Hund's rule states that
When there are two or more atomic orbitals with the same energy, an electron will occupy an empty orbital before it will pair up with another elecron.
In this way, elecron repulsion is minimized.
The sixth elecron of a carbon atom, therefore, goes into an empty 2p orbitals, rather than pairing up with the elecron already occupying a 2p orbital see in the table.
. The electronic configuration of the smallest atom
There is one more empty 2p orbital, so that is where nitrogen's seventh electron goes. The eighth electron of an oxygen atom pairing up with an electron occupying a 2p orbital rather than going into the higher-energy 3s orbital.
The location of the electrons in the remaining elements can be assigned using these three rules.
The elecrons in inner shells (those below the outermost shell) are called core electrons. Core electrons do not participate in chemical bonding.the electrons in the outermost shell are called valence electrons.
Carbon has two core electrons and four valence electrons Lithium and Sodium each have one valence electron. If you examine the periodic table, you will see that and Sodium are in the same column. Elements in the same column of the periodic table have the same number of valence electrons. Because the number of Valence electrons in the major factor determining an element's chemical properties, elements in the same column of the periodic table have similar chemical properties. Thus, the chemical behaviour of an element depends on its electronic configuration.
* Some important notes
• core electrons are electrons in inner shells.
• Valence electrons are electrons in the outermost shell.
• The chemical behaviour of an element depends on its electronic configuration.
