbital have radial nodes, have a greater probability of spherical volume and can only hold two electrons. However, they are located at different energy levels and thus occupy different spaces around the nucleus.

Location in the Periodic Table of Elements

Each of the electronic configurations of the elements are unique, which is why they determine their position in the Periodic Table of Elements. This position is defined by the period of each element and its atomic number by the amount of electrons that has the atom of the element.

Thus, using the periodic table to determine the configuration of electrons in atoms is key. The elements are divided into groups according to their electronic configurations as follows:

Each orbital is represented in specific blocks within the Periodic Table of Elements. For example, the orbital block S is the region of the alkali metals, the first group in the table and where six elements are found: Lithium (Li), Rubidium (Rb), Potassium (K), Sodium (Na), Fr) and Cesium (Cs) and also hydrogen (H), which is not a metal, but a gas.

This group of elements has an electron, which is often easily lost to form a positively charged ion. They are the most active and most reactive metals.

Hydrogen, in this case is a gas, but is within group 1 of the Periodic Table of Elements since it also has only one electron. Hydrogen can form ions with a single positive charge, but achieving its single electron requires much more energy than removing the electrons from other alkaline metals. In forming compounds, hydrogen often generates covalent bonds.

However, under very high pressures, the hydrogen becomes metallic and behaves like the rest of the elements of its group. This occurs, for example, inside the nucleus of the planet Jupiter.

Group 2 corresponds to alkaline earth metals, since their oxides have alkaline properties. Among the elements of this group are Magnesium (Mg) and Calcium (Ca). Their orbitals also belong to level S.

Transition metals, which correspond to groups 3 through 12 in the Periodic Table, have D-type orbitals.

The elements from group 13 to 18 in the table correspond to P. orbitals. And finally the elements known as lanthanides and actinides have orbital names F.

Location of the electron in orbitals

Electrons are found in the orbitals of the atom as a way to decrease energy. Therefore, if they seek to increase energy, the electrons will fill the main orbital levels, moving away from the nucleus of the atom.

Consider that the electrons have an intrinsic property known as spin. This is a quantum concept that determines among other things, the spin of the electron within the orbital. This is critical to determining your position in the energy sub-levels.

The rules that determine the position of the electrons in the orbitals of the atom are the following:

  • Principle of Aufbau: Electrons enter the orbital with lower energy first. This principle is based on the diagrams of the energy levels of certain atoms.
  • Principle of Exclusion of Pauli: An atomic orbital can describe at least two electrons. This means that only two electrons with different electron spin can occupy an atomic orbital.

This implies that an atomic orbital is an energetic state.

  • Hund Rule: When electrons occupy orbitals of the same energy, the electrons enter the first empty orbitals. This means that electrons prefer parallel spins in separate orbitals of energy sub-levels.

The electrons will fill all the orbitals in the sub-levels before encountering opposing spins.

Special electronic configurations

There are also atoms with special cases of energy sub-levels. When two electrons occupy the same orbital, not only must they have different spins (as indicated by the principle of Exclusion of Pauli), but the coupling of the electrons slightly elevates the energy.

In the case of energy sub-levels, a half-filled and a full-filled sub-level reduce the energy of the atom. This leads the atom to have greater stability.


  1. Electron Configuration. Retrieved from Wikipedia.com.
  2. Electronic Configurations Intro. Retrieved from chem.libretexts.org.
  3. Orbitals and Bonds. Retrieved from chem.fsu.edu.
  4. Periodic table, main group elements. Retrieved from newworldencyclopedia.org.
  5. Electron Configuration Principles. Retrieved from sartep.com.
  6. Electronic Configuration of Elements. Retrieved from science.uwaterloo.ca.
  7. Electron Spin. Retrieved from hyperphysics.phy-astr.gsu.edu.

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