Final answer:
Valency is defined by the number of valence electrons an atom must gain, lose, or share to fill its outer shell. Valence electrons are essential in determining an element's chemical properties and reactivity. The octet rule often governs the stability achieved through chemical bonds.
Step-by-step explanation:
The valency of an element refers to the number of valence electrons that an atom possesses in its outer shell. These are the electrons available for forming bonds with other atoms, either by losing, gaining, or sharing them to achieve a full outer shell, which often means having eight electrons to fulfill the octet rule. Atoms tend to be more stable when this happens because they achieve a noble gas electron configuration.
The presence of valence electrons is critical for determining an element's chemical properties and its ability to bond with other elements. For example, when two hydrogen atoms each share their one valence electron, they create a stable H2 molecule.
Chemical bonding is intricately related to the concept of valence electrons. The more open or incomplete the valence shell, the more reactive the atom, and thus, more likely to form chemical bonds. Conversely, if an atom's valence shell is full, it is generally less reactive.