The valence number of an element, also known as the valence electron number, refers to the number of electrons in the outermost shell (or energy level) of an atom. These electrons are called valence electrons and play a crucial role in determining an element's chemical properties and its ability to form chemical bonds with other atoms.
The valence number is often associated with the group number of an element in the periodic table. Elements in the same group typically have the same valence number. For example, elements in Group 1 (such as hydrogen, lithium, sodium) have one valence electron, while elements in Group 17 (such as fluorine, chlorine, bromine) have seven valence electrons.
The valence number is significant because it helps predict an element's reactivity and the types of chemical bonds it can form. Elements tend to gain, lose, or share electrons in order to achieve a stable electron configuration, usually with a full outer shell of electrons. This interaction between valence electrons of different atoms leads to the formation of compounds and the wide variety of chemical reactions observed in chemistry.