Atoms can have multiple valencies due to the presence of multiple energy levels and orbitals in their electron configuration. Valency refers to the number of electrons an atom can gain, lose, or share to attain a stable configuration.
The valency of an atom depends on its outermost energy level, known as the valence shell. The valence shell determines the number of available orbitals for electrons and their capacity to form chemical bonds. In general, the valency of an atom corresponds to the number of electrons needed to fill or empty the outermost energy level to achieve the stable electron configuration of a noble gas.
To calculate the valency of an atom using its atomic number, you can look at the position of the atom in the periodic table. The group number of an element (also known as the column number) indicates its valency in many cases.
For example, in the main group elements (also called representative elements), the valency often corresponds to the group number. For instance, elements in Group 1 (such as hydrogen, H, and sodium, Na) have a valency of 1, while elements in Group 2 (such as magnesium, Mg) have a valency of 2.
However, for transition metals and other elements in the d- and f-blocks of the periodic table, determining valency solely based on the atomic number is not as straightforward. These elements often exhibit multiple valencies due to the involvement of inner d- or f-orbitals in bonding. In such cases, the valency of a transition metal can vary depending on the specific compound or context.
Determining the valency of a particular atom in a compound often involves understanding its electron configuration and the bonding patterns it exhibits in different chemical environments.