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💎💎Electrons are the negatively charged subatomic particles in an atom and are located outside of the nucleus in various energy levels, called shells. These shells are then divided into subshells, which in turn have their own orbitals.

💎💎Subshells are a grouping within a shell, each carrying slightly different energies. The subshells are denoted by the symbols s,p,d, and f.

There are 2 electrons for each s subshell, 6 for each p subshell, 10 for each d subshell, and 14 for each f subshell. This information is summarized in the table below:

💎💎This can also be expressed in a graph form, as shown below, known as an energy-level diagram.

These energy levels are filled from the lowest energy levels, according to the Aufbau principle.

The orbital filling pattern is shown here:

It is important to remember that, when filling orbitals of equal energy, electrons are added to the orbitals to half fill each one before any pairing occurs, according to Hund's rule.

Here is an example of an energy-level diagram of oxygen.

Note how the orbitals fill from lowest energy to highest energy due to the Aufbau principle, and how in the 2p subshell two electrons remain unpaired due to Hund's rule.

💎💎To condense all of this information, the electron configuration system was developed. This uses the Aufbau build up principle.
--Here is the electron-configuration of oxygen as shown in the energy-level diagram:


--Electron-configurations can also be written for cations and anions. For the purpose of example, I will simply write the electron-configurations of oxygen if it were to have a +1 or -1 charge:

oxygen as a cation, charge +1, lost one electron


oxygen as an anion, charge -1, gained one electron


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