Valence electrons are the electrons found in the outermost energy level, or shell, of an atom. These electrons are responsible for the chemical properties and behavior of elements because they participate in the formation of chemical bonds.
Elements can have different numbers of valence electrons due to their electron configuration, which follows certain rules. The electron configuration is determined by the way electrons occupy specific energy levels or orbitals within an atom.
Elements in the s- and p-blocks of the periodic table can have multiple valence electrons. The s-block elements, located in the leftmost two columns of the periodic table, have one or two valence electrons. For example, sodium (Na) has one valence electron in its 3s orbital, while calcium (Ca) has two valence electrons in its 4s orbital.
The p-block elements, found in groups 13 to 18 on the periodic table, have between one and six valence electrons. These elements fill their p orbitals, and the number of valence electrons corresponds to the group number minus ten. For instance, carbon (C) has four valence electrons in its 2p orbital because it is in group 14, while nitrogen (N) has five valence electrons in its 2p orbital because it is in group 15.
On the other hand, some elements, such as the noble gases in group 18, have a stable electron configuration with a complete outermost energy level. They are considered to have a full complement of valence electrons, and therefore, they do not readily form chemical bonds. For example, helium (He) has two valence electrons in its 1s orbital, which is a complete shell, making it chemically inert.
The number of valence electrons an element possesses affects its reactivity and the types of chemical bonds it can form. Elements with incomplete outermost energy levels tend to gain, lose, or share electrons to achieve a stable electron configuration, usually by acquiring a full set of valence electrons. This process allows elements to attain a more stable, lower-energy state and form compounds with other elements.